What makes the autumnal equinox special?

autumnal equinox

A few days ago, on September 22, the autumnal equinox happened, announcing the beginning of fall. Of course, this is the case in the Northern Hemisphere. For those in the Southern Hemisphere, the September equinox brings springtime. The autumnal equinox marks the date when the sun’s angles and earth are perfect for that day and night to be of equal length. However, that is not entirely accurate. Let’s look at the equinox in more detail.

Does the autumnal equinox last an entire day?

Although we celebrate or observe the day as the autumnal equinox, the equinox is actually something that happens for just a moment. There is an imaginary line up in the sky above the equator, named the celestial equator. The moment  the equinox occurs is when the sun crosses that line. Furthermore, the sideways angle of the earth at that moment is perfectly balanced on its axis so that it does not tilt away nor toward the sun. Importantly, this is true for only a moment.

This year, that moment was at 9:31 a.m. EDT. From that moment on, sunrise will be progressively later and sunset earlier in the Northern Hemisphere. That will change gradually as we move through the winter until March next year when spring arrives.

Is the equal night and day idea valid on the autumnal equinox?

Most people believe the day and night are each 12 hours long on the autumnal equinox day. The truth is that your distance from the equator can allow you to see the sun rising earlier or later than people at other vantage points. However, the difference will be minutes rather than hours. Your location and latitude in the Northern Hemisphere will play a role in the length of the day and night where you are.

Why is the autumnal equinox not on the same date every year?

This will also explain why every fourth year has an extra day. Although we refer to the length of a year as 365 days, it actually takes the earth 365-and-one-quarter day to travel around the sun. Therefore, every fourth year has that extra day made up of the four quarter days. Also, that one-quarter day means that next year’s autumnal equinox will occur six hours later than this year. After four years, the added hours will accumulate and bring the equinox one day later. Most frequently, the autumnal equinox falls on either September 22 or 23. However, on rare occasions, it could be on September 21 or 24. The rarity is underscored by the fact that the next time it will fall on September 21 is in 2092, and the last time it happened on that date was 1000 C.E.

Northern lights
Aurora Borealis

With the equinox comes the Northern Lights

From the day of the autumnal equinox and throughout October, those spectacular Northern Lights in the sky are visible for those who are in Alaska during that time.

What causes the aurora borealis? Solar winds, which are particles of escaped plasma from the sun. As they pass through the earth’s magnetic shield, they mingle with molecules and atoms of nitrogen, oxygen, and other elements. The result is the dazzling lights displayed in the sky.

Unfortunately, Alaska is the only state where you can experience the spectacle live. For the rest of us, we’ll have to experience it second hand.

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