The September Equinox of 2019

It is time to bid farewell to the summers in northern hemisphere and welcome the fall. For people living in southern hemisphere, replace summers by winters and fall by spring. The September equinox, also known as the autumnal equinox or the fall equinox is on Monday at 7:50 UTC (13:20 IST). Here are 5 things to know about this important day.

1. What is September Equinox?

The September equinox is that moment when the Sun crosses the celestial equator heading southward. On this day, if you stand on equator, you'll see the Sun rise exactly in East and set in the West. Before this fall equinox, the Sun rises and sets more to the north, and afterwards, it rises and sets more to the south.

2. Sun and celestial equator

This is the second and the last time in the year when Sun will pass through the celestial equator. The first time was at the Vernal equinox in March, when Sun headed northwards after the equinox. The point where the Sun crosses the celestial equator southwards is called the first point of Libra. However, due to the precession of the equinoxes, this point is no longer in the constellation Libra, but rather in Virgo. The September equinox passed from Libra into Virgo in year −729, will pass into Leo in year 2439.

Also Read: All the 30 articles of Basics of Astrophysics series.

3. Day and night on september equinox

The length of day and night are same at the equinox. This year on September equinox, there will be approximately 10 minutes of day over night, depending upon where you live. This happens because of the following reason: Although the sun is perfectly over the equator, we mark sunrises and sunsets at the first and last minute the tip of the disk appears. Also, because of atmospheric refraction, light is bent which makes it appear like the sun is rising or setting earlier.

September Equinox
The position of Earth at September Equinox.

4. What's Next?

From hereon, nights will start getting longer and days will start getting shorter in the Northern Hemisphere while in the Southern Hemisphere, it will be vice-verse. This will continue till 21st December, the day of winter solstice, also known as the shortest day in the northern region.

Fall Equinox
The trees start changing their shade in fall

5. September Equinox In Nature

Where should you look for the signs of September Equinox? Answer is: everywhere. The days are getting shorter (Northern Hemisphere) and the mercury has started dipping. There's a chill in the air as the shorter days are bringing cooler weather. Also notice the arc of the sun across the sky each day. You’ll find it’s shifting toward the south. Birds and butterflies are migrating southward, too, along with the path of the sun. Creatures of the wild are putting on their winter coats. In the night sky, Fomalhaut – the Autumn Star – is making its way across the heavens each night.

Also Read: The 3 types of celestial coordinate systems

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