Autumn is nearly here. This year, the season officially begins Monday at 8:45 a.m. PDT.
Many people think that the beginning of autumn occurs when the sun rises due east and sets due west, and when the length of our day and night are equal. And, as long as we don't concern ourselves with fine details, those are both reasonably true statements.
So why, then, do we cite the exact time for the beginning of autumn? That's because astronomers know the beginning of autumn, the autumnal equinox, to be 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; in fact, it's 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; in our wintertime, it crosses relatively low in the southern sky. This perspective comes from the fact the Earth's axis is tilted and, as we orbit the sun, our summertime sun shines more directly onto our planet's Northern Hemisphere, while the wintertime sun shines more directly onto the Southern Hemisphere.
At some point during the year, the sun – traveling along its orbit, called the ecliptic – must cross the equator on its way southward, and that defines the moment known as the autumnal equinox
Now, try to imagine standing on the equator. On the first day of autumn you would 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.
Another way of thinking about it is to visualize 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 the sun crosses this great circle on its way south marks the onset of autumn in the Earth's Northern Hemisphere.
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