Solar noon, which indicates the moment when the sun crosses the meridian and when it's at its highest elevation in the sky, at 12 o'clock apparent solar time, does not occur every day at the same moment. This because of the equation of time, which indicates the gap, variable during the year, between the mean solar time and true solar time. These differences are due to what the earth does not have the same speed during the year. Sometimes it accelerates, sometimes it slows down. If this is due to the approximation (perihelion) and the distance from the sun (aphelion), we'll have only one acceleration phase (perihelion) and one slowdown (aphelion). However, the annual curve of the equation of time ( http://www.hartrao.ac.za/other/sundial/eqtime.gif ) shows four phases: Acceleration then deceleration then acceleration and finally deceleration. This is due to two effects: the earth's elliptical orbit and the tilt of earth axis.
Can someone explain how the tilt of earth axis generates four phases of acceleration and deceleration.
Thank you very much.
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