Tuesday, September 22, 2009
Jimmy Westlake's Celestial News column appears Tuesdays in the Steamboat Today.
Find more columns by Westlake here.
The season of autumn officially arrives for the northern hemisphere at 3:22 p.m. today. How do astronomers determine the precise moment the season begins?
Fall begins the instant the sun crosses the equator on its way south. Thanks to the 23.5-degree tilt of the Earth's axis of rotation, the sun spends half of the year shining directly onto the northern hemisphere and the other half of the year shining directly onto the southern hemisphere. It reaches its highest point in our sky June 21 - the summer solstice - and its lowest point Dec. 21 - the winter solstice. Separating these two extremes are two days during the year called the equinoxes - six months apart - when the sun shines directly down on the Earth's equator. Equinox is a word that means "equal night" and is used to describe these two special days of the year when every location on Earth experiences exactly 12 hours of sunshine and 12 hours of darkness, and the sun rises exactly in the east and sets exactly in the west.
The equinoxes have been revered by many cultures throughout the course of history. The ancient Druids went to great lengths to align the gigantic stones at Stonehenge to point out the position of the rising sun on the dates of the equinoxes and solstices. The Great Sphinx of Egypt also faces the direction of the rising sun on the date of the equinox. A coincidence? Probably not.
You might have noticed recently that the sun is rising later in the morning and setting earlier in the evening than it did in mid-summer, and if you are very observant, you might have also noticed that it is rising and setting much farther to the south than it was in mid-summer. Here in the northern hemisphere, we have just enjoyed six months of long, sun-filled days. Now it's the southern hemisphere's turn. With each passing day, the noontime sun will sink lower and lower in our sky from now until the winter solstice Dec. 21. Then, it will begin moving northward again and cross the equator on the vernal equinox, March 21, bringing spring with it.
Oh, and one more thing - that business about balancing an egg on its end on the day of the equinox? It's only a myth. You can just as easily balance that egg on any day of the year. But don't take my word for it. Give it a try and see for yourself.
Jimmy Westlake teaches astronomy and physics at Colorado Mountain College's Alpine Campus. He is an avid astronomer whose photographs and articles have been published all around the world. His "Celestial News" column appears weekly in the Steamboat Pilot & Today newspaper. Check out Jimmy's Web site at www.jwestlake.com.