太陽連續2次經過子午線(meridian)的時間間隔叫作一天，英文名稱謂之sundial，但是一天的長度並不一定是等於24小時喔。

在春分、秋分的時候，一天的長度剛好為24小時，但從春分開始，一天的時間就慢慢地縮短，到了夏天的第一天(約為7月21日)，一天的時間為最短，之後慢慢地增長，到了秋分，時間又變為24小時。

從秋分之後，sundial的時間會大於24小時，到了冬天的第一天(約為12月21日)，一天的時間為最長，之後就慢慢地減少，到了春分，時間又變為24小時了。

Equation of time

原理

由於2個因素，導致sundial的產生:

1. 地球的傾鈄角(obliguity)
2. 地球的橢圓形(elliptical)軌道

obliguity的影響

The obliquity causes the sun to be highest in the sky on the first day of summer (around June 21st) and lowest in the sky on the first day of winter (around December 21st).會影響>太陽的高度角。

橢形軌道的影響

近日點(Perihelion): The Earth is closest to the Sun around January 4th. Consequently, the orbital velocity is highest on this date. As a result, the length of the day, the period from noon to noon for example, is longer than the average of 24 hours. (The additional motion of the Earth in its revolution around the Sun during the 4 minutes between steps 2 and 3 is not shown.)

遠日點(Aphelion): The Earth is farthest from the Sun around July 3rd. Because the orbital speed is slowest at this time of year, the length of the day is shorter than the average of 24 hours. (The angle between the arrows in steps 2 and 3 is smaller in this figure than between steps 2 and 3 in the perihelion figure.)