When is autumnal equinox 2017

The word equinox literally means "equal nights" in Latin, although day and night won't be exactly equal to the second. It actually only takes place during the moment in time when the sun crosses the celestial equator, an imaginary line in the sky above the real equator. For astronomers, the equinox marks the end of summer and the start of autumn, meaning fall has officially fallen.

Meteorologists, meanwhile, prefer to divide the year a bit differently, with autumn taking place from 1 September to the end of November. Now, you can enjoy your pumpkin spice latte without feeling like you're cheating on summer: Today officially marks the Autumnal equinox, which is to say it's the first day of fall. Also known as the September Equinox , the autumnal equinox occurs when the sun crosses the equator. According to The Farmer's Almanac , both the Southern and Northern hemispheres receive the same amount of sunshine at this time, and day and night are of nearly equal length.

The Northern Hemisphere will start seeing shorter days and longer nights, whereas the Southern Hemisphere will see a reversal in seasons due to the Earth's tilt. September 22 marks the fall equinox in the United States and the start of autumn across the northern hemisphere—at the same time in the southern hemisphere the vernal equinox marks the start of spring. In theory the orientation of the earth towards the sun on the day of the equinox means across the planet we will experience a day and a night of equal length—equinox translates from Latin to mean 'equal night.

Read more: As fall equinox approaches, was Stonehenge used to predict astronomical events? As the Earth makes its orbit around the sun, the relative position of the northern and southern hemispheres change over the course of the year. This is the result of the changes to the planet's rotational axis which sits at an angle.

However, today is only the first day of the astronomical fall. The meteorological fall started on September 1. According to the Metro , meteorological seasons are based on the annual temperature cycle and are split into four seasons of three months.

The Fall Equinox as seen from a satellite in space. Courtesy NASA. The fall equinox lands on either September 22, 23, or 24 every year in the northern hemisphere. It is one of the four seasonal astronomical events that impact earth. The other three are the winter solstice, the spring equinox, and the summer solstice. The fall equinox is also known as the autumnal equinox. The spring equinox is one of the two annual equinoxes where the sun shines directly on the equator and the length of day and night is nearly the same.