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    Question ID:   4017         Current Version: 1
Question: Why are sunrise and sunset not exactly 12 hours apart on the Equinox?
Category: Science > Earth Sciences
Keywords: March, September, Spring, Fall, day, night, exactly, 12 hours, Equinox
Type: why
Rating:(0 ratings)    Views: 277    Discussions: 0   In Watch Lists: 1  

 
    Answer:
Day and night are not exactly of equal length at the time of the March and September equinoxes. The dates on which day and night are each 12 hours occur a few days before and after the equinoxes. The specific dates for this occurrence are different for different latitudes.

On the day of the equinox, the geometric center of the Sun's disk crosses the equator, and this point is above the horizon for 12 hours everywhere on the Earth. However, the Sun is not simply a geometric point. Sunrise is defined as the instant when the leading edge of the Sun's disk becomes visible on the horizon, whereas sunset is the instant when the trailing edge of the disk disappears below the horizon. At these times, the center of the disk is already below the horizon. Furthermore, atmospheric refraction (or bending) of the Sun's rays cause the Sun's disk to appear higher in the sky than it would if the Earth had no atmosphere. Thus, in the morning, the upper edge of the disk is visible for several minutes before the geometric edge of the disk reaches the horizon. Similarly, in the evening, the upper edge of the disk disappears several minutes after the geometric disk has passed below the horizon.

For observers within a couple of degrees of the equator, the period from sunrise to sunset is always several minutes longer than the night. At higher latitudes in the Northern Hemisphere, the date of equal day and night occurs before the March equinox. Daytime continues to be longer than nighttime until after the September equinox. In the Southern Hemisphere, the dates of equal day and night occur before the September equinox and after the March equinox. 

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