Why aren't there exactly 12 hours of daylight and dark on the first day of the season?

The first day of spring is March 20

DENVER - 7NEWS' meteorologists often receive questions on the subject of sunrise and set times around the equinox periods in the spring and fall.

"Folks want to know why there is not exactly 12 hours of daylight and night right on the first day of the season," said Mike Nelson, 7NEWS' Chief Meteorologist. 

There is some effect that the atmosphere has on the apparent sunset and sunrise as the air tends to "bend" the light rays slightly. This is something that would not happen without an atmosphere -- on the Moon for instance.

The phenomenon of the "real" equinoxes (12 hours sunshine, 12 hours dark) preceding in the spring and following in the fall is due in part to the index of refraction of the atmosphere.

The atmosphere bends sunlight over the horizon, making the sun appear to rise up to a couple of minutes before it has theoretically cleared the horizon.

At sunset, the refractive effect causes the observed sunset to occur up to a couple of minutes later than theory would allow. This causes the days of 12 hours of sun and 12 hours of dark to occur a few days before the equinox in spring and a few days after in fall.

Due to the shape of the Earth's orbit and to the Earth's tilt on its axis, the Sun's apparent motion through the heavens varies slightly through the year. The analemma -- a figure eight sometimes drawn on globes or maps -- is a visual representation of this.

In essence, the real Sun runs anywhere from 20 minutes fast or slow, depending on the season. The analemma provides a correction to sundials to determine how much the real Sun is off from the clock-time Sun.

The first day of spring is Thurs., March 20.

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