Analemma
The analemma is a plot of the different positions of the Sun in the celestial sphere recorded at a certain point in time(at 24 hour intervals) and from the same location along the calendar year. Due to the earth's tilt on its axis (23.5°) 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 vertical coordinate of a point corresponding to a date corresponds to the declination of the sun on that date, while the horizontal coordinate indicates whether the sun is "fast" or "slow" compared to mean time.
The deviation between solar time and mean time is known as the equation of time and is due to the variation in the length of the synodic day, which is due to variation in the distance between the earth and the sun. (See aphelion, perihelion.)
On Earth, the analemma shows a "8"-shape figure, but on Mars it has a teardrop shape. Other planets also have analemmas with various shapes [1] (http://www.analemma.com/Pages/OtherAnalemmas/OtherAnalemmas.html).
External links
- Figure-8 in the Sky (http://astro.isi.edu/games/analemma.html)
- Astronomy Picture of the Day, 2002 July 9: Analemma (http://antwrp.gsfc.nasa.gov/apod/ap020709.html)
- Astronomy Picture of the Day, 2003 March 20: Sunrise Analemma (http://antwrp.gsfc.nasa.gov/apod/ap030320.html)
- Astronomy Picture of the Day, Analemma over Ancient Nemea (http://antwrp.gsfc.nasa.gov/apod/ap040621.html)
- Analemma.com (http://www.analemma.com/) (incorrectly claims Mercury and Venus lack analemmae)
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Categories: Celestial mechanics