Friday, March 20, 2009

The vernal equinox (First day of spring!)

Today is the official first day of spring, or the vernal equinox. Welcome to Spring!

Though "equinox" connotes an equal amount of day and night, that is not exactly so for Memphis at 35 degrees north latitude. The actual time of the equinox is 6:44am, when the sun passes directly over the Equator. Sunrise in Memphis is at 7:03am and sunset at 7:12pm. Thus, the length of the day is 12 hrs., 9 min.! Memphis actually experienced an equal amount of day and night 4 days ago, on March 16, when the sun rose at 7:09am and set at 7:09pm. Pretty cool, huh?

So, for the first day of spring, we're looking for an absolutely gorgeous day with temps right at normal in the mid 60s. A small rain chance shows up Saturday but temps should remain near normal through then before starting to rise to slightly above normal levels to end the weekend and start next week. The next major frontal system won't affect the area until Tuesday or Wednesday of next week.


Anonymous said...

"Pretty cool"? No. Explains nothing ... I can see where maybe the length of the day on the equinox should be 12:04 minutes ... to make up for the fact the earth goes around the sun once a year ... but the other 5 minutes? where does that come from? If you comeup with something ... please send me a note.

Meteorologist Erik Proseus said...

An equinox actually means the sun is directly over the Equator, with neither of the poles tilted towards or away from the sun. The term "equal day and night" is probably better said this way: a point at the same altitude north and south of the Equator have equal amounts of day and night because the sun is exactly between those two points. Memphis is at 35North, so a location at 35 South has the same amount of daylight as Memphis on this day.

As the earth's tilt causes the sun to move north of the Equator after the spring equinox, our days get longer. The day that we happened to pass that 12-hr-dark/12-hr-light point actually occurred a couple of days ago due to our latitude north of the Equator.