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126-465: HD 87884 is separated from Regulus by 176 ″ and is itself a close pair. Regulus, along with five slightly dimmer stars ( Zeta Leonis , Mu Leonis , Gamma Leonis , Epsilon Leonis , and Eta Leonis ) have collectively been called 'the Sickle', which is an asterism that marks the head of Leo. α Leonis (Latinized to Alpha Leonis ) is the star system's Bayer designation . The traditional name Rēgulus

252-529: A 1 MOA rifle should be capable, under ideal conditions, of repeatably shooting 1-inch groups at 100 yards. Most higher-end rifles are warrantied by their manufacturer to shoot under a given MOA threshold (typically 1 MOA or better) with specific ammunition and no error on the shooter's part. For example, Remington's M24 Sniper Weapon System is required to shoot 0.8 MOA or better, or be rejected from sale by quality control . Rifle manufacturers and gun magazines often refer to this capability as sub-MOA , meaning

378-596: A dust ring-cloud , with a suspected origin either from Venus–trailing asteroids, interplanetary dust migrating in waves, or the remains of the Solar System's original circumstellar disc that formed the planetary system . Earth and Venus have a near orbital resonance of 13:8 (Earth orbits eight times for every 13 orbits of Venus). Therefore, they approach each other and reach inferior conjunction in synodic periods of 584 days, on average. The path that Venus makes in relation to Earth viewed geocentrically draws

504-493: A habitable environment , before a runaway greenhouse effect evaporated any water and turned Venus into its present state. The rotation of Venus has been slowed and turned against its orbital direction ( retrograde ) by the currents and drag of its atmosphere. It takes 224.7 Earth days for Venus to complete an orbit around the Sun, and a Venusian solar year is just under two Venusian days long. The orbits of Venus and Earth are

630-408: A pentagram over five synodic periods, shifting every period by 144°. This pentagram of Venus is sometimes referred to as the petals of Venus due to the path's visual similarity to a flower. When Venus lies between Earth and the Sun in inferior conjunction, it makes the closest approach to Earth of any planet at an average distance of 41 million km (25 million mi). Because of

756-495: A visual angle of one minute of arc, from a distance of twenty feet . A 20/20 letter subtends 5 minutes of arc total. The deviation from parallelism between two surfaces, for instance in optical engineering , is usually measured in arcminutes or arcseconds. In addition, arcseconds are sometimes used in rocking curve (ω-scan) x ray diffraction measurements of high-quality epitaxial thin films. Some measurement devices make use of arcminutes and arcseconds to measure angles when

882-451: A 4" telescope. Although naked eye visibility of Venus's phases is disputed, records exist of observations of its crescent. When Venus is sufficiently bright with enough angular distance from the sun, it is easily observed in a clear daytime sky with the naked eye, though most people do not know to look for it. Astronomer Edmund Halley calculated its maximum naked eye brightness in 1716, when many Londoners were alarmed by its appearance in

1008-414: A Venusian year (243 versus 224.7 Earth days). Slowed by its strong atmospheric current the length of the day also fluctuates by up to 20 minutes. Venus's equator rotates at 6.52 km/h (4.05 mph), whereas Earth's rotates at 1,674.4 km/h (1,040.4 mph). Venus's rotation period measured with Magellan spacecraft data over a 500-day period is smaller than the rotation period measured during

1134-459: A change that would have occurred over the course of billions of years. The rotation period of Venus may represent an equilibrium state between tidal locking to the Sun's gravitation, which tends to slow rotation, and an atmospheric tide created by solar heating of the thick Venusian atmosphere. The 584-day average interval between successive close approaches to Earth is almost exactly equal to 5   Venusian solar days (5.001444 to be precise), but

1260-687: A circle with a diameter of 1.047 inches (which is often rounded to just 1 inch) at 100 yards (2.66 cm at 91 m or 2.908 cm at 100 m), a traditional distance on American target ranges . The subtension is linear with the distance, for example, at 500 yards, 1 MOA subtends 5.235 inches, and at 1000 yards 1 MOA subtends 10.47 inches. Since many modern telescopic sights are adjustable in half ( ⁠ 1 / 2 ⁠ ), quarter ( ⁠ 1 / 4 ⁠ ) or eighth ( ⁠ 1 / 8 ⁠ ) MOA increments, also known as clicks , zeroing and adjustments are made by counting 2, 4 and 8 clicks per MOA respectively. For example, if

1386-459: A degree) and specify locations within about 120 metres (390 feet). For navigational purposes positions are given in degrees and decimal minutes, for instance The Needles lighthouse is at 50º 39.734’N 001º 35.500’W. Related to cartography, property boundary surveying using the metes and bounds system and cadastral surveying relies on fractions of a degree to describe property lines' angles in reference to cardinal directions . A boundary "mete"

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1512-570: A degree, ⁠ 1 / 1 296 000 ⁠ of a turn, and ⁠ π / 648 000 ⁠ (about ⁠ 1 / 206 264 .8 ⁠ ) of a radian. These units originated in Babylonian astronomy as sexagesimal (base 60) subdivisions of the degree; they are used in fields that involve very small angles, such as astronomy , optometry , ophthalmology , optics , navigation , land surveying , and marksmanship . To express even smaller angles, standard SI prefixes can be employed;

1638-511: A degree/day in the Earth's annual rotation around the Sun, which is off by roughly 1%. The same ratios hold for seconds, due to the consistent factor of 60 on both sides. The arcsecond is also often used to describe small astronomical angles such as the angular diameters of planets (e.g. the angular diameter of Venus which varies between 10″ and 60″); the proper motion of stars; the separation of components of binary star systems ; and parallax ,

1764-574: A depression. These features are volcanic in origin. Most Venusian surface features are named after historical and mythological women. Exceptions are Maxwell Montes, named after James Clerk Maxwell , and highland regions Alpha Regio , Beta Regio , and Ovda Regio . The last three features were named before the current system was adopted by the International Astronomical Union , the body which oversees planetary nomenclature . The longitude of physical features on Venus

1890-467: A dynamo at its core. A dynamo requires three things: a conducting liquid, rotation, and convection . The core is thought to be electrically conductive and, although its rotation is often thought to be too slow, simulations show it is adequate to produce a dynamo. This implies that the dynamo is missing because of a lack of convection in Venus's core. On Earth, convection occurs in the liquid outer layer of

2016-422: A fraction of a mrad) are collectively called a mrad reticle. If the markings are round they are called mil-dots . In the table below conversions from mrad to metric values are exact (e.g. 0.1 mrad equals exactly 10 mm at 100 metres), while conversions of minutes of arc to both metric and imperial values are approximate. In humans, 20/20 vision is the ability to resolve a spatial pattern separated by

2142-484: A group measuring 0.7 inches followed by a group that is 1.3 inches, this is not statistically abnormal. The metric system counterpart of the MOA is the milliradian (mrad or 'mil'), being equal to 1 ⁄ 1000 of the target range, laid out on a circle that has the observer as centre and the target range as radius. The number of milliradians on a full such circle therefore always is equal to 2 × π × 1000, regardless

2268-400: A gun consistently shooting groups under 1 MOA. This means that a single group of 3 to 5 shots at 100 yards, or the average of several groups, will measure less than 1 MOA between the two furthest shots in the group, i.e. all shots fall within 1 MOA. If larger samples are taken (i.e., more shots per group) then group size typically increases, however this will ultimately average out. If a rifle

2394-455: A line running from the starting point 85.69 feet in a direction 65° 39′ 18″ (or 65.655°) away from north toward the west. The arcminute is commonly found in the firearms industry and literature, particularly concerning the precision of rifles , though the industry refers to it as minute of angle (MOA). It is especially popular as a unit of measurement with shooters familiar with the imperial measurement system because 1 MOA subtends

2520-420: A minute, for example, written as 42° 25.32′ or 42° 25.322′. This notation has been carried over into marine GPS and aviation GPS receivers, which normally display latitude and longitude in the latter format by default. The average apparent diameter of the full Moon is about 31 arcminutes, or 0.52°. One arcminute is the approximate distance two contours can be separated by, and still be distinguished by,

2646-431: A modern second. Since antiquity, the arcminute and arcsecond have been used in astronomy : in the ecliptic coordinate system as latitude (β) and longitude (λ); in the horizon system as altitude (Alt) and azimuth (Az); and in the equatorial coordinate system as declination (δ). All are measured in degrees, arcminutes, and arcseconds. The principal exception is right ascension (RA) in equatorial coordinates, which

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2772-449: A more massive primary atmosphere from solar nebula have been proposed to explain the enrichment. However, the atmosphere is depleted of radiogenic argon, a proxy for mantle degassing, suggesting an early shutdown of major magmatism. Studies have suggested that billions of years ago, Venus's atmosphere could have been much more like the one surrounding the early Earth, and that there may have been substantial quantities of liquid water on

2898-424: A multiple star system consisting of at least four stars. Regulus A is the dominant star, with a binary companion 177" distant that is thought to be physically related. Regulus D is a 12th magnitude companion at 212", but is an unrelated background object. Regulus A is a binary star consisting of a blue-white subgiant star of spectral type B8, which is orbited by a star of at least 0.3 solar masses, which

3024-528: A period at the end of a sentence in the Apollo mission manuals left on the Moon as seen from Earth. One nanoarcsecond is about the size of a penny on Neptune 's moon Triton as observed from Earth. Also notable examples of size in arcseconds are: The concepts of degrees, minutes, and seconds—as they relate to the measure of both angles and time—derive from Babylonian astronomy and time-keeping. Influenced by

3150-535: A person with 20/20 vision . One arcsecond is the approximate angle subtended by a U.S. dime coin (18 mm) at a distance of 4 kilometres (about 2.5 mi). An arcsecond is also the angle subtended by One milliarcsecond is about the size of a half dollar, seen from a distance equal to that between the Washington Monument and the Eiffel Tower . One microarcsecond is about the size of

3276-455: A planet was on July 7, 1959, by Venus. The next will occur on October 1, 2044, also by Venus. Other planets will not occult Regulus over the next few millennia because of their node positions . An occultation of Regulus by the asteroid 166 Rhodope was filmed in Italy on October 19, 2005. Differential bending of light was measured to be consistent with general relativity . Regulus was occulted by

3402-468: A precision-oriented firearm's performance will be measured in MOA. This simply means that under ideal conditions (i.e. no wind, high-grade ammo, clean barrel, and a stable mounting platform such as a vise or a benchrest used to eliminate shooter error), the gun is capable of producing a group of shots whose center points (center-to-center) fit into a circle, the average diameter of circles in several groups can be subtended by that amount of arc. For example,

3528-417: A thick, global sulfuric acid cloud cover. At the surface it has a mean temperature of 737 K (464 °C; 867 °F) and a pressure 92 times that of Earth's at sea level. These extreme conditions compress carbon dioxide into a supercritical state at Venus's surface. Internally, Venus has a core ,  mantle , and  crust . Venus lacks an internal dynamo, and its weakly induced magnetosphere

3654-399: A whole is the twenty-first brightest star in the night sky with an apparent magnitude of +1.35. The light output is dominated by Regulus A. Regulus B, if seen in isolation, would be a binocular object of magnitude +8.1, and its companion, Regulus C, the faintest of the three stars that has been directly observed, would require a substantial telescope to be seen, at magnitude +13.5. Regulus A

3780-531: Is Latin for 'prince' or 'little king'. In 2016, the International Astronomical Union organized a Working Group on Star Names (WGSN) to catalog and standardize proper names for stars. The WGSN's first bulletin of July 2016 included a table of the first two batches of names approved by the WGSN; which included Regulus for this star. It is now so entered in the IAU Catalog of Star Names. The Regulus system as

3906-557: Is δ (Delta) Geminorum , of magnitude +3.53. As Regulus closely aligns to the mean orbits of large bodies of the Solar System and involves more light reaching the Earth than such other stars, the system has advanced telescopic use (to study and identify objects occulting and casting their shadow on a telescope, including known or unknown asteroids of the Solar System such as Trojans , being in line by definition with their associated planetary plane). The last occultation of Regulus by

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4032-422: Is 96.5% carbon dioxide, with most of the remaining 3.5% being nitrogen . The surface pressure is 9.3 megapascals (93 bars ), and the average surface temperature is 737 K (464 °C; 867 °F), above the critical points of both major constituents and making the surface atmosphere a supercritical fluid out of mainly supercritical carbon dioxide and some supercritical nitrogen. The Venusian surface

4158-524: Is a K2V star, while the second is about M4V. The companion pair has an orbital period of about 600 years with a separation of 2.5" in 1942. Rēgulus is Latin for 'prince' or 'little king'; its Greek equivalent is Basiliskos or, in Latinised form, Basiliscus. The name Regulus first appeared in the early 16th century. It is also known as Qalb al-Asad, from the Arabic قلب الأسد, meaning 'the heart of

4284-540: Is a rocky body like Earth. It is similar to Earth in size and mass and is often described as Earth's "sister" or "twin". Venus is close to spherical due to its slow rotation. Venus has a diameter of 12,103.6 km (7,520.8 mi)—only 638.4 km (396.7 mi) less than Earth's—and its mass is 81.5% of Earth's, making it the third-smallest planet in the Solar System . Conditions on the Venusian surface differ radically from those on Earth because its dense atmosphere

4410-434: Is also abbreviated as arcmin or amin . Similarly, double prime ″ (U+2033) designates the arcsecond, though a double quote " (U+0022) is commonly used where only ASCII characters are permitted. One arcsecond is thus written as 1″. It is also abbreviated as arcsec or asec . In celestial navigation , seconds of arc are rarely used in calculations, the preference usually being for degrees, minutes, and decimals of

4536-430: Is caused by atmospheric interactions with the solar wind . Internal heat escapes through active volcanism , resulting in resurfacing instead of plate tectonics . Venus is one of two planets in the Solar System , the other being Mercury , that have no moons . Conditions perhaps favourable for life on Venus have been identified at its cloud layers. Venus may have had liquid surface water early in its history with

4662-405: Is currently volcanically active, specifically the detection of olivine , a volcanic product that would weather quickly on the planet's surface. This massive volcanic activity is fuelled by a superheated interior, which models say could be explained by energetic collisions from when the planet was young. Impacts would have had significantly higher velocity than on Earth, both because Venus's orbit

4788-486: Is described with a beginning reference point, the cardinal direction North or South followed by an angle less than 90 degrees and a second cardinal direction, and a linear distance. The boundary runs the specified linear distance from the beginning point, the direction of the distance being determined by rotating the first cardinal direction the specified angle toward the second cardinal direction. For example, North 65° 39′ 18″ West 85.69 feet would describe

4914-487: Is expressed relative to its prime meridian . The original prime meridian passed through the radar-bright spot at the centre of the oval feature Eve, located south of Alpha Regio. After the Venera missions were completed, the prime meridian was redefined to pass through the central peak in the crater Ariadne on Sedna Planitia . The stratigraphically oldest tessera terrains have consistently lower thermal emissivity than

5040-636: Is far from certain. Studies reported on 26 October 2023 suggest for the first time that Venus may have had plate tectonics during ancient times and, as a result, may have had a more habitable environment , possibly one capable of sustaining life . Venus has gained interest as a case for research into the development of Earth-like planets and their habitability . Much of the Venusian surface appears to have been shaped by volcanic activity. Venus has several times as many volcanoes as Earth, and it has 167 large volcanoes that are over 100 km (60 mi) across. The only volcanic complex of this size on Earth

5166-576: Is faster due to its closer proximity to the Sun and because objects would require higher orbital eccentricities to collide with the planet. In 2008 and 2009, the first direct evidence for ongoing volcanism was observed by Venus Express , in the form of four transient localized infrared hot spots within the rift zone Ganis Chasma , near the shield volcano Maat Mons . Three of the spots were observed in more than one successive orbit. These spots are thought to represent lava freshly released by volcanic eruptions. The actual temperatures are not known, because

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5292-462: Is formed by sulphur dioxide and water through a chemical reaction resulting in sulfuric acid hydrate. Additionally, the clouds consist of approximately 1% ferric chloride . Other possible constituents of the cloud particles are ferric sulfate , aluminium chloride and phosphoric anhydride . Clouds at different levels have different compositions and particle size distributions. These clouds reflect, similar to thick cloud cover on Earth, about 70% of

5418-404: Is induced by an interaction between the ionosphere and the solar wind , rather than by an internal dynamo as in the Earth's core . Venus's small induced magnetosphere provides negligible protection to the atmosphere against solar and cosmic radiation . The lack of an intrinsic magnetic field on Venus was surprising, given that it is similar to Earth in size and was expected to contain

5544-496: Is itself a spectroscopic binary; the secondary star has not yet been directly observed as it is much fainter than the primary. The BC pair lies at an angular distance of 177 arc-seconds from Regulus A, making them visible in amateur telescopes. Regulus is 0.465 degrees from the ecliptic , the closest of the bright stars, and is often occulted by the Moon . This occurs in spates every 9.3 years, due to lunar precession . The last spate

5670-423: Is measured in time units of hours, minutes, and seconds. Contrary to what one might assume, minutes and seconds of arc do not directly relate to minutes and seconds of time, in either the rotational frame of the Earth around its own axis (day), or the Earth's rotational frame around the Sun (year). The Earth's rotational rate around its own axis is 15 minutes of arc per minute of time (360 degrees / 24 hours in day);

5796-404: Is most likely at least partially liquid because the two planets have been cooling at about the same rate, although a completely solid core cannot be ruled out. The slightly smaller size of Venus means pressures are 24% lower in its deep interior than Earth's. The predicted values for the moment of inertia based on planetary models suggest a core radius of 2,900–3,450 km. This is in line with

5922-432: Is not known with certainty, but speculation has ranged from elemental tellurium to lead sulfide ( galena ). Although Venus has no seasons, in 2019 astronomers identified a cyclical variation in sunlight absorption by the atmosphere, possibly caused by opaque, absorbing particles suspended in the upper clouds. The variation causes observed changes in the speed of Venus's zonal winds and appears to rise and fall in time with

6048-514: Is probably a white dwarf . The two stars take approximately 40 days to complete an orbit around their common centre of mass. Given the extremely distorted shape of the primary, the relative orbital motion may be notably altered with respect to the two-body purely Keplerian scenario because of non-negligible long-term orbital perturbations affecting, for example, its orbital period . In other words, Kepler's third law , which holds exactly only for two point-like masses, would no longer be valid for

6174-863: Is recent evidence of lava flow on Venus (2024), such as flows on Sif Mons, a shield volcano, and on Niobe Planitia, a flat plain. There are visible calderas . The planet has few impact craters , demonstrating that the surface is relatively young, at 300–600   million years old. Venus has some unique surface features in addition to the impact craters, mountains, and valleys commonly found on rocky planets. Among these are flat-topped volcanic features called " farra ", which look somewhat like pancakes and range in size from 20 to 50 km (12 to 31 mi) across, and from 100 to 1,000 m (330 to 3,280 ft) high; radial, star-like fracture systems called "novae"; features with both radial and concentric fractures resembling spider webs, known as " arachnoids "; and "coronae", circular rings of fractures sometimes surrounded by

6300-503: Is roughly 30 metres (98 feet). The exact distance varies along meridian arcs or any other great circle arcs because the figure of the Earth is slightly oblate (bulges a third of a percent at the equator). Positions are traditionally given using degrees, minutes, and seconds of arcs for latitude , the arc north or south of the equator, and for longitude , the arc east or west of the Prime Meridian . Any position on or above

6426-406: Is speculation on the possibility that life exists in the upper cloud layers of Venus, 50 km (30 mi) up from the surface, where the atmospheric conditions are the most Earth-like in the Solar System, with temperatures ranging between 303 and 353 K (30 and 80 °C; 86 and 176 °F), and the pressure and radiation being about the same as at Earth's surface, but with acidic clouds and

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6552-626: Is that some MOA scopes, including some higher-end models, are calibrated such that an adjustment of 1 MOA on the scope knobs corresponds to exactly 1 inch of impact adjustment on a target at 100 yards, rather than the mathematically correct 1.047 inches. This is commonly known as the Shooter's MOA (SMOA) or Inches Per Hundred Yards (IPHY). While the difference between one true MOA and one SMOA is less than half of an inch even at 1000 yards, this error compounds significantly on longer range shots that may require adjustment upwards of 20–30 MOA to compensate for

6678-420: Is that the absence of a late, large impact on Venus ( contra the Earth's "Moon-forming" impact) left the core of Venus stratified from the core's incremental formation, and without the forces to initiate/sustain convection, and thus a "geodynamo". The weak magnetosphere around Venus means that the solar wind is interacting directly with its outer atmosphere. Here, ions of hydrogen and oxygen are being created by

6804-465: Is the Big Island of Hawaii. More than 85,000 volcanoes on Venus were identified and mapped. This is not because Venus is more volcanically active than Earth, but because its crust is older and is not subject to the same erosion process. Earth's oceanic crust is continually recycled by subduction at the boundaries of tectonic plates, and has an average age of about 100 million years, whereas

6930-450: Is visible in dark skies long after sunset. As the brightest point-like object in the sky, Venus is a commonly misreported " unidentified flying object ". As it orbits the Sun, Venus displays phases like those of the Moon in a telescopic view. The planet appears as a small and "full" disc when it is on the opposite side of the Sun (at superior conjunction ). Venus shows a larger disc and "quarter phase" at its maximum elongations from

7056-417: Is −4.14 with a standard deviation of 0.31. The brightest magnitude occurs during the crescent phase about one month before or after an inferior conjunction. Venus fades to about magnitude −3 when it is backlit by the Sun. The planet is bright enough to be seen in broad daylight, but is more easily visible when the Sun is low on the horizon or setting. As an inferior planet , it always lies within about 47° of

7182-487: The Magellan spacecraft imaged a highly reflective substance at the tops of the highest mountain peaks, a " Venus snow " that bore a strong resemblance to terrestrial snow. This substance likely formed from a similar process to snow, albeit at a far higher temperature. Too volatile to condense on the surface, it rose in gaseous form to higher elevations, where it is cooler and could precipitate. The identity of this substance

7308-485: The Sumerians , the ancient Babylonians divided the Sun's perceived motion across the sky over the course of one full day into 360 degrees. Each degree was subdivided into 60 minutes and each minute into 60 seconds. Thus, one Babylonian degree was equal to four minutes in modern terminology, one Babylonian minute to four modern seconds, and one Babylonian second to ⁠ 1 / 15 ⁠ (approximately 0.067) of

7434-467: The Sun . Venus "overtakes" Earth every 584 days as it orbits the Sun. As it does so, it changes from the "Evening Star", visible after sunset, to the "Morning Star", visible before sunrise. Although Mercury, the other inferior planet, reaches a maximum elongation of only 28° and is often difficult to discern in twilight, Venus is hard to miss when it is at its brightest. Its greater maximum elongation means it

7560-405: The Sun's mass. It is spinning extremely rapidly, with a rotation period of only 15.9 hours (for comparison, the rotation period of the Sun is 25 days), which causes it to have a highly oblate shape. This results in so-called gravity darkening : the photosphere at Regulus' poles is considerably hotter, and five times brighter per unit surface area, than its equatorial region. The star's surface at

7686-401: The decreasing eccentricity of Earth's orbit , the minimum distances will become greater over tens of thousands of years. From the year   1 to 5383, there are 526 approaches less than 40 million km (25 million mi); then, there are none for about 60,158 years. While Venus approaches Earth the closest, Mercury is more often the closest to Earth of all planets. Venus has

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7812-422: The dissociation of water molecules from ultraviolet radiation. The solar wind then supplies energy that gives some of these ions sufficient velocity to escape Venus's gravity field. This erosion process results in a steady loss of low-mass hydrogen, helium, and oxygen ions, whereas higher-mass molecules, such as carbon dioxide, are more likely to be retained. Atmospheric erosion by the solar wind could have led to

7938-400: The milliarcsecond (mas) and microarcsecond (μas), for instance, are commonly used in astronomy. For a three-dimensional area such as on a sphere, square arcminutes or seconds may be used. The prime symbol ′ ( U+ 2032 ) designates the arcminute, though a single quote ' (U+0027) is commonly used where only ASCII characters are permitted. One arcminute is thus written as 1′. It

8064-497: The 16-year period between the Magellan spacecraft and Venus Express visits, with a difference of about 6.5   minutes. Because of the retrograde rotation, the length of a solar day on Venus is significantly shorter than the sidereal day, at 116.75 Earth days (making the Venusian solar day shorter than Mercury 's 176 Earth days — the 116-day figure is close to the average number of days it takes Mercury to slip underneath

8190-558: The Earth in its orbit [the number of days of Mercury's synodic orbital period]). One Venusian year is about 1.92   Venusian solar days. To an observer on the surface of Venus, the Sun would rise in the west and set in the east, although Venus's opaque clouds prevent observing the Sun from the planet's surface. Venus may have formed from the solar nebula with a different rotation period and obliquity, reaching its current state because of chaotic spin changes caused by planetary perturbations and tidal effects on its dense atmosphere,

8316-451: The Earth's reference ellipsoid can be precisely given with this method. However, when it is inconvenient to use base -60 for minutes and seconds, positions are frequently expressed as decimal fractional degrees to an equal amount of precision. Degrees given to three decimal places ( ⁠ 1 / 1000 ⁠ of a degree) have about ⁠ 1 / 4 ⁠ the precision of degrees-minutes-seconds ( ⁠ 1 / 3600 ⁠ of

8442-606: The Earth's atmosphere but are diffraction limited . For example, the Hubble Space Telescope can reach an angular size of stars down to about 0.1″. Minutes (′) and seconds (″) of arc are also used in cartography and navigation . At sea level one minute of arc along the equator equals exactly one geographical mile (not to be confused with international mile or statute mile) along the Earth's equator or approximately one nautical mile (1,852 metres ; 1.151 miles ). A second of arc, one sixtieth of this amount,

8568-475: The Earth's rotational rate around the Sun (not entirely constant) is roughly 24 minutes of time per minute of arc (from 24 hours in day), which tracks the annual progression of the Zodiac. Both of these factor in what astronomical objects you can see from surface telescopes (time of year) and when you can best see them (time of day), but neither are in unit correspondence. For simplicity, the explanations given assume

8694-468: The Lion's breast". Second of arc A minute of arc , arcminute ( arcmin ), arc minute , or minute arc , denoted by the symbol ′ , is a unit of angular measurement equal to ⁠ 1 / 60 ⁠ of one degree . Since one degree is ⁠ 1 / 360 ⁠ of a turn, or complete rotation , one arcminute is ⁠ 1 / 21 600 ⁠ of a turn. The nautical mile (nmi)

8820-515: The Regulus system. Regulus A was long thought to be fairly young, only 50–100 million years old, calculated by comparing its temperature, luminosity, and mass. The existence of a white dwarf companion would mean that the system is at least 1 billion years old, just to account for the formation of the white dwarf. The discrepancy can be accounted for by a history of mass transfer onto a once-smaller Regulus A. The primary of Regulus A has about 3.8 times

8946-487: The Sun and thus receives only 25% of Mercury's solar irradiance , of 2,600 W/m (double that of Earth). Because of its runaway greenhouse effect , Venus has been identified by scientists such as Carl Sagan as a warning and research object linked to climate change on Earth. Venus's atmosphere is rich in primordial noble gases compared to that of Earth. This enrichment indicates an early divergence from Earth in evolution. An unusually large comet impact or accretion of

9072-477: The Sun's 11-year sunspot cycle . The existence of lightning in the atmosphere of Venus has been controversial since the first suspected bursts were detected by the Soviet Venera probes . In 2006–07, Venus Express clearly detected whistler mode waves , the signatures of lightning. Their intermittent appearance indicates a pattern associated with weather activity. According to these measurements,

9198-422: The Sun, and appears at its brightest in the night sky. The planet presents a much larger thin "crescent" in telescopic views as it passes along the near side between Earth and the Sun. Venus displays its largest size and "new phase" when it is between Earth and the Sun (at inferior conjunction). Its atmosphere is visible through telescopes by the halo of sunlight refracted around it. The phases are clearly visible in

9324-402: The Venusian surface is covered by smooth, volcanic plains, consisting of 70% plains with wrinkle ridges and 10% smooth or lobate plains. Two highland "continents" make up the rest of its surface area, one lying in the planet's northern hemisphere and the other just south of the equator. The northern continent is called Ishtar Terra after Ishtar , the Babylonian goddess of love, and is about

9450-590: The Venusian surface is estimated to be 300–600   million years old. Several lines of evidence point to ongoing volcanic activity on Venus. Sulfur dioxide concentrations in the upper atmosphere dropped by a factor of 10 between 1978 and 1986, jumped in 2006, and again declined 10-fold. This may mean that levels had been boosted several times by large volcanic eruptions. It has been suggested that Venusian lightning (discussed below) could originate from volcanic activity (i.e. volcanic lightning ). In January 2020, astronomers reported evidence that suggests that Venus

9576-451: The angle, measured in arcseconds, of the object's apparent movement caused by parallax. The European Space Agency 's astrometric satellite Gaia , launched in 2013, can approximate star positions to 7 microarcseconds (μas). Apart from the Sun, the star with the largest angular diameter from Earth is R Doradus , a red giant with a diameter of 0.05″. Because of the effects of atmospheric blurring , ground-based telescopes will smear

9702-422: The asteroid 163 Erigone in the early morning of March 20, 2014. The center of the shadow path passed through New York and eastern Ontario , but no one is known to have seen it, due to cloud cover. The International Occultation Timing Association recorded no observations at all. Although best seen in the evening in the northern hemisphere's late winter and spring, Regulus appears at some time of night throughout

9828-411: The atmosphere before reaching the ground. Without data from reflection seismology or knowledge of its moment of inertia , little direct information is available about the internal structure and geochemistry of Venus. The similarity in size and density between Venus and Earth suggests that they share a similar internal structure: a core , mantle , and crust . Like that of Earth, the Venusian core

9954-478: The atmosphere of Venus. On 29 January 2013, ESA scientists reported that the ionosphere of Venus streams outwards in a manner similar to "the ion tail seen streaming from a comet under similar conditions." In December 2015, and to a lesser extent in April and May 2016, researchers working on Japan's Akatsuki mission observed bow-shaped objects in the atmosphere of Venus. This was considered direct evidence of

10080-443: The atmosphere. Later research attributed the spectroscopic signal that was interpreted as phosphine to sulphur dioxide, or found that in fact there was no absorption line. Thermal inertia and the transfer of heat by winds in the lower atmosphere mean that the temperature of Venus's surface does not vary significantly between the planet's two hemispheres, those facing and not facing the Sun, despite Venus's slow rotation. Winds at

10206-628: The bullet drop. If a shot requires an adjustment of 20 MOA or more, the difference between true MOA and SMOA will add up to 1 inch or more. In competitive target shooting, this might mean the difference between a hit and a miss. The physical group size equivalent to m minutes of arc can be calculated as follows: group size = tan( ⁠ m / 60 ⁠ ) × distance. In the example previously given, for 1 minute of arc, and substituting 3,600 inches for 100 yards, 3,600 tan( ⁠ 1 / 60 ⁠ ) ≈ 1.047 inches. In metric units 1 MOA at 100 metres ≈ 2.908 centimetres. Sometimes,

10332-517: The carbon dioxide air. Venus's atmosphere could also have a potential thermal habitable zone at elevations of 54 to 48 km, with lower elevations inhibiting cell growth and higher elevations exceeding evaporation temperature. The putative detection of an absorption line of phosphine in Venus's atmosphere, with no known pathway for abiotic production, led to speculation in September 2020 that there could be extant life currently present in

10458-408: The closest between any two Solar System planets, approaching each other in synodic periods of 1.6 years. Venus and Earth have the lowest difference in gravitational potential of any pair of Solar System planets. This allows Venus to be the most accessible destination and a useful gravity assist waypoint for interplanetary flights from Earth. Venus figures prominently in human culture and in

10584-420: The core because the bottom of the liquid layer is much higher in temperature than the top. On Venus, a global resurfacing event may have shut down plate tectonics and led to a reduced heat flux through the crust. This insulating effect would cause the mantle temperature to increase, thereby reducing the heat flux out of the core. As a result, no internal geodynamo is available to drive a magnetic field. Instead,

10710-552: The crust. Then, over a period of about 100   million years, subduction occurs on an enormous scale, completely recycling the crust. Venusian craters range from 3 to 280 km (2 to 174 mi) in diameter. No craters are smaller than 3   km, because of the effects of the dense atmosphere on incoming objects. Objects with less than a certain kinetic energy are slowed so much by the atmosphere that they do not create an impact crater. Incoming projectiles less than 50 m (160 ft) in diameter will fragment and burn up in

10836-412: The daytime with overcast clouds". Strong 300 km/h (185 mph) winds at the cloud tops go around Venus about every four to five Earth days. Winds on Venus move at up to 60 times the speed of its rotation, whereas Earth's fastest winds are only 10–20% rotation speed. The surface of Venus is effectively isothermal ; it retains a constant temperature not only between the two hemispheres but between

10962-596: The daytime. French emperor Napoleon Bonaparte once witnessed a daytime apparition of the planet while at a reception in Luxembourg . Another historical daytime observation of the planet took place during the inauguration of the American president Abraham Lincoln in Washington, D.C., on 4   March 1865. A transit of Venus is the appearance of Venus in front of the Sun, during inferior conjunction . Since

11088-578: The early solar system orbital dynamics have shown that the eccentricity of the Venus orbit may have been substantially larger in the past, reaching values as high as 0.31 and possibly impacting early climate evolution. All planets in the Solar System orbit the Sun in an anticlockwise direction as viewed from above Earth's north pole. Most planets rotate on their axes in an anticlockwise direction, but Venus rotates clockwise in retrograde rotation once every 243 Earth days—the slowest rotation of any planet. This Venusian sidereal day lasts therefore longer than

11214-442: The equator and the poles. Venus's minute axial tilt —less than 3°, compared to 23° on Earth—also minimizes seasonal temperature variation. Altitude is one of the few factors that affect Venusian temperatures. The highest point on Venus, Maxwell Montes , is therefore the coolest point on Venus, with a temperature of about 655 K (380 °C; 715 °F) and an atmospheric pressure of about 4.5 MPa (45 bar). In 1995,

11340-454: The equator rotates at about 320 kilometres per second (199 miles per second), or 96.5% of its critical angular velocity for break-up. It is emitting polarized light because of this. Regulus BC is 5,000  AU from Regulus A. A and BC share a common proper motion and are thought to orbit each other taking several million years. Designated Regulus B and Regulus C, the pair has Henry Draper Catalogue number HD 87884. The first

11466-415: The existence of perhaps the largest stationary gravity waves in the solar system. Venus orbits the Sun at an average distance of about 0.72  AU (108 million  km ; 67 million  mi ), and completes an orbit every 224.7 days. Although all planetary orbits are elliptical , Venus's orbit is currently the closest to circular, with an eccentricity of less than 0.01. Simulations of

11592-621: The first observation-based estimate of 3,500 km. The principal difference between the two planets is the lack of evidence for plate tectonics on Venus, possibly because its crust is too strong to subduct without water to make it less viscous . This results in reduced heat loss from the planet, preventing it from cooling and providing a likely explanation for its lack of an internally generated magnetic field . Instead, Venus may lose its internal heat in periodic major resurfacing events. In 1967, Venera 4 found Venus's magnetic field to be much weaker than that of Earth. This magnetic field

11718-400: The first soft landing on another planet by Venera 7 in 1970. These probes demonstrated the extreme surface conditions, an insight that has informed predictions about global warming on Earth. This finding ended the theories and then popular science fiction about Venus being a habitable or inhabited planet. Venus is one of the four terrestrial planets in the Solar System, meaning that it

11844-399: The first week of September, or in the second week. Every 8 years, Venus passes very near the star system around or a few days before the heliacal rising, as on 5 September 2022 (the superior conjunction of Venus happens about two days earlier with each turn of its 8-year cycle, so as this cycle continues Venus will more definitely pass Regulus before the star's heliacal rising). Regulus is

11970-474: The following 200 years , but most were determined to be stars in the vicinity. Alex Alemi's and David Stevenson 's 2006 study of models of the early Solar System at the California Institute of Technology shows Venus likely had at least one moon created by a huge impact event billions of years ago. About 10   million   years later, according to the study, another impact reversed

12096-429: The heat from the core is reheating the crust. One possibility is that Venus has no solid inner core, or that its core is not cooling, so that the entire liquid part of the core is at approximately the same temperature. Another possibility is that its core has already been completely solidified. The state of the core is highly dependent on the concentration of sulphur , which is unknown at present. Another possibility

12222-472: The history of astronomy. Orbiting inferiorly (inside of Earth's orbit), it always appears close to the Sun in Earth's sky, as either a "morning star" or an "evening star". While this is also true for Mercury , Venus appears more prominent, since it is the third brightest object in Earth's sky after the Moon and the Sun. In 1961, Venus became the target of the first interplanetary flight, Venera 1 , followed by many essential interplanetary firsts , such as

12348-399: The hypothesis of a spin-orbit resonance with Earth has been discounted. Venus has no natural satellites. It has several trojan asteroids : the quasi-satellite 524522 Zoozve and two other temporary trojans, 2001 CK 32 and 2012 XE 133 . In the 17th century, Giovanni Cassini reported a moon orbiting Venus, which was named Neith and numerous sightings were reported over

12474-405: The image of a star to an angular diameter of about 0.5″; in poor conditions this increases to 1.5″ or even more. The dwarf planet Pluto has proven difficult to resolve because its angular diameter is about 0.1″. Techniques exist for improving seeing on the ground. Adaptive optics , for example, can produce images around 0.05″ on a 10 m class telescope. Space telescopes are not affected by

12600-402: The lightning rate is at least half that on Earth, however other instruments have not detected lightning at all. The origin of any lightning remains unclear, but could originate from clouds or Venusian volcanoes . In 2007, Venus Express discovered that a huge double atmospheric polar vortex exists at the south pole. Venus Express discovered, in 2011, that an ozone layer exists high in

12726-760: The lion', a name already attested in the Greek Kardia Leontos whose Latin equivalent is Cor Leōnis. The Arabic phrase is sometimes approximated as Kabelaced. In Chinese it is known as 軒轅十四, the Fourteenth Star of Xuanyuan, the Yellow Emperor . In Indian astronomy , Regulus corresponds to the Nakshatra Magha ("the bountiful"). Babylonians called it Sharru ("the King"), and it marked the 15th ecliptic constellation. In India it

12852-492: The loss of most of Venus's water during the first billion years after it formed. However, the planet may have retained a dynamo for its first 2–3 billion years, so the water loss may have occurred more recently. The erosion has increased the ratio of higher-mass deuterium to lower-mass hydrogen in the atmosphere 100 times compared to the rest of the solar system. Venus has a dense atmosphere composed of 96.5% carbon dioxide , 3.5% nitrogen—both exist as supercritical fluids at

12978-470: The lowest gravitational potential difference to Earth than any other planet, needing the lowest delta-v to transfer between them. Tidally Venus exerts the third strongest tidal force on Earth, after the Moon and the Sun, though significantly less. To the naked eye , Venus appears as a white point of light brighter than any other planet or star (apart from the Sun). The planet's mean apparent magnitude

13104-462: The object being measured is too small for direct visual inspection. For instance, a toolmaker's optical comparator will often include an option to measure in "minutes and seconds". Venus Venus is the second planet from the Sun . It is a terrestrial planet and is the closest in mass and size to its orbital neighbour Earth . Venus has by far the densest atmosphere of the terrestrial planets, composed mostly of carbon dioxide with

13230-404: The orbit of Venus is slightly inclined relative to Earth's orbit, most inferior conjunctions with Earth, which occur every synodic period of 1.6 years, do not produce a transit of Venus above Earth. Consequently, Venus transits above Earth only occur when an inferior conjunction takes place during some days of June or December, the time where the orbits of Venus and Earth cross a straight line with

13356-405: The planet underwent a global resurfacing event 300–600   million years ago, followed by a decay in volcanism. Whereas Earth's crust is in continuous motion, Venus is thought to be unable to sustain such a process. Without plate tectonics to dissipate heat from its mantle, Venus instead undergoes a cyclical process in which mantle temperatures rise until they reach a critical level that weakens

13482-421: The planet's spin direction and the resulting tidal deceleration caused the Venusian moon gradually to spiral inward until it collided with Venus. If later impacts created moons, these were removed in the same way. An alternative explanation for the lack of satellites is the effect of strong solar tides, which can destabilize large satellites orbiting the inner terrestrial planets. The orbital space of Venus has

13608-581: The planet's surface with a density 6.5% that of water —and traces of other gases including sulphur dioxide . The mass of its atmosphere is 92 times that of Earth's, whereas the pressure at its surface is about 93 times that at Earth's—a pressure equivalent to that at a depth of nearly 1 km ( 5 ⁄ 8  mi) under Earth's ocean surfaces. The density at the surface is 65 kg/m (4.1 lb/cu ft), 6.5% that of water or 50 times as dense as Earth's atmosphere at 293 K (20 °C; 68 °F) at sea level. The CO 2 -rich atmosphere generates

13734-403: The point of impact is 3 inches high and 1.5 inches left of the point of aim at 100 yards (which for instance could be measured by using a spotting scope with a calibrated reticle, or a target delineated for such purposes), the scope needs to be adjusted 3 MOA down, and 1.5 MOA right. Such adjustments are trivial when the scope's adjustment dials have a MOA scale printed on them, and even figuring

13860-470: The right number of clicks is relatively easy on scopes that click in fractions of MOA. This makes zeroing and adjustments much easier: Another common system of measurement in firearm scopes is the milliradian (mrad). Zeroing an mrad based scope is easy for users familiar with base ten systems. The most common adjustment value in mrad based scopes is ⁠ 1 / 10 ⁠  mrad (which approximates 1 ⁄ 3 MOA). One thing to be aware of

13986-479: The size of Australia. Maxwell Montes , the highest mountain on Venus, lies on Ishtar Terra. Its peak is 11 km (7 mi) above the Venusian average surface elevation. The southern continent is called Aphrodite Terra , after the Greek mythological goddess of love, and is the larger of the two highland regions at roughly the size of South America. A network of fractures and faults covers much of this area. There

14112-528: The size of the hot spots could not be measured, but are likely to have been in the 800–1,100 K (527–827 °C; 980–1,520 °F) range, relative to a normal temperature of 740 K (467 °C; 872 °F). In 2023, scientists reexamined topographical images of the Maat Mons region taken by the Magellan orbiter. Using computer simulations, they determined that the topography had changed during an 8-month interval, and concluded that active volcanism

14238-411: The small change of position of a star or Solar System body as the Earth revolves about the Sun. These small angles may also be written in milliarcseconds (mas), or thousandths of an arcsecond. The unit of distance called the parsec , abbreviated from the par allax angle of one arc sec ond, was developed for such parallax measurements. The distance from the Sun to a celestial object is the reciprocal of

14364-410: The strongest greenhouse effect in the Solar System, creating surface temperatures of at least 735 K (462 °C; 864 °F). This makes the Venusian surface hotter than Mercury 's, which has a minimum surface temperature of 53 K (−220 °C; −364 °F) and maximum surface temperature of 700 K (427 °C; 801 °F), even though Venus is nearly twice Mercury's distance from

14490-443: The sunlight that falls on them back into space, and since they cover the whole planet they prevent visual observation of Venus's surface. The permanent cloud cover means that although Venus is closer than Earth to the Sun, it receives less sunlight on the ground, with only 10% of the received sunlight reaching the surface, resulting in average daytime levels of illumination at the surface of 14,000 lux , comparable to that on Earth "in

14616-459: The surface are slow, moving at a few kilometres per hour, but because of the high density of the atmosphere at the surface, they exert a significant amount of force against obstructions, and transport dust and small stones across the surface. This alone would make it difficult for a human to walk through, even without the heat, pressure, and lack of oxygen. Above the dense CO 2 layer are thick clouds, consisting mainly of sulfuric acid , which

14742-503: The surface. After a period of 600 million to several billion years, solar forcing from rising luminosity of the Sun and possibly large volcanic resurfacing caused the evaporation of the original water and the current atmosphere. A runaway greenhouse effect was created once a critical level of greenhouse gases (including water) was added to its atmosphere. Although the surface conditions on Venus are no longer hospitable to any Earth-like life that may have formed before this event, there

14868-420: The surrounding basaltic plains measured by Venus Express and Magellan , indicating a different, possibly a more felsic , mineral assemblage. The mechanism to generate a large amount of felsic crust usually requires the presence of water ocean and plate tectonics , implying that habitable condition had existed on early Venus with large bodies of water at some point. However, the nature of tessera terrains

14994-419: The target range. Therefore, 1 MOA ≈ 0.2909 mrad. This means that an object which spans 1 mrad on the reticle is at a range that is in metres equal to the object's linear size in millimetres (e.g. an object of 100 mm subtending 1 mrad is 100 metres away). So there is no conversion factor required, contrary to the MOA system. A reticle with markings (hashes or dots) spaced with a one mrad apart (or

15120-408: The year except for about a month (depending on ability to compensate for the sun's glare, ideally done so in twilight) on either side of August 22–24, when the Sun is too close. The star can be viewed the whole night, crossing the sky, in late February. Regulus passes through SOHO 's LASCO C3 every August. For Earth observers, the heliacal rising (pre-sunrise appearance) of Regulus occurs late in

15246-442: Was a subject of speculation until some of its secrets were revealed by planetary science in the 20th century. Venera landers in 1975 and 1982 returned images of a surface covered in sediment and relatively angular rocks. The surface was mapped in detail by Magellan in 1990–91. The ground shows evidence of extensive volcanism, and the sulphur in the atmosphere may indicate that there have been recent eruptions. About 80% of

15372-414: Was around 2017, with occultations every month from December 2016 till July 2017, each one limited to certain areas on Earth. Occultations by Mercury and Venus are possible but rare, as are occultations by asteroids . Seven other stars which have a Bayer designation are less than 0.9° from the ecliptic (perfected, mean plane of Earth's orbit and mean apparent path of the Sun) the next brightest of which

15498-843: Was known as Maghā ("the Mighty"), in Sogdiana Magh ("the Great"), in Persia Miyan ("the Centre") and also as one of the four ' royal stars ' of the Persian monarchy. It was one of the fifteen Behenian stars known to medieval astrologers , associated with granite , mugwort , and the kabbalistic symbol [REDACTED] . In the Babylonian MUL.APIN , Regulus is listed as Lugal , meaning king, with co-descriptor, "star of

15624-405: Was originally defined as the arc length of a minute of latitude on a spherical Earth, so the actual Earth's circumference is very near 21 600  nmi . A minute of arc is ⁠ π / 10 800 ⁠ of a radian . A second of arc , arcsecond (arcsec), or arc second , denoted by the symbol ″ , is ⁠ 1 / 60 ⁠ of an arcminute, ⁠ 1 / 3600 ⁠ of

15750-458: Was the cause. Almost a thousand impact craters on Venus are evenly distributed across its surface. On other cratered bodies, such as Earth and the Moon, craters show a range of states of degradation. On the Moon, degradation is caused by subsequent impacts, whereas on Earth it is caused by wind and rain erosion. On Venus, about 85% of the craters are in pristine condition. The number of craters, together with their well-preserved condition, indicates

15876-446: Was truly a 1 MOA rifle, it would be just as likely that two consecutive shots land exactly on top of each other as that they land 1 MOA apart. For 5-shot groups, based on 95% confidence , a rifle that normally shoots 1 MOA can be expected to shoot groups between 0.58 MOA and 1.47 MOA, although the majority of these groups will be under 1 MOA. What this means in practice is if a rifle that shoots 1-inch groups on average at 100 yards shoots

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