Sky Spy: Equinox marks spring's official start

2013-03-21T00:00:00Z 2014-07-01T16:49:38Z Sky Spy: Equinox marks spring's official startTim Hunter Special To The Arizona Daily Star Arizona Daily Star

Spring officially began Wednesday at 4:02 a.m. Tucson time. This brief moment in time is officially known as the spring or vernal equinox.

Because the Earth moves around the sun once a year, the sun appears to move slowly across the sky from day to day. When the sun crosses directly over the equator from the southern to the northern part of the sky, winter gives way to spring.

"Equinox" means "equal night" - the hours of day and night are equal. It's a common misunderstanding that they are equal time frames at the vernal equinox around March 21 and the autumnal equinox around Sept. 21, but that is not true.

The hours of day and night are close but not exactly equal at these times. For Tucson's latitude of about 32 degrees north, the times of equal daylight (from sunrise to sunset) and night (from sunset to sunrise) are five to six days before March 21 and five to six days after Sept. 21.

Even so, there is somewhat more daylight than night, because the atmosphere acts as a giant lens and bends the sun's light up above the horizon a few minutes before sunrise and a few minutes after sunset.

Around the times of the vernal and autumnal equinox, the sun rises due east and sets due west.

Check this out Friday morning when the sun rises at 6:24 a.m. almost due east and sets at 6:37 p.m. almost due west.

After sunset Friday, check out the gibbous moon high in the southeastern sky.

It will be halfway between Procyon in Canis Minor, the Lesser Dog, on the right (west) and Regulus in Leo the Lion on the left (east). That is a good way to start spring.

Moon Watch

The moon is in a waxing (growing larger) gibbous (more than half-lit) phase. Full moon is Wednesday.

Contact Tim Hunter at skyspy@azstarnet.com

Copyright 2014 Arizona Daily Star. All rights reserved. This material may not be published, broadcast, rewritten or redistributed.

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