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42-575: The Bortle dark-sky scale (usually referred to as simply the Bortle scale ) is a nine-level numeric scale that measures the night sky 's brightness of a particular location. It quantifies the astronomical observability of celestial objects and the interference caused by light pollution . Amateur astronomer John E. Bortle created the scale and published it in the February 2001 edition of Sky & Telescope magazine to help skywatchers evaluate

84-530: A decade. They tend to be visible only shortly before sunrise or after sunset because those are the times they are close enough to the Sun to show a tail. Clouds obscure the view of other objects in the sky, though varying thicknesses of cloud cover have differing effects. A very thin cirrus cloud in front of the moon might produce a rainbow-colored ring around the moon . Stars and planets are too small or dim to take on this effect and are instead only dimmed (often to

126-558: A distance from other objects because their navigation lights blink. Beside the Solar System objects changing in the course of them and Earth orbiting and changing orbits over time around the Sun and in the case of the Moon around Earth, appearing over time smaller by expanding its orbit, the night sky also changes over the course of the years with stars having a proper motion and changing brightness because of being variable stars , by

168-480: A dome. Orion is among the most prominent and recognizable constellations. The Big Dipper (which has a wide variety of other names) is helpful for navigation in the northern hemisphere because it points to Polaris , the north star. The pole stars are special because they are approximately in line with the Earth's axis of rotation so they appear to stay in one place while the other stars rotate around them through

210-423: A few hundred kilometres. It is not noticeable during the daytime due to the glare and scattering of sunlight . Even at the best ground-based observatories, airglow limits the photosensitivity of optical telescopes . Partly for this reason, space telescopes like Hubble can observe much fainter objects than current ground-based telescopes at visible wavelengths . Airglow at night may be bright enough for

252-399: A ground observer to notice and appears generally bluish. Although airglow emission is fairly uniform across the atmosphere, it appears brightest at about 10° above the observer's horizon , since the lower one looks, the greater the mass of atmosphere one is looking through. Very low down, however, atmospheric extinction reduces the apparent brightness of the airglow. One airglow mechanism

294-564: A measurement of the intensity of the airglow phenomenon in the near-infrared . The range measured is from 500 to 61400 photons , with a resolution of 500 photons. The Venus Express spacecraft contains an infrared sensor which has detected near-IR emissions from the upper atmosphere of Venus . The emissions come from nitric oxide (NO) and from molecular oxygen. Scientists had previously determined in laboratory testing that during NO production, ultraviolet emissions and near-IR emissions were produced. The UV radiation had been detected in

336-400: A patch the size of the seeing-enlarged image of the star, 35 photons arrive from the star and 3500 from air-glow. So, over an hour, roughly 1.3 × 10 arrive from the air-glow, and approximately 1.3 × 10 arrive from the source; so the S / N ratio is about: We can compare this with "real" answers from exposure time calculators. For an 8 m unit Very Large Telescope telescope, according to

378-451: Is about the size of a thumbnail at arm's length, and is readily identified. Over 29.53 days on average, the moon goes through a full cycle of lunar phases . People can generally identify phases within a few days by looking at the Moon. Unlike stars and most planets, the light reflected from the Moon is bright enough to be seen during the day. Some of the most spectacular moons come during the full moon phase near sunset or sunrise. The Moon on

420-446: Is brighter than the stars and often the only "star" visible near sunrise or sunset, depending on its location in its orbit. Because of its brightness, Venus can sometimes be seen after sunrise. Mercury , Mars , Jupiter and Saturn are also visible to the naked eye in the night sky. The Moon appears as a grey disc in the sky with cratering visible to the naked eye. It spans, depending on its exact location, 29–33 arcminutes – which

462-435: Is rather small (10 cm × 10 cm × 10 cm) and weighs less than 1 kg, it carries a small telescope for obtaining images of the airglow. The first SwissCube-1 image came down on 18 February 2011 and was quite black with some thermal noise on it. The first airglow image came down on 3 March 2011. This image has been converted to the human optical range (green) from its near-infrared measurement. This image provides

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504-407: Is when an atom of nitrogen combines with an atom of oxygen to form a molecule of nitric oxide (NO). In the process, a photon is emitted. This photon may have any of several different wavelengths characteristic of nitric oxide molecules. The free atoms are available for this process, because molecules of nitrogen (N 2 ) and oxygen (O 2 ) are dissociated by solar energy in the upper reaches of

546-549: The FORS exposure time calculator, 40 hours of observing time are needed to reach V = 28 , while the 2.4 m Hubble only takes 4 hours according to the ACS exposure time calculator. A hypothetical 8 m Hubble telescope would take about 30 minutes. It should be clear from this calculation that reducing the view field size can make fainter objects more detectable against the airglow; unfortunately, adaptive optics techniques that reduce

588-517: The International Space Station (ISS) and Iridium Satellites . Meteors streak across the sky infrequently. During a meteor shower , they may average one a minute at irregular intervals, but otherwise their appearance is a random surprise. The occasional meteor will make a bright, fleeting streak across the sky, and they can be very bright in comparison to the night sky. Aircraft are also visible at night, distinguishable at

630-516: The backscatter of sunlight by interplanetary dust . Shortly after sunset and before sunrise, artificial satellites often look like stars – similar in brightness and size – but move relatively quickly. Those that fly in low Earth orbit cross the sky in a couple of minutes. Some satellites, including space debris , appear to blink or have a periodic fluctuation in brightness because they are rotating. Satellite flares can appear brighter than Venus, with notable examples including

672-425: The rod cells without triggering the cone cells . If it is particularly dark and a particularly faint celestial object is of interest, averted vision may be helpful. The stars of the night sky cannot be counted unaided because they are so numerous and there is no way to track which have been counted and which have not. Further complicating the count, fainter stars may appear and disappear depending on exactly where

714-456: The advent of artificial light sources, however, light pollution has been a growing problem for viewing the night sky. Optical filters and modifications to light fixtures can help to alleviate this problem, but for optimal views, both professional and amateur astronomers seek locations far from urban skyglow . The fact that the sky is not completely dark at night, even in the absence of moonlight and city lights, can be easily observed, since if

756-442: The atmosphere and may encounter each other to form NO. Other chemicals that can create air glow in the atmosphere are hydroxyl (OH), atomic oxygen (O), sodium (Na), and lithium (Li). The sky brightness is typically measured in units of apparent magnitude per square arcsecond of sky. In order to calculate the relative intensity of airglow, we need to convert apparent magnitudes into fluxes of photons; this clearly depends on

798-426: The belief that relationships between heavenly bodies influence or explain events on Earth. The scientific study of objects in the night sky takes place in the context of observational astronomy . Visibility of celestial objects in the night sky is affected by light pollution . The presence of the Moon in the night sky has historically hindered astronomical observation by increasing the amount of sky brightness . With

840-460: The case of Earth's atmosphere , this optical phenomenon causes the night sky never to be completely dark, even after the effects of starlight and diffused sunlight from the far side are removed. This phenomenon originates with self-illuminated gases and has no relationship with Earth's magnetism or sunspot activity. The airglow phenomenon was first identified in 1868 by Swedish physicist Anders Ångström . Since then, it has been studied in

882-417: The classes. For some classes, there can be drastic differences from one class to the next, e.g, Bortle 4 to 5. Night sky Natural light sources in a night sky include moonlight , starlight , and airglow , depending on location and timing. Aurorae light up the skies above the polar circles . Occasionally, a large coronal mass ejection from the Sun or simply high levels of solar wind may extend

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924-452: The course of a night (or a year). Planets , named for the Greek word for 'wanderer', process through the starfield a little each day, executing loops with time scales dependent on the length of the planet's year or orbital period around the Sun. Planets, to the naked eye, appear as points of light in the sky with variable brightness. Planets shine due to sunlight reflecting or scattering from

966-504: The dark adaptation. Star charts are produced to aid stargazers in identifying constellations and other celestial objects. Constellations are prominent because their stars tend to be brighter than other nearby stars in the sky. Different cultures have created different groupings of constellations based on differing interpretations of the more-or-less random patterns of dots in the sky. Constellations were identified without regard to distance to each star, but instead as if they were all dots on

1008-416: The darkness of an observing site, and secondarily, to compare the darkness of observing sites. The scale ranges from Class 1, the darkest skies available on Earth, through to Class 9, inner-city skies. It gives several criteria for each level beyond naked-eye limiting magnitude (NELM). The accuracy and utility of the scale have been questioned in 2014 research. The table summarizes Bortle's descriptions of

1050-406: The diameter of the view field of an Earth-based telescope by an order of magnitude only as yet work in the infrared, where the sky is much brighter. A space telescope isn't restricted by the view field, since it is not affected by airglow. Scientific experiments have been conducted to induce airglow by directing high-power radio emissions at the Earth's ionosphere . These radiowaves interact with

1092-597: The distance to them getting larger or other celestial events like supernovas . Over a timescale of tens of billions of years the night sky in the Local Group will significantly change when the coalescence of the Andromeda Galaxy and the Milky Way merge into a single elliptical galaxy. Airglow Airglow (also called nightglow ) is a faint emission of light by a planetary atmosphere . In

1134-421: The emission from airglow over the area of the image corresponds to about V = 23 . This gives the number of photons from airglow, N a : The signal-to-noise for an ideal ground-based observation with a telescope of area A (ignoring losses and detector noise), arising from Poisson statistics, is only: If we assume a 10 m diameter ideal ground-based telescope and an unresolved star: every second, over

1176-453: The horizon benefits from the Moon illusion which makes it appear larger. The Sun's light reflected from the Moon traveling through the atmosphere also appears to color the Moon orange and/or red. Comets come to the night sky only rarely. Comets are illuminated by the Sun, and their tails extend away from the Sun. A comet with a visible tail is quite unusual – a great comet appears about once

1218-474: The ionosphere to induce faint but visible optical light at specific wavelengths under certain conditions. The effect is also observable in the radio frequency band, using ionosondes . SwissCube-1 is a Swiss satellite operated by Ecole Polytechnique Fédérale de Lausanne . The spacecraft is a single unit CubeSat , which was designed to conduct research into airglow within the Earth's atmosphere and to develop technology for future spacecraft. Though SwissCube-1

1260-420: The laboratory, and various chemical reactions have been observed to emit electromagnetic energy as part of the process. Scientists have identified some of those processes that would be present in Earth's atmosphere, and astronomers have verified that such emissions are present. Simon Newcomb was the first person to scientifically study and describe airglow, in 1901. Airglow existed in pre-industrial society and

1302-529: The number of photons we receive per square centimeter of telescope aperture per second from the source is N s : (where h is the Planck constant ; hν is the energy of a single photon of frequency ν ). At V band, the emission from airglow is V = 22 per square arc-second at a high-altitude observatory on a moonless night; in excellent seeing conditions, the image of a star will be about 0.7 arc-second across with an area of 0.4 square arc-second, and so

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1344-466: The observer is looking. The result is an impression of an extraordinarily vast star field. Because stargazing is best done from a dark place away from city lights, dark adaptation is important to achieve and maintain. It takes several minutes for eyes to adjust to the darkness necessary for seeing the most stars, and surroundings on the ground are hard to discern. A red flashlight can be used to illuminate star charts and telescope parts without undoing

1386-418: The period from −12° to −18°. When the Sun drops more than 18° below the horizon, the sky generally attains its minimum brightness. Several sources can be identified as the source of the intrinsic brightness of the sky, namely airglow , indirect scattering of sunlight, scattering of starlight, and artificial light pollution . Depending on local sky cloud cover, pollution, humidity, and light pollution levels,

1428-514: The phenomenon toward the Equator . The night sky and studies of it have a historical place in both ancient and modern cultures. In the past, for instance, farmers have used the status of the night sky as a calendar to determine when to plant crops. Many cultures have drawn constellations between stars in the sky, using them in association with legends and mythology about their deities . The history of astrology has generally been based on

1470-403: The planets' surface or atmosphere. Thus, the relative Sun-planet-Earth positions determine the planet's brightness. With a telescope or good binoculars, the planets appear as discs demonstrating finite size, and it is possible to observe orbiting moons which cast shadows onto the host planet's surface. Venus is the most prominent planet, often called the "morning star" or "evening star" because it

1512-459: The point of invisibility). Thicker cloud cover obscures celestial objects entirely, making the sky black or reflecting city lights back down. Clouds are often close enough to afford some depth perception, though they are hard to see without moonlight or light pollution. On clear dark nights in unpolluted areas, when the Moon appears thin or below the horizon, the Milky Way, a band of what looks like white dust, can be seen. The Magellanic Clouds of

1554-436: The situation is more complicated and a further differentiation is required. Twilight is divided in three segments according to how far the Sun is below the horizon in segments of 6°. After sunset the civil twilight sets in, and ends when the Sun drops more than 6° below the horizon. This is followed by the nautical twilight , when the Sun reaches heights of −6° and −12°, after which comes the astronomical twilight defined as

1596-419: The sky were absolutely dark, one would not be able to see the silhouette of an object against the sky. The intensity of the sky brightness varies greatly over the day and the primary cause differs as well. During daytime when the Sun is above the horizon direct scattering of sunlight ( Rayleigh scattering ) is the overwhelmingly dominant source of light. In twilight, the period of time between sunset and sunrise,

1638-425: The southern sky are easily mistaken to be Earth-based clouds (hence the name) but are in fact collections of stars found outside the Milky Way known as dwarf galaxies . Zodiacal light is a glow that appears near the points where the Sun rises and sets, and is caused by sunlight interacting with interplanetary dust . Gegenschein is a faint bright spot in the night sky centered at the antisolar point , caused by

1680-484: The spectrum of the source, but we will ignore that initially. At visible wavelengths, we need the parameter S 0 ( V ), the power per square centimetre of aperture and per micrometre of wavelength produced by a zeroth-magnitude star, to convert apparent magnitudes into fluxes – S 0 ( V ) = 4.0 × 10  W⋅cm ⋅µm . If we take the example of a V = 28 star observed through a normal V band filter ( B = 0.2 μm bandpass, frequency ν ≈ 6 × 10  Hz ),

1722-515: The stars visible to the unaided naked eye appear as hundreds, thousands or tens of thousands of white pinpoints of light in an otherwise near black sky together with some faint nebulae or clouds of light. In ancient times the stars were often assumed to be equidistant on a dome above the Earth because they are much too far away for stereopsis to offer any depth cues. Visible stars range in color from blue (hot) to red (cold), but with such small points of faint light, most look white because they stimulate

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1764-579: Was known to the ancient Greeks. " Aristotle and Pliny described the phenomena of Chasmata , which can be identified in part as auroras, and in part as bright airglow nights." Airglow is caused by various processes in the upper atmosphere of Earth , such as the recombination of atoms which were photoionized by the Sun during the day, luminescence caused by cosmic rays striking the upper atmosphere, and chemiluminescence caused mainly by oxygen and nitrogen reacting with hydroxyl free radicals at heights of

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