Celestial News: Autumnal Equinox arrives Thursday
September 20, 2016
This year, the season of autumn officially arrives for the northern hemisphere at 7:21 a.m. Thursday, Colorado time. Our season of autumn 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 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 around June 21, the summer solstice, and its lowest point around 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 on the Earth's equator.
Equinox is a word that means "equal nights" and is used to describe these two special days of the year when every location on Earth experiences 12 hours of Sunshine and 12 hours of darkness. These are also the only two days of the year when the Sun rises exactly in the east and sets exactly in the west.
OK — this is oversimplified quite a bit. The Sun is actually above the horizon for slightly more than 12 hours on the equinoxes. This is an optical illusion caused by the Earth's atmosphere bending the image of the Sun and placing it slightly higher above the horizon than it actually is. Consequently, we are able to see the Sun briefly, even after it has set. Strange, but true.
You might have noticed recently that the Sun is rising later in the mornings and setting earlier in the evenings than it did in mid-summer and, if you are very observant, you also might have noticed it is rising and setting much farther to the south than it did in mid-summer.
Here in the northern hemisphere, we have just enjoyed six months of long, Sun-filled days, but now, it's the southern hemisphere's turn. We are rapidly exchanging minutes of daylight for minutes of darkness each day as we move toward the December solstice.
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After the solstice, the Sun will begin moving northward once again and cross the equator on the vernal equinox, around March 21, bringing spring with it.
Oh, and that business about balancing eggs and broomsticks only on the day of the equinox? That's a myth. You can balance that egg and broomstick just as easily on any day of the year. But don't take my word for it. Give it a try, and debunk this equinox myth for yourself.
Professor Jimmy Westlake teaches astronomy and physics at Colorado Mountain College’s Steamboat Springs Campus. His “Celestial News” column appears weekly in Steamboat Today. Check out Westlake's astrophotography website at jwestlake.com.
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