Summer Is Over, Autumn Is Here

Summer is now officially over. On Friday, September 23, at around 5:05 am EDT (0905 GMT), the autumn equinox finally arrived, marking the end of the warm season and the arrival of autumn, for the Northern Hemisphere.

In the Southern Hemisphere, the moment marked the arrival of spring after a long winter. The second of two equinoxes occurring in a year takes place in March, and marches in spring for the Northern Hemisphere, while marking autumn in the Southern Hemisphere.

During the two equinoxes, days and night have equal lengths, scientists say. The very name comes from Latin, meaning equal nights. Beyond this date, nights begin to get longer for Europe, most of Asia, Japan, and North America, while the days get shorter.

The exact opposite happens for a large swath of Africa, Australia, some parts of Asia, New Zealand, South America and the entire continent of Antarctica. In the Southern Hemisphere, days begin to get longer than nights starting September 23, Space reports.

Equinoxes occur yearly on September 22 or 23 (autumn) and March 20 or 21 (spring). Other significant moments are the two solstices, in June and December, marking the longest and the shortest days of the year, respectively.

Even though we marked the arrival of autumn today, days and nights will not appear to be of equal length for another few days. This is due to an optical illusion caused by our planet's atmosphere, which tends to refract sunlight in a way that makes the star appear to be rising earlier, and setting later.

However, this is not actually happening, as any scientific measurement can indicate. Interestingly enough, the equinox itself is not a day proper, but rather a very brief moment in time when the Sun moves past Earth's cosmic equator.

This threshold is made up of projections rising upwards from our planet's geographical equator. When solstices occur, the Sun appears to be in the northernmost or southernmost extremes from the equator, respectively.

The two equinoxes and two solstices do not occur the same day every year because most of the world uses Gregorian calendars, which holds that a year has 365 days. In reality, Earth orbits the Sun once ever 365.25 days, which is why we have an extra day in February every four years.