The September Equinox

~2 min
The September Equinox

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 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 13:31 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.

Astronomical definition

In astronomy, all four seasons start along with the equinoxes and solstices. For example, in the Northern Hemisphere, the September equinox marks the first day of autumn, and in the Southern Hemisphere — the beginning of spring. Astronomical autumn in the Northern Hemisphere will last till the winter solstice representing the start of winter.

Meteorological definition

By contrast, meteorologists divide a year into four seasons, three months each, based on average monthly temperatures with summer as the warmest and winter as the coldest season. According to this definition, in the Northern Hemisphere, the autumn starts on September 1 and ends on November 30. For the Southern Hemisphere — on March 1 until May 31.

Phenological definition

The third definition is based on phenological indicators. These include a range of ecological and biological signs, like leaves falling off the trees and birds’ migration to warmer countries. Of course, weather and climate influence these events, and, within this definition, seasons might move in the future due to climate change.

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