According to my paper, there were 11 hours 59 min. of daylight on Friday, March 16; and 12:04 today, March 18. But the Vernal Equinox isn’t until March 20.
I always thought that on the definition of Equinox was 12 hours of daylight–that the day marked on the calendar as the First Day of Spring had sunrise & sunset exactly 12 hours apart. What the heck is going on?
Could you quote exactly what your paper said? This will help the Teeming Millions help you out better.
Also, where do you live? Latitude has an effect upon the length of the day, I think (correct me if I’m wrong).
The vernal equinox is the moment in time when the sun is directly above the equator. There would be approximately 12 hours of daylight, but not exactly, since the atomosphere refracts (bends) sunlight. The sun rises a few minutes earlier and sets a few minutes later than it would without the atmosphere.
Daylight begins slightly before sunrise, and continues for a while after sunset. The time between the moment when the first edge of the sun comes above the horizon, and the last edge drops below the horizon is longer than exactly one half of the day by the length of time it takes for the entire disk of the sun to pass through the line of the horizon.
The answer to this question can be found in today’s USA Today, on the weater page. Both bibliophage and Triskadecamus are correct.
The online version of USA Today doesn’t provide much help, but it did provide a link to the US Naval Observatory. USNO says I live at 37.5 N (which should be irrelevant–if it’s an Equinox, it’s 12 hours between sunrise & sunset at any latitude) and that March 19 sunrise is at 6:13, sunset at 6:20; and that the Equinox is March 20.
The sun is bigger than the Earth. Because of this, without taking atmospheric refraction into account, 50.14% of the Earth is exposed to the sun at any given time. According to my calcualtions, this would account for almost exactly 4 minutes difference between day and night at the equinox. Add the refractive effect of the atmosphere on top of that and I’d expect that you’d get numbers almost exactly like what actually happens. This works out good.
This site has the straight dope on how refraction affects sunrise and sunset.
http://apollo.lsc.vsc.edu/classes/met130/notes/chapter4/sunrise_set.html