The September Equinox

Star Walk
3 min readSep 21, 2020
Image credit: Bessi

Astronomical autumn in the Northern Hemisphere is coming and the Southern Hemisphere is ready for spring. From this article, you’ll learn the difference between astronomical and meteorological autumn and more interesting astronomical facts about this event.

What is an equinox?

The name “equinox” comes from Latin “aequus” (equal) and “nox” (night) and gives us a pretty accurate definition. The Sun illuminates the Southern or Northern Hemisphere more due to the Earth being tilted on its axis. However, twice a year, the Sun shines on both hemispheres equally. This phenomenon is called an equinox.

It happens when the Sun is exactly above the Earth’s equator. The dividing line between daytime and nighttime areas (called the terminator) becomes vertical and passes through the planet’s north and south poles. At an equinox, the Earth experiences approximately the same amount of daylight and darkness a day.

This year, the September equinox (or autumnal equinox) will occur on September 22 at 9:31 a.m. EDT (1:31 p.m. UTC). We prepared a fun and educational quiz about equinoxes and solstices for you. Take it right now or wait until the end of the article. Check if you can tell the difference between these two astronomical events!

Interesting facts

Here are some noteworthy facts about this annual astronomical event:

  • Equinoxes cause geostationary satellite signals’ breaks. During equinoxes, the Sun goes directly behind the Earth satellites, and its radiation swamps the radio signals from the satellites. That is why in India, exchanges are closed for several hours at this time. Ordinary people may lose TV or radio signals.
  • On the equinox day, the Sun rises precisely on the east and sets strictly on the west.
  • Equinoxes occur on any planet with a tilted rotational axis. For example, it happens every 15 years on Saturn, since for this planet, it takes almost 30 years to orbit the Sun. Saturn’s most recent equinox was on 11 August 2009, and the next one will take place on May 6, 2025.

When does autumn start?

There are three main definitions of the beginning of autumn: astronomical, meteorological, and phenological.

Star Walk

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