September Equinox 2019

At 03:50 a.m. EDT on Monday, September 23, the Equinox occurs. What is an Equinox, why does it happen, and why is it called so? Let’s see.

The equinox in September is known as the autumnal (fall) equinox in the Northern Hemisphere, where it marks the beginning of autumn. In the Southern Hemisphere it is known as the vernal (spring) equinox and marks the first day of spring.

The equinox occurs when the Sun crosses the celestial equator – the imaginary line in the sky above Earth’s equator – from north to south. This happens every year either on September 22, 23, or 24. On the day of the Equinox, everyone worldwide experiences 12 hours each of daytime and night-time. This is where the term, Latin for “equal night”, comes from.

Because the Earth’s axis of rotation is tilted 23.5° away from the Ecliptic, the equator circle and the ecliptic circle behave like two hula hoops with the same center, but with one tilted so that they intersect at only two spots. At the moment of the Autumnal Equinox, the sun is situated at one of the intersection points — and its apparent motion through the stars is carrying it into the southern bowl of the sky — as if it is “stepping over” the equator. Six months from now, on the Vernal Equinox, the sun will again cross the Celestial Equator at the other intersection point — this time heading into the northern bowl of the sky. At that moment, Spring will begin for the Northern Hemisphere.

The Equinox triggers a few interesting effects. At either equinox, the sun rises due east and sets due west. For the next six months, the sun will spend all of each day in front of the southern hemisphere stars, and also high overhead of the lucky folks who live there! With the sun higher in their sky, they experience more daylight hours and receive more highly concentrated solar radiation, producing warmer weather. At the same time, North Americans, Europeans, and Asians have to accept shorter, colder days and longer nights.

The nights around the equinoxes offer better chances to see the aurorae at high northern and southern latitudes, too.

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