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Wednesday, December 12 , 2018, 12:46 am | Fair 45º


Dennis Mammana: Fall Is in Line with Upcoming Autumnal Equinox

Ready or not, autumn is coming this week. Click to view larger
Ready or not, autumn is coming this week. (Creators.com illustration)

Autumn is nearly here, and for this desert rat, it can’t come soon enough.

This year, autumn officially begins at 1:02 p.m. PDT Sept. 22. Many people think that autumn starts on the day that the sun rises due east and sets due west, and when the length of our day and night are equal. If we don’t concern ourselves with fine details, both are reasonably true statements.

So why, then, did I cite the exact time of the beginning of autumn? That’s because the beginning of autumn, the autumnal equinox, is not actually a day but rather a moment in time.

It’s marked by the sun’s passage in the heavens from the Northern Hemisphere sky into the Southern Hemisphere sky; it is, in fact, the moment when the sun’s center crosses the celestial equator on its journey from north to south.

Confused? Well, try to imagine what’s going on.

During summertime, we in the Northern Hemisphere see the sun cross our daytime sky high overhead. But during wintertime, it crosses relatively low in the southern sky. This happens because Earth’s equator is tipped about 23.4 degrees to the plane of its orbit around the sun.

At some point during the year, however, as the sun travels along its orbit (called the “ecliptic”), it must cross the equator on its way southward. That defines the moment known as the autumnal equinox. When this occurs, our sun appears directly overhead for viewers on the equator.

Now try to imagine standing at the equator. On the first day of autumn, you’d see the sun pass directly overhead. A few days or weeks earlier, the sun would have crossed your northern sky, and a few days or weeks later, it would cross your southern sky. But on that day, the sun stands directly over the equator so you would watch it pass directly overhead at midday.

Another way to think about it is to imagine extending a projection of the Earth’s equator into the starry heavens. This would create in the sky what astronomers call the “celestial equator.” The moment when the sun crosses this great circle on its way south marks the autumnal equinox and the onset of autumn in the Earth’s Northern Hemisphere.

An easy way to watch how the sun’s position changes during the year is to notice the sunset point along the horizon. To do this, it’s only necessary to have a fixed observing location and to note the sun’s setting or rising position from week to week.

To watch it, find a location where the sun sets behind distant landmarks such as trees, houses or mountain peaks. Each day this week or next, return to that same spot late in the afternoon and sketch the sunset position relative to those landmarks. Be careful not to look directly at the sun itself, or you may suffer irreversible eye damage.

You’ll need only a few days to notice the southerly drift of the sunset point along the horizon and the decreasing length of daylight, both factors that lead to more pleasant temperatures coming to our part of planet Earth.

Dennis Mammana is an astronomy writer, author, lecturer and photographer working from under the clear dark skies of the Anza-Borrego Desert in the San Diego County backcountry. Contact him at [email protected] and follow him on Twitter: @dennismammana. Click here to read previous columns. The opinions expressed are his own.

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