On 2009.03.19, at 19:56 , Brij Bhushan Vij wrote:
However, Vernal Equinox is the point in time when SUN is directly over head and causing EQUAL 12-hours in TWO hemispheres of Earth.
The only way that you'll reckon a 12-hour day and 12-hour night on the day of the vernal equinox is if you use an equatorial sundial to reckon the time!
(Some other types of sundials may also yield such readings also, but I'm less familiar with them.)
The ancients considered day and night equal on the day of an equinox because they used sundials to reckon time.
If you use a clock that proceeds at the rate of mean solar time, then the daytime will be approximately 30 minutes longer than the nighttime.
Daytime will be about 12h 15m, nighttime about 11h 45m.
The exact amounts depend on the observer's latitude, and when is the actual moment of the equinox in relation to sunrise and sunset at the observer's locale.
Due to atmospheric refraction, when Sun is at the horizon it appears to be higher than its actual geometric position.
Also, Sun appears as a disk almost 1/2° in diameter, and it is daytime when any part of that disk is above the horizon.
If Earth had no air, and if Sun were a bright point of light, then yes, on the day of an equinox the length of day and night would be equal.
OK, so 3000 days have now ELAPSED since the beginning of the 3rd millennium, and the northward equinox happened about an hour ago as I write this...
-- Irv Bromberg, Toronto, Canada
