When observation hours grow longer

Have you noticed it yet? Have you seen it? The sun peeking directly in the east window in the morning and shining into your face? And sunlight blaring through the west curtains in the evening? How about the sun extending its light into the south windows of your house in the middle of the day?

It is that time of year. When the sun's creep southward along the horizon reaches the point where it rises due east and sets due west--the time of the equinox, or the equality of day and night.

The astronomical time of seasonal change -- going from summer into fall -- unless you happen to live in the southern hemisphere where it will be going from winter into spring. The equinox occurs on Sept. 23 at 3:52 am MDT. At this time, the sun will cross the celestial equator going from north to south in its annual march.

Or perhaps you have noticed the days getting shorter, the sun rising later than it used to and setting earlier than it did during the summer.

Just a month ago, on Aug. 12, the sun rose at 5:53 a.m. MDT and sat at 7:48 p.m. Wednesday, on Sept. 12, it will rise at 6:23 a.m. and will set at 7:02 p.m. -- a loss of more than an hour of daylight in the past month.

We can thank the 23 and one-half-degree tilt of the Earth on its axis for this change.

As we orbit the sun this tilt causes different portions of the Earth's surface to be exposed to direct sunlight. As we enter this particular part of the orbit, this tilt of the Earth leans the northern hemisphere away from the sun causing the shorter, and ultimately, colder days and longer nights.

We astronomers welcome this lengthening of night because it gives us more observing time than was previously available.

For example, on Aug. 12 the average observing time from the end of astronomical twilight (astronomical twilight is the time when the sky if dark enough to observe) in the evening to its beginning before dawn the next day was about seven hours and 30 minutes. Now, on Sept. 12, the average observing time is about eight hours.

Something else you should notice: the autumnal constellations are slowly climbing over the eastern horizon a little more each night and the summer constellations creep westward.

If you really want a jolt, get up early and look to the east. There you will find the winter stars holding forth getting ready for their turn on the cosmic stage.

Enter Pegasus with attending Andromeda and trailing Perseus, Cassiopeia, and Cepheus. Throw in Pisces, Aquarius, and Cetus for good measure, and don't forget the loneliest star in the sky, bright Formalhaut (pronounced FORM-a-low) in Pisces Austrinus, a constellation so dim that only Formalhaut is visible in the southeastern sky.

SKY WATCH: Look in the west about a half hour after sunset on Sept. 13 for a very slender crescent moon near the bright star Spica in Virgo, the Maiden. The bright dot just to the Moon's right is the planet Mercury.

Both are very low to the horizon and set soon after the sun. The next night look for a growing moon to the left of Spica. On Sept. 17 and 18 watch the moon play tag with Antares and Jupiter in the south.

Next time: More astronomical blatherings.