When day and night get equal time

Time once again for all things to equal out, time wise that is. On Sept. 22, the sun will cross the celestial equator at 9:44 a.m. MDT, from the northern hemisphere into the southern and summer will end for us and fall, or autumn, will begin.

The name for this crossing is the autumnal equinox, referring to the idea of "equal time" or daytime and night time will be equal.

If you lived right on the equator that would be true, however, living where we do, about 40 degrees north of the equator here in Southwest Nebraska, equal time will be on Friday, Sept. 26, when the sun will rise at 6:37 a.m. and set at 6:37 p.m. giving an equal 12 hours of daylight and darkness.

Also on that day the sun will rise due east and set due west.

I have been watching the sunrises for the last few weeks and the progress of the sun southward along the horizon has been quite obvious. In fact, if you pull back the curtains on your south-facing windows at noon you will see the sunshine coming in and making blobs of sunlight on your floor -- something you would not have seen a few weeks ago as the sun was much higher overhead.

This solar transit is due to the 23.5 degree tilt of the Earth on its axis. That tilt means that as the Earth orbits the sun, different parts of the planet's surface will be receiving direct sunlight at different times. This tilt is responsible for giving us our seasons.

In a few weeks, this tilt is going to lean the northern hemisphere away from direct sunlight which means temperatures are going to cool, something we are already experiencing, and the days will grow shorter and the nights longer.

There is one particularly large cottonwood tree here in town that I watch for the first signs of autumn. Small areas of leaves on the tree will turn yellow while all the rest of the leaves remain green. I saw the first splotch of yellow last week.

SKYWATCH: Bright Venus is still continuing to play tag with Mars and Mercury in the west just after sunset although Mars is becoming more difficult to find. On Sept. 20, the bright star Spica will join the trio to the upper left.

During the next few days Venus and company will move closer to Spica. Jupiter is the bright object in the south during the early evening hours. Use your binoculars to find its four continuously moving inner moons.

On this Wednesday evening, the moon will be playing in amongst the stars of the Pleiades star cluster, this will be the fourth time this year for such a grouping. It may be hard to find the cluster stars as the moon will be just past full and thus still quite bright, blotting out most everything else in the night sky. In the coming weeks, I will have information about the upcoming annual star count. This is a fun event where the whole family can get involved.

NEXT TIME: More astronomical blathering.