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jordan1989 · Mar 18, 2009, 07:48 PM

The spring equinox is supposed to be on March 21, but by definition of an equinox, it's the day when the time from sunrise to sunset is exactly 12 hours. And according to the Weather Channel website, that day was yesterday (March 17).

Daily Averages for Livonia, MI (48150) - weather.com

Here you see that March 17 is the first day where the day is longer than the night.

Capuchin · Mar 19, 2009, 03:56 AM

Firstly, the vernal equinox occurs on the 20th of March this year. (and for many years to come).

Secondly, your definition of equinox is wrong. An equinox is defined as the time at which the center of the sun is directly over the equator. (this year the vernal equinox is at 11:44, 20th March UTC.)

If the sun were a point source of light, then you would be right, the day should be 12 hours long. However the sun is a disc, as such it takes 12 hours between the center of the sun appearing and disappearing, however sunrise and sunset (and therefore the length of the day) are defined as when any part of the sun appears or disappears (when light directly from the sun hits ground level) - so in reality the day is always slightly longer than 12 hours on the equinoxes.

Remember that the equinox is not defined by the length of day, but by the position of the sun.

Hopes this helps to clear up your confusion! Let me know if you have any further questions.

sarnian · Mar 19, 2009, 04:32 AM

Dear Capuchin

An equinox is defined as the time at which the center of the sun is directly over the equator. (this year the vernal equinox is at 11:44, 20th March UTC.)

True, but I suggest a slightly different description : that an equinox results from the tilt of the earth's axis against our orbit around the sun, and occurs when the center of the sun is directly over the equator.

The result is the same, but indicates that the variance is not related to the sun, but to the direction of the earth axis in our solar orbit.

ebaines · Mar 19, 2009, 06:37 AM

One other point - the published times of sunrise and sunset also take into account the fact that refraction of light through the atmosphere makes it so that you see the sun even when it is a degree or so below the horizon. This refraction can actually add several additional minutes to the length of daylight, compared to what it would be if there was no atmosphere. Hence sunrise times are a minute or two earlier than you might think based purely on geometry, and sunset is a minute or two later.

Capuchin · Mar 19, 2009, 07:22 AM

Originally Posted by sarnian

Dear Capuchin

True, but I suggest a slightly different description : that an equinox results from the tilt of the earth's axis against our orbit around the sun, and occurs when the center of the sun is directly over the equator.

The result is the same, but indicates that the variance is not related to the sun, but to the direction of the earth axis in our solar orbit.

Maybe, from the sun's point of view ;D