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April 15, 2024

Backyard Astronomy: Signs of Autumn (2003-09-22)

By Cheryl Haimann

The autumnal equinox will occur on September 22, but you don't need a calendar to know that the date is near. The sky gives you two unmistakable clues -- one at night, and the other during the day.

If you have had to drive straight east in the early morning, or straight west in the late afternoon, you have probably seen one of the clues -- the sun shining straight into your eyes.

At summer solstice, Earth's North Pole was leaning toward the sun. The days were long because the sun rose and set far to the north, and was high in the sky at midday. After solstice, the sun rose a little bit farther south each day as Earth's orbit caused the axis to point less directly toward the sun.
The equinox ("equal night") is at the midpoint between the summer and winter solstices, and marks the point where the axis is parallel to the sun rather than pointing towards it. At solstice, the length of daylight varies depending on your latitude. At equinox, though, half of the earth at every latitude is facing the sun, and half is in the dark. Thus, at every point on earth, daylight lasts exactly as long as night, twelve hours each.

Also, because every latitude has the same orientation to the sun, the sun rises at due east for all of us, and sets directly in the west. This is why the early morning sun is so blinding if you are driving east. You are indeed driving directly towards the sun. The sun's high point in the sky, though, still varies according to your latitude. At the equator, the midday sun will be directly overhead. Farther north, the midpoint will be lower in the sky.
Meanwhile, in the early evening, Vega is directly overhead and the Summer Triangle is heading off to the west. Another asterism dominates the eastern sky in autumn. The Great Square is comprised of three stars in Pegasus and one star in Andromeda. More than 10 degrees wide (one fist-width), it looks like a square tipped over on its point.

In legend, when Perseus killed the Gorgon Medusa, a winged horse sprang forth from her blood. Perseus was riding home on this horse, Pegasus, when he chanced upon the hapless Andromeda strung up to a rock.

Originally appeared 2003-09-22

Article © Cheryl Haimann. All rights reserved.
Published on 2017-09-18
Image(s) © Cheryl Haimann. All rights reserved.
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