AskDefine | Define analemma

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  1. A scale on a terrestrial globe representing the sun's declination, usually drawn on or near the equator and in a roughly figure eight shape

Extensive Definition

Analemma was also a book by Ptolemy.
In astronomy, an analemma (IPA: /ˌænəˈlɛmə/, Latin for the pedestal of a sundial) is a curve representing the angular offset of a celestial body (usually the Sun) from its mean position on the celestial sphere as viewed from another celestial body (usually the Earth). For instance, knowing that Earth's average solar day is almost exactly 24 hours, an analemma can be traced by plotting the position of the Sun as viewed from a fixed position on Earth at the same time every day for an entire year. The resulting curve resembles a figure of eight. This curve is commonly printed on globes. 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).
There are three parameters that affect the size and shape of the analemma: obliquity, eccentricity, and the angle between the apse line and the line of solstices. For 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. For an object with a circular orbit but axial tilt similar to Earth's, the analemma would be a figure of eight with northern and southern lobes equal in size. For an object with eccentricity similar to Earth's, but no axial tilt, the analemma would be a straight east-west line along the equator.
The vertical component of the analemma is the declination, or how far north or south from the equator an observer sees the sun directly overhead. The horizontal component is the equation of time, or the difference between solar time and local mean time. This can be interpreted as how "fast" or "slow" the sun is compared to clock time.

Earth's Analemma

Due to the earth's tilt on its axis (23.439°) and its elliptical orbit around the sun, the relative location of the sun above the horizon is not constant from day to day when observed at the same time on each day. Depending on one's geographical latitude, this loop will be inclined at different angles.
The figure on the left is an example of an Earth analemma. It is a plot of the position of the sun at 12:00 noon at Royal Observatory, Greenwich, England (latitude 51.4791 deg north, longitude 0) during the year 2006. The horizontal axis is the azimuth angle in degrees (180 degrees is facing south). The vertical axis is the altitude in degrees above the horizon. The first day of each month is shown in black, and the solstices and equinoxes are shown in green. It can be seen that the equinoxes occur at altitude φ=90 - 51.4791 = 38.5209 degrees, and the solstices occur at altitudes φ±ε where ε is the axial tilt of the earth, 23.439 degrees. The analemma is plotted with width highly exaggerated, which permits noticing that it is very slightly asymmetrical (due to the two-week misalignment of the apsides of the Earth's orbit and its solstices).
See equation of time for an in-depth description of the east-west characteristics of the analemma.

Other analemmas

On Earth, the analemma appears as a figure eight, but on other solar system bodies it may be very different The variation is due to the interplay between the tilt of each body's axis and the elliptical shape of its orbit.
In the following list, "day" and "year" refer to the synodic day and sidereal year of the particular body.
  • Mercury: Because the day is exactly two years long (due to orbital resonance), the method of plotting the sun's position at the same time each day would only yield a single point. However, the equation of time can still be calculated for any time of the year, so an analemma can be graphed with this information. The resulting curve is a nearly straight east-west line. An interesting phenomenon occurs because of the relationship between Mercury's day and year (see Mercury (planet)#Orbit and rotation).
  • Venus: There are slightly less than two days per year, so it would take several years to accumulate a complete analemma by the usual method. The resulting curve is an ellipse.
  • Mars: teardrop
  • Jupiter: ellipse
  • Saturn: technically a figure 8, but the northern loop is so small that it more closely resembles a teardrop
  • Uranus: figure 8
  • Neptune: figure 8
  • Pluto: figure 8

See also

External links

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