The March Equinox, heralding the beginning of spring in the Northern Hemisphere and autumn in the Southern Hemisphere, is a few hours early this year, despite 2012 being a leap year. The equinox will take place on March 19 in a good section of the of the world as it officially occurs at 05:14 UTC on March 20 (i.e., every time zone west of U.S. Central Daylight Time will begin spring on the 19th). On the equinox the sun is directly above the equator at local noon. The entire planet experiences twelve hours of daylight and twelve hours of darkness. Sunrise is 6 a.m. local time (uncorrected for Daylight Saving Time) and sunset is at 6 p.m. On March 20, the North Pole emerges into the light while the South Pole slips into darkness for the next six months.
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You said that on that day “Sunrise is 6 a.m. local time (uncorrected for Daylight Saving Time) and sunset is at 6 p.m.” I think this is not the case. You forgot the fact that sunrises and sunsets change continuously over space, while local times change discretely according to time zones. True, at Greenwich the sunrise is 6 a.m. on that day, and this is also true for other standard meridians for each time zone (with few exceptions), but the farther away a place from its standard meridian, the larger the deviation of its sunrise from 6 a.m.
The daylight on the equinox is a few minutes longer than 12 hrs because 1) atmospheric refraction makes the sun visible a bit below the horizon, and 2) I believe that sunrise and set are reckoned from the times of first light at sunrise to last light at sunset which adds the angular diameter of the sun to its 180-degree arc.
I know Matt is too polite to say this, so I’ll say it for him: Adi and George, you both are high level nit-pickers.
Thanks Matt–interesting post. I knew that where I live we don’t have 12 hours of light and dark on any day of the year, but did not know why.
Pro. Joe
I’m another ‘nit-picker’. The sun neither rises nor sets; the earth passes through “The Terminator”.
It goes like this, ON OR ABOUT: Jan 4 latest Sunrise, March 21 vernal equinox, June 14 earliest Sunrise, June 22 longest day (vernal solstice), June 26 Latest Sunset Day, September 21 autumnal equinox, October 23 earth and I created, December 7 earliest Sunset (why the Japanese picked that day to attack.), December 22 Solstice.
There are two equinoxes (equal day and night everywhere) There is one day when days stop getting longer and start getting shorter and one vies versa. These are the Solstices and mark the change of season, along with the equinoxes. The lack of symmetry with earliest sunsets, etc. is because the earth’s orbit is not exactly a circle. This screws up sun dials and why globes have an alemna, looks like a distorted figure 8.
We celebrate Christmas on December 25 (it was supposed to be on the Solstice) because that’s when several solstice celebrations were already widely observed.
I should have added “in the northern hemisphere. Sorry.
At what time (CST) is the March equinox?
At 12:14 AM CDT on March 20. That is if you had sprung forward. If you were still on CST it would be March 19 at 11:14 PM.
During this period the sun has apparently moved from the southern hemisphere to the equator. There is increase in temperatures in areas around the equatorial region abut fact is is it really the sun that moves?. It can’t holly be true but is attributed to the shape and multi movement of the earth.