The analemma is a very cool figure eight shape which describes how sundial time varies with clock time, sometimes it is shown between the tropics on a globe. If you were to take a multiple-exposure photograph of the same part of the sky at the same time (not counting Daylight Saving Time), every day for a year, the positions of the sun in the image would show the shape of the analemma.

The sun reaches a different altitude at noon depending on the time of year, and some days it actually moves faster across the sky than other days. The change in altitude is due to the fact that the spin of the Earth is tilted at an angle of approximately 23.5 degrees to the ecliptic. The change in apparent angular speed is the combination of the effect of the Earth's tilt, and of the effect of the eccentricity of the Earth's orbit.

At perihelion, when the Earth is closest to the sun, the Earth has a greater angular speed (by Kepler's second law), so the sun has a greater angular speed relative to the Earth. The speed-up is in fact in the same direction as the Earth's spin, so the sun actually appears to move slower near perihelion than near aphelion.

At solstices, the ecliptic is farthest from the celestial equator but not moving relative to it. This means the rotation of the sun as observed from the Earth around the ecliptic is most 'in sync' with the spin of the Earth projected on the celestial equator. Thus, the sun has a greater apparent angular speed during the solstices than during the equinoxes.

Specially prepared sundials have gnomons to take all these effects into account, so the sundial time is always very close to clock time. However, due to the precession of the equinoxes, the shape of the analemma is changing, very very slowly…

Source: http://www.analemma.com/

An`a*lem"ma (#), n. [L. analemma a sun dial on a pedestal, showing the latitude and meridian of a place, Gr. a support, or thing supported, a sun dial, fr. to take up; + to take.]

1. Chem.

An orthographic projection of the sphere on the plane of the meridian, the eye being supposed at an infinite distance, and in the east or west point of the horizon.

2.

An instrument of wood or brass, on which this projection of the sphere is made, having a movable horizon or cursor; -- formerly much used in solving some common astronomical problems.

3.

A scale of the sun's declination for each day of the year, drawn across the torrid zone on an artificial terrestrial globe.

 

© Webster 1913.

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