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76-431: A series of missions, designed by military experts, test the participants' physical stamina, mental strength, and cooperation to determine the last team standing. Episodes also include concurrent commentary from a panel of hosts, including the "Master" who runs the competition events. Season 1 aired every Tuesday at 22:30 ( KST ) from March 23, 2021, to July 20, 2021. This season featured 24 participants from six units across

152-520: A water clock for telling time. A canonical sundial is one that indicates the canonical hours of liturgical acts. Such sundials were used from the 7th to the 14th centuries by the members of religious communities. The Italian astronomer Giovanni Padovani published a treatise on the sundial in 1570, in which he included instructions for the manufacture and laying out of mural (vertical) and horizontal sundials. Giuseppe Biancani 's Constructio instrumenti ad horologia solaria (c. 1620) discusses how to make

228-537: A broad shadow; the shadow of the style shows the time. The gnomon may be a rod, wire, or elaborately decorated metal casting. The style must be parallel to the axis of the Earth's rotation for the sundial to be accurate throughout the year. The style's angle from horizontal is equal to the sundial's geographical latitude . The term sundial can refer to any device that uses the Sun's altitude or azimuth (or both) to show

304-404: A circle on the equatorial plane, and the radius of the circle measures the declination of the sun. The ends of the gnomon bar may be used as the nodus, or some feature along its length. An ancient variant of the equatorial sundial has only a nodus (no style) and the concentric circular hour-lines are arranged to resemble a spider-web. In the horizontal sundial (also called a garden sundial ),

380-477: A horizontal and analemmatic dial, mounted together on one plate. In these designs, their times agree only when the plate is aligned properly. Sundials may indicate the local solar time only. To obtain the national clock time, three corrections are required: The principles of sundials are understood most easily from the Sun 's apparent motion. The Earth rotates on its axis, and revolves in an elliptical orbit around

456-460: A horizontal dial they run anticlockwise (US: counterclockwise) rather than clockwise. Sundials which are designed to be used with their plates horizontal in one hemisphere can be used with their plates vertical at the complementary latitude in the other hemisphere. For example, the illustrated sundial in Perth , Australia , which is at latitude 32° South, would function properly if it were mounted on

532-500: A line of light may be formed by allowing the Sun's rays through a thin slit or focusing them through a cylindrical lens . A spot of light may be formed by allowing the Sun's rays to pass through a small hole, window, oculus , or by reflecting them from a small circular mirror. A spot of light can be as small as a pinhole in a solargraph or as large as the oculus in the Pantheon. Sundials also may use many types of surfaces to receive

608-529: A much later "official" time at the western edge of a time-zone, compared to sunrise and sunset times at the eastern edge. If a sundial is located at, say, a longitude 5° west of the reference longitude, then its time will read 20 minutes slow, since the Sun appears to revolve around the Earth at 15° per hour. This is a constant correction throughout the year. For equiangular dials such as equatorial, spherical or Lambert dials, this correction can be made by rotating

684-408: A perfect sundial. They have been commonly used since the 16th century. In general, sundials indicate the time by casting a shadow or throwing light onto a surface known as a dial face or dial plate . Although usually a flat plane, the dial face may also be the inner or outer surface of a sphere, cylinder, cone, helix, and various other shapes. The time is indicated where a shadow or light falls on

760-519: A shadow or the edge of a shadow while others use a line or spot of light to indicate the time. The shadow-casting object, known as a gnomon , may be a long thin rod or other object with a sharp tip or a straight edge. Sundials employ many types of gnomon. The gnomon may be fixed or moved according to the season. It may be oriented vertically, horizontally, aligned with the Earth's axis, or oriented in an altogether different direction determined by mathematics. Given that sundials use light to indicate time,

836-514: A south-facing vertical wall at latitude 58° (i.e. 90° − 32°) North, which is slightly further north than Perth, Scotland . The surface of the wall in Scotland would be parallel with the horizontal ground in Australia (ignoring the difference of longitude), so the sundial would work identically on both surfaces. Correspondingly, the hour marks, which run counterclockwise on a horizontal sundial in

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912-413: A style and a nodus to determine the time and date. The gnomon is usually fixed relative to the dial face, but not always; in some designs such as the analemmatic sundial, the style is moved according to the month. If the style is fixed, the line on the dial plate perpendicularly beneath the style is called the substyle , meaning "below the style". The angle the style makes with the plane of the dial plate

988-559: A sundial designed for a latitude of 40° can be used at a latitude of 45°, if the sundial plane is tilted upwards by 5°, thus aligning the style with the Earth's rotational axis. Many ornamental sundials are designed to be used at 45 degrees north. Some mass-produced garden sundials fail to correctly calculate the hourlines and so can never be corrected. A local standard time zone is nominally 15 degrees wide, but may be modified to follow geographic or political boundaries. A sundial can be rotated around its style (which must remain pointed at

1064-410: A sundial must have been designed for the local geographical latitude and its style must be parallel to the Earth's rotational axis; the style must be aligned with true north and its height (its angle with the horizontal) must equal the local latitude. To adjust the style height, the sundial can often be tilted slightly "up" or "down" while maintaining the style's north-south alignment. Some areas of

1140-404: A time zone known as Pyongyang Standard Time , but the change was reverted to promote Korean unity. The IANA time zone database contains one zone for South Korea in the file zone.tab , named Asia/Seoul. Sundial A sundial is a horological device that tells the time of day (referred to as civil time in modern usage) when direct sunlight shines by the apparent position of

1216-462: Is abbreviated KST . South Korea currently does not observe daylight saving time . From May 8 to October 9 in 1988, daylight saving time was tested to better accommodate the calendar of competitions held during the 1988 Summer Olympics in Seoul . In 1434, inventor Jang Yeong-sil developed Korea 's first automatic water clock , which King Sejong adapted as Korea's standard timekeeper. It

1292-409: Is at Jaipur , raised 26°55′ above horizontal, reflecting the local latitude. On any given day, the Sun appears to rotate uniformly about this axis, at about 15° per hour, making a full circuit (360°) in 24 hours. A linear gnomon aligned with this axis will cast a sheet of shadow (a half-plane) that, falling opposite to the Sun, likewise rotates about the celestial axis at 15° per hour. The shadow

1368-453: Is called the substyle height, an unusual use of the word height to mean an angle . On many wall dials, the substyle is not the same as the noon line (see below). The angle on the dial plate between the noon line and the substyle is called the substyle distance , an unusual use of the word distance to mean an angle . By tradition, many sundials have a motto . The motto is usually in the form of an epigram : sometimes sombre reflections on

1444-430: Is equal worldwide: it does not depend on the local latitude or longitude of the observer's position. It does, however, change over long periods of time, (centuries or more, ) because of slow variations in the Earth's orbital and rotational motions. Therefore, tables and graphs of the equation of time that were made centuries ago are now significantly incorrect. The reading of an old sundial should be corrected by applying

1520-476: Is likely that Koreans used water clocks to keep time prior to this invention, but no concrete records of them exist. In 1437, Jang Yeong-sil, with Jeong Cho, created a bowl-shaped sundial called the angbu ilgu ( 앙부일구 ), which King Sejong had placed in public so anyone could use it. Geographically, the western parts of Korea , including the South Korean capital city, Seoul , are GMT+08:00 . In 1908,

1596-409: Is made via the relation Near the equinoxes in spring and autumn, the sun moves on a circle that is nearly the same as the equatorial plane; hence, no clear shadow is produced on the equatorial dial at those times of year, a drawback of the design. A nodus is sometimes added to equatorial sundials, which allows the sundial to tell the time of year. On any given day, the shadow of the nodus moves on

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1672-433: Is opaque, both sides of the equatorial dial must be marked, since the shadow will be cast from below in winter and from above in summer. With translucent dial plates (e.g. glass) the hour angles need only be marked on the sun-facing side, although the hour numberings (if used) need be made on both sides of the dial, owing to the differing hour schema on the sun-facing and sun-backing sides. Another major advantage of this dial

1748-444: Is seen by falling on a receiving surface that is usually flat, but which may be spherical, cylindrical, conical or of other shapes. If the shadow falls on a surface that is symmetrical about the celestial axis (as in an armillary sphere, or an equatorial dial), the surface-shadow likewise moves uniformly; the hour-lines on the sundial are equally spaced. However, if the receiving surface is not symmetrical (as in most horizontal sundials),

1824-507: Is symmetrical about that axis; examples include the equatorial dial, the equatorial bow, the armillary sphere, the cylindrical dial and the conical dial. However, other designs are equiangular, such as the Lambert dial, a version of the analemmatic sundial with a moveable style. A sundial at a particular latitude in one hemisphere must be reversed for use at the opposite latitude in the other hemisphere. A vertical direct south sundial in

1900-418: Is that equation of time (EoT) and daylight saving time (DST) corrections can be made by simply rotating the dial plate by the appropriate angle each day. This is because the hour angles are equally spaced around the dial. For this reason, an equatorial dial is often a useful choice when the dial is for public display and it is desirable to have it show the true local time to reasonable accuracy. The EoT correction

1976-412: Is the angle between a given hour-line and the noon hour-line (which always points due north) on the plane, and t is the number of hours before or after noon. For example, the angle   H V   {\displaystyle \ H_{V}\ } of the 3  P.M. hour-line would equal the arctangent of cos L , since tan 45° = 1 . The shadow moves counter-clockwise on

2052-499: Is the angle between a given hour-line and the noon hour-line (which always points towards true north ) on the plane, and t is the number of hours before or after noon. For example, the angle   H H   {\displaystyle \ H_{H}\ } of the 3  PM hour-line would equal the arctangent of sin L , since tan 45° = 1. When   L = 90 ∘   {\displaystyle \ L=90^{\circ }\ } (at

2128-421: Is the intersection of the cone of light rays with the flat surface. This cone and its conic section change with the seasons, as the Sun's declination changes; hence, sundials that follow the motion of such light-spots or shadow-tips often have different hour-lines for different times of the year. This is seen in shepherd's dials, sundial rings, and vertical gnomons such as obelisks. Alternatively, sundials may change

2204-650: The Korean Empire adopted a standard time , GMT+08:30. In 1912, during the Japanese occupation of Korea , the Governor-General of Korea changed standard time to GMT+09:00 to align with Japan Standard Time . However, in 1954, the South Korean government under President Syngman Rhee reverted the standard time to GMT+08:30. Then in 1961, under the military government of President Park Chung-hee ,

2280-480: The North Pole ), the horizontal sundial becomes an equatorial sundial; the style points straight up (vertically), and the horizontal plane is aligned with the equatorial plane; the hour-line formula becomes   H H = 15 ∘ × t   , {\displaystyle \ H_{H}=15^{\circ }\times t\ ,} as for an equatorial dial. A horizontal sundial at

2356-725: The Northern Hemisphere becomes a vertical direct north sundial in the Southern Hemisphere . To position a horizontal sundial correctly, one has to find true north or south . The same process can be used to do both. The gnomon, set to the correct latitude, has to point to the true south in the Southern Hemisphere as in the Northern Hemisphere it has to point to the true north. The hour numbers also run in opposite directions, so on

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2432-701: The Old Testament describes a sundial—the "dial of Ahaz" mentioned in Isaiah 38:8 and 2 Kings 20:11 . By 240 BC Eratosthenes had estimated the circumference of the world using an obelisk and a water well and a few centuries later Ptolemy had charted the latitude of cities using the angle of the sun. The people of Kush created sun dials through geometry. The Roman writer Vitruvius lists dials and shadow clocks known at that time in his De architectura . The Tower of Winds constructed in Athens included sundial and

2508-683: The Republic of Korea Army and Republic of Korea Navy . It was also released on Netflix under the title The Iron Squad and gained a high viewership ranking on release. Season 2 aired every Tuesday at 21:20 ( KST ) from February 22, 2022, to June 7, 2022. The new season introduces two new units not featured in Season 1, including one from the Republic of Korea Air Force , for a total of 32 participants hailing from eight units. Season 3 premiered on September 19, 2023 and airs every Tuesday at 22:30 ( KST ). Another two new units were introduced, including

2584-418: The Sun in the sky . In the narrowest sense of the word, it consists of a flat plate (the dial ) and a gnomon , which casts a shadow onto the dial. As the Sun appears to move through the sky, the shadow aligns with different hour-lines, which are marked on the dial to indicate the time of day. The style is the time-telling edge of the gnomon, though a single point or nodus may be used. The gnomon casts

2660-430: The equation of time . This compensates for the slight eccentricity in the Earth's orbit and the tilt of the Earth's axis that causes up to a 15 minute variation from mean solar time. This is a type of dial furniture seen on more complicated horizontal and vertical dials. Prior to the invention of accurate clocks, in the mid 17th century, sundials were the only timepieces in common use, and were considered to tell

2736-409: The fixed stars , the Sun changes its position on the celestial sphere, being (in the northern hemisphere) at a positive declination in spring and summer, and at a negative declination in autumn and winter, and having exactly zero declination (i.e., being on the celestial equator ) at the equinoxes . The Sun's celestial longitude also varies, changing by one complete revolution per year. The path of

2812-497: The "right" time. The equation of time was not used. After the invention of good clocks, sundials were still considered to be correct, and clocks usually incorrect. The equation of time was used in the opposite direction from today, to apply a correction to the time shown by a clock to make it agree with sundial time. Some elaborate " equation clocks ", such as one made by Joseph Williamson in 1720, incorporated mechanisms to do this correction automatically. (Williamson's clock may have been

2888-408: The Earth's equator , where   L = 0 ∘   , {\displaystyle \ L=0^{\circ }\ ,} would require a (raised) horizontal style and would be an example of a polar sundial (see below). The chief advantages of the horizontal sundial are that it is easy to read, and the sunlight lights the face throughout the year. All the hour-lines intersect at

2964-399: The Sun is actually on the meridian at official clock time of 3  PM ). This occurs in the far west of Alaska , China , and Spain . For more details and examples, see time zones . Although the Sun appears to rotate uniformly about the Earth, in reality this motion is not perfectly uniform. This is due to the eccentricity of the Earth's orbit (the fact that the Earth's orbit about

3040-426: The Sun is not perfectly circular, but slightly elliptical ) and the tilt (obliquity) of the Earth's rotational axis relative to the plane of its orbit. Therefore, sundial time varies from standard clock time . On four days of the year, the correction is effectively zero. However, on others, it can be as much as a quarter-hour early or late. The amount of correction is described by the equation of time . This correction

3116-426: The Sun on the celestial sphere is called the ecliptic . The ecliptic passes through the twelve constellations of the zodiac in the course of a year. This model of the Sun's motion helps to understand sundials. If the shadow-casting gnomon is aligned with the celestial poles , its shadow will revolve at a constant rate, and this rotation will not change with the seasons. This is the most common design. In such cases,

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3192-414: The Sun. An excellent approximation assumes that the Sun revolves around a stationary Earth on the celestial sphere , which rotates every 24 hours about its celestial axis. The celestial axis is the line connecting the celestial poles . Since the celestial axis is aligned with the axis about which the Earth rotates, the angle of the axis with the local horizontal is the local geographical latitude . Unlike

3268-412: The angle or position (or both) of the gnomon relative to the hour lines, as in the analemmatic dial or the Lambert dial. The earliest sundials known from the archaeological record are shadow clocks (1500 BC or BCE ) from ancient Egyptian astronomy and Babylonian astronomy . Presumably, humans were telling time from shadow-lengths at an even earlier date, but this is hard to verify. In roughly 700 BC,

3344-402: The celestial pole) to adjust to the local time zone. In most cases, a rotation in the range of 7.5° east to 23° west suffices. This will introduce error in sundials that do not have equal hour angles. To correct for daylight saving time , a face needs two sets of numerals or a correction table. An informal standard is to have numerals in hot colors for summer, and in cool colors for winter. Since

3420-435: The competition switched to a single-elimination style without a Death Match. Each stage also featured a "Benefit Mission" where the winner would be given an advantage in the main mission that followed. In the case of the second elimination stage, the lowest-ranking team was also sent directly to Death Match, bypassing the main mission. The second season largely continued with the format of main missions and Death Matches until

3496-435: The dial face, which is usually inscribed with hour lines. Although usually straight, these hour lines may also be curved, depending on the design of the sundial (see below). In some designs, it is possible to determine the date of the year, or it may be required to know the date to find the correct time. In such cases, there may be multiple sets of hour lines for different months, or there may be mechanisms for setting/calculating

3572-466: The dial surface by an angle equaling the difference in longitude, without changing the gnomon position or orientation. However, this method does not work for other dials, such as a horizontal dial; the correction must be applied by the viewer. However, for political and practical reasons, time-zone boundaries have been skewed. At their most extreme, time zones can cause official noon, including daylight savings, to occur up to three hours early (in which case

3648-501: The equatorial bow, offsetting its time measurement. In other cases, the hour lines may be curved, or the equatorial bow may be shaped like a vase, which exploits the changing altitude of the sun over the year to effect the proper offset in time. A heliochronometer is a precision sundial first devised in about 1763 by Philipp Hahn and improved by Abbé Guyoux in about 1827. It corrects apparent solar time to mean solar time or another standard time . Heliochronometers usually indicate

3724-718: The first non-Korean team, composed of former retired US special forces members, alongside new participants from the top four units of Season 2. Each team consists of four reservists representing their respective special forces unit, with a team leader (indicated in bold ) selected by the team members. Season 1 (육군특수전사령부) (해병대특수수색대) (제707특수임무단) (해군특수전전단) (군사경찰특임대) (해난구조전대) Season 2 (육군특수전사령부) (해병대특수수색대) (제707특수임무단) (해군특수전전단) (군사경찰특임대) (해난구조전대) (특수탐색구조대대) (국군정보사령부특임대) (ROK Defence Intelligence Command) Season 3 (육군특수전사령부) (제707특수임무단) (해군특수전전단) (육군첩보부대) ( ROK Defence Intelligence Command ) (해군첩보부대) ( ROK Defence Intelligence Command ) In

3800-432: The first-ever device to use a differential gear.) Only after about 1800 was uncorrected clock time considered to be "right", and sundial time usually "wrong", so the equation of time became used as it is today. The most commonly observed sundials are those in which the shadow-casting style is fixed in position and aligned with the Earth's rotational axis, being oriented with true north and south, and making an angle with

3876-436: The gnomon's style is aligned with the Earth's axis of rotation. As in the horizontal dial, the line of shadow does not move uniformly on the face; the sundial is not equiangular . If the face of the vertical dial points directly south, the angle of the hour-lines is instead described by the formula where L is the sundial's geographical latitude ,   H V   {\displaystyle \ H_{V}\ }

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3952-575: The horizontal equal to the geographical latitude. This axis is aligned with the celestial poles , which is closely, but not perfectly, aligned with the pole star Polaris . For illustration, the celestial axis points vertically at the true North Pole , whereas it points horizontally on the equator . The world's largest axial gnomon sundial is the mast of the Sundial Bridge at Turtle Bay in Redding, California . A formerly world's largest gnomon

4028-427: The hour angles are not evenly spaced, the equation of time corrections cannot be made via rotating the dial plate about the gnomon axis. These types of dials usually have an equation of time correction tabulation engraved on their pedestals or close by. Horizontal dials are commonly seen in gardens, churchyards and in public areas. In the common vertical dial , the shadow-receiving plane is aligned vertically; as usual,

4104-478: The hour lines must be corrected accordingly. The rays of light that graze the tip of a gnomon, or which pass through a small hole, or reflect from a small mirror, trace out a cone aligned with the celestial poles. The corresponding light-spot or shadow-tip, if it falls onto a flat surface, will trace out a conic section , such as a hyperbola , ellipse or (at the North or South Poles) a circle . This conic section

4180-400: The hour-lines to be calculated for various types of sundial. In some cases, the calculations are simple; in others they are extremely complicated. There is an alternative, simple method of finding the positions of the hour-lines which can be used for many types of sundial, and saves a lot of work in cases where the calculations are complex. This is an empirical procedure in which the position of

4256-521: The light or shadow. Planes are the most common surface, but partial spheres , cylinders , cones and other shapes have been used for greater accuracy or beauty. Sundials differ in their portability and their need for orientation. The installation of many dials requires knowing the local latitude , the precise vertical direction (e.g., by a level or plumb-bob), and the direction to true north . Portable dials are self-aligning: for example, it may have two dials that operate on different principles, such as

4332-405: The minutes to within 1 minute of Universal Time . The Sunquest sundial , designed by Richard L. Schmoyer in the 1950s, uses an analemmic-inspired gnomon to cast a shaft of light onto an equatorial time-scale crescent. Sunquest is adjustable for latitude and longitude, automatically correcting for the equation of time, rendering it "as accurate as most pocket watches". Similarly, in place of

4408-406: The month. In addition to the hour lines, the dial face may offer other data—such as the horizon, the equator and the tropics—which are referred to collectively as the dial furniture. The entire object that casts a shadow or light onto the dial face is known as the sundial's gnomon . However, it is usually only an edge of the gnomon (or another linear feature) that casts the shadow used to determine

4484-423: The passing of time and the brevity of life, but equally often humorous witticisms of the dial maker. One such quip is, I am a sundial, and I make a botch, Of what is done much better by a watch. A dial is said to be equiangular if its hour-lines are straight and spaced equally. Most equiangular sundials have a fixed gnomon style aligned with the Earth's rotational axis, as well as a shadow-receiving surface that

4560-465: The plane that receives the shadow is aligned horizontally, rather than being perpendicular to the style as in the equatorial dial. Hence, the line of shadow does not rotate uniformly on the dial face; rather, the hour lines are spaced according to the rule. Or in other terms: where L is the sundial's geographical latitude (and the angle the gnomon makes with the dial plate),   H H   {\displaystyle \ H_{H}\ }

4636-428: The point where the gnomon's style crosses the horizontal plane. Since the style is aligned with the Earth's rotational axis, the style points true north and its angle with the horizontal equals the sundial's geographical latitude L  . A sundial designed for one latitude can be adjusted for use at another latitude by tilting its base upwards or downwards by an angle equal to the difference in latitude. For example,

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4712-435: The present-day equation of time, not one from the period when the dial was made. In some sundials, the equation of time correction is provided as an informational plaque affixed to the sundial, for the observer to calculate. In more sophisticated sundials the equation can be incorporated automatically. For example, some equatorial bow sundials are supplied with a small wheel that sets the time of year; this wheel in turn rotates

4788-401: The ratings below, the highest rating for the season will be in red and the lowest rating for the season will be in blue . Season 1 Season 2 The first three elimination stages followed a double-elimination tournament style, with a main mission (indicated in bold ) determining the participants of the "Death Match" elimination round. Following the revival round,

4864-778: The round of 4, with the exception of the first stage which was a direct elimination round. Other notable changes include: (1:2) (2:1) As of the second elimination stage, the competition format remains unchanged. This season opened with a notably higher calibre team introductions segment, where the customary pull-up friendly contest in a filming studio was upgraded to live firing at sea on a Korean Coast Guard vessel. Other show firsts included: Benefit Mission: Integrated Mobile Shooting 1.Lost 2. Win 3. Lost 1.Win 2. Lost 3. Win Time in South Korea South Korea has one time zone, Korea Standard Time ( GMT+9 ), which

4940-446: The same hour lines may be used throughout the year. The hour-lines will be spaced uniformly if the surface receiving the shadow is either perpendicular (as in the equatorial sundial) or circular about the gnomon (as in the armillary sphere ). In other cases, the hour-lines are not spaced evenly, even though the shadow rotates uniformly. If the gnomon is not aligned with the celestial poles, even its shadow will not rotate uniformly, and

5016-402: The shadow of a gnomon the sundial at Miguel Hernández University uses the solar projection of a graph of the equation of time intersecting a time scale to display clock time directly. An analemma may be added to many types of sundials to correct apparent solar time to mean solar time or another standard time . These usually have hour lines shaped like "figure eights" ( analemmas ) according to

5092-402: The shadow of the gnomon of a real sundial is marked at hourly intervals. The equation of time must be taken into account to ensure that the positions of the hour-lines are independent of the time of year when they are marked. An easy way to do this is to set a clock or watch so it shows "sundial time" which is standard time , plus the equation of time on the day in question. The hour-lines on

5168-554: The southern hemisphere, also do so on a vertical sundial in the northern hemisphere. (See the first two illustrations at the top of this article.) On horizontal northern-hemisphere sundials, and on vertical southern-hemisphere ones, the hour marks run clockwise. The most common reason for a sundial to differ greatly from clock time is that the sundial has not been oriented correctly or its hour lines have not been drawn correctly. For example, most commercial sundials are designed as horizontal sundials as described above. To be accurate, such

5244-493: The standard time was changed back to GMT+09:00 once again. In order to accommodate American television viewers, South Korea observed daylight saving time (GMT+10:00) when Seoul hosted the 1988 Summer Olympics . The one-hour time change meant that many daytime events could be broadcast live from South Korea when it was prime time on the U.S. east coast . North Korea also uses Korea Standard Time. From August 2015 to May 2018, North Korea changed its time zone to GMT+08:30,

5320-399: The sun's apparent rotation about the Earth casts a uniformly rotating sheet of shadow from the gnomon; this produces a uniformly rotating line of shadow on the equatorial plane. Since the Earth rotates 360° in 24 hours, the hour-lines on an equatorial dial are all spaced 15° apart (360/24). The uniformity of their spacing makes this type of sundial easy to construct. If the dial plate material

5396-453: The sundial are marked to show the positions of the shadow of the style when this clock shows whole numbers of hours, and are labelled with these numbers of hours. For example, when the clock reads 5:00, the shadow of the style is marked, and labelled "5" (or "V" in Roman numerals ). If the hour-lines are not all marked in a single day, the clock must be adjusted every day or two to take account of

5472-413: The surface shadow generally moves non-uniformly and the hour-lines are not equally spaced; one exception is the Lambert dial described below. Some types of sundials are designed with a fixed gnomon that is not aligned with the celestial poles like a vertical obelisk. Such sundials are covered below under the section, "Nodus-based sundials". The formulas shown in the paragraphs below allow the positions of

5548-531: The time. Sundials are valued as decorative objects, metaphors , and objects of intrigue and mathematical study. The passing of time can be observed by placing a stick in the sand or a nail in a board and placing markers at the edge of a shadow or outlining a shadow at intervals. It is common for inexpensive, mass-produced decorative sundials to have incorrectly aligned gnomons, shadow lengths, and hour-lines, which cannot be adjusted to tell correct time. There are several different types of sundials. Some sundials use

5624-408: The time; this linear feature is known as the sundial's style . The style is usually aligned parallel to the axis of the celestial sphere, and therefore is aligned with the local geographical meridian. In some sundial designs, only a point-like feature, such as the tip of the style, is used to determine the time and date; this point-like feature is known as the sundial's nodus . Some sundials use both

5700-422: The variation of the equation of time. The distinguishing characteristic of the equatorial dial (also called the equinoctial dial ) is the planar surface that receives the shadow, which is exactly perpendicular to the gnomon's style. This plane is called equatorial, because it is parallel to the equator of the Earth and of the celestial sphere. If the gnomon is fixed and aligned with the Earth's rotational axis,

5776-483: The world practice daylight saving time , which changes the official time, usually by one hour. This shift must be added to the sundial's time to make it agree with the official time. A standard time zone covers roughly 15° of longitude, so any point within that zone which is not on the reference longitude (generally a multiple of 15°) will experience a difference from standard time that is equal to 4 minutes of time per degree. For illustration, sunsets and sunrises are at

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