![]() The east–west component shows the equation of time, or the difference between solar time and local mean time. The north–south component of the analemma shows the Sun's declination, its latitude on the celestial sphere, or the latitude on the Earth at which the Sun is directly overhead. For an object with an eccentric orbit but no axial tilt, the analemma would be a straight east–west line along the celestial equator. For an object with a circular orbit but significant axial tilt, the analemma would be a figure of eight with northern and southern lobes equal in size. ![]() Viewed from an object with a perfectly circular orbit and no axial tilt, the Sun would always appear at the same point in the sky at the same time of day throughout the year and the analemma would be a dot. There are three parameters that affect the size and shape of the analemma- obliquity, eccentricity, and the angle between the northward equinox and the periapsis. They also occur a mere couple of weeks after solstices, which in turn causes slight tilt of the figure eight and its minor lateral asymmetry. The difference in size of the lobes of the figure-eight form arises mainly from the fact that the perihelion and aphelion occur far from the equinoxes. This width of the analemma is approximately 7.7°, so the length of the figure is more than six times its width. The "width" of the figure is due to the equation of time, and its angular extent is the difference between the greatest positive and negative deviations of local solar time from local mean time when this time-difference is related to angle at the rate of 15° per hour, i.e., 360° in 24 h. The component along this axis of the Sun's apparent motion is a result of the familiar seasonal variation of the declination of the Sun through the year. The long axis of the figure-the line segment joining the northernmost point on the analemma to the southernmost-is bisected by the celestial equator, to which it is approximately perpendicular, and has a "length" of twice the obliquity of the ecliptic, i.e., about 47°. It is possible, though challenging, to photograph the analemma, by leaving the camera in a fixed position for an entire year and snapping images on 24-hour intervals (or some multiple thereof) see section below. This curve is commonly printed on terrestrial globes, usually in the eastern Pacific Ocean, the only large tropical region with very little land. The resulting curve resembles a long, slender figure-eight with one lobe much larger than the other. Īlthough the term analemma usually refers to Earth's solar analemma, it can be applied to other celestial bodies as well.Īnalemma on Earth as the position of the Sun is directly overhead every 24 hours over one year.Īn analemma can be traced by plotting the position of the Sun as viewed from a fixed position on Earth at the same clock time every day for an entire year, or by plotting a graph of the Sun's declination against the equation of time. ![]() ![]() Before this, the term had a more generic meaning that referred to a graphical procedure of representing three-dimensional objects in two dimensions, now known as orthographic projection. One can photograph an analemma by keeping a camera at a fixed location and orientation and taking multiple exposures throughout the year, always at the same time of day (disregarding daylight saving time).Īnalemmas (in the modern sense of the term) have been used in conjunction with sundials since the 18th century to convert between apparent and mean solar time. The east–west component results from the nonuniform rate of change of the Sun's right ascension, governed by the combined effects of Earth's axial tilt and its orbital eccentricity. The north–south component of the analemma results from the change in the Sun's declination due to the tilt of Earth's axis of rotation. Globes of Earth often display an analemma as a two-dimensional figure of equation of time vs. The diagram will resemble a figure eight. In astronomy, an analemma ( / ˌ æ n ə ˈ l ɛ m ə/ from Ancient Greek ἀνάλημμα (analēmma) 'support') is a diagram showing the position of the Sun in the sky as seen from a fixed location on Earth at the same mean solar time, as that position varies over the course of a year. Analemma with date marks, printed on a globe, Globe Museum, Vienna, Austria
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