Fall Equinox Comes As Gas Giants Jupiter, Saturn Pair up in the Sky Before Harvest Moon

Today marks the fall equinox in the Northern Hemisphere and the spring equinox in the Southern Hemisphere—an astronomical event that, this year, coincides with gas giants Jupiter and Saturn pairing up in the night sky.

Just like summer and winter solstices, equinoxes occur as a result of the fact that Earth's axis is tilted at an angle of around 23.4 degrees relative to the plane of its orbit around the sun.

Solstices refer to the moments in time when Earth's poles are tilted directly towards or away from the sun. When the North Pole is tilted away from the sun, the Northern Hemisphere enters winter—in astronomical terms—while summer in the Southern Hemisphere begins.

When the North Pole is tilted toward our star, the opposite is true: the Northern Hemisphere enters summer and winter begins in the Southern Hemisphere.

The March and September equinoxes, on the other hand, occur in between the two solstices at the two points in the year when Earth's axis is tilted neither towards or away from the sun.

At these moments, the sun shines directly over Earth's equator, and the length of day and night is roughly equal.

In the Northern Hemisphere, the September equinox marks the beginning of fall in astronomical terms, while in the Southern Hemisphere, it heralds the start of spring.

Meteorologists define the start of the seasons using a different system based on average temperatures rather than the location of Earth in its orbit around the sun. By this meteorological definition, fall in the autumn hemisphere begins earlier on September 1 and runs until November 30.

The September equinox occurs anywhere between September 21 and 24 every year. The reason it doesn't always fall on the same date is because there is a slight discrepancy between the Gregorian calendar—the most widely used calendar system in the world—and the "solar year."

The solar year is the length of time it takes for Earth to complete one revolution around the sun—which is 365 days, 5 hours, 48 minutes and 45 seconds. The Gregorian calendar, meanwhile, simply defines the year as 365 days long, meaning it does not synchronize exactly with the solar year.

This year, the fall equinox—which occurs at 9:31 a.m. ET—comes in a week when the gas giants Jupiter and Saturn are both visible in the night sky, almost level with each other, to the west of the constellation Sagittarius, Sky & Telescope reported.

fall equinox
Sunset over lower Manhattan and One World Trade Center on the fall equinox in New York City on September 22, 2017, as seen from Jersey City, New Jersey. Gary Hershorn/Getty Images

For the best views, use binoculars or a telescope and look towards the south at dusk. Over the course of the night the planets will shift to the right, before setting in the southwest around midnight or 1 a.m.

Over the next few months the two planets, which on Tuesday will be just under 8 degrees apart in the sky, will move closer to each other, until they appear to meet on December 21—an event that only takes place once every 20 years.

The fall equinox also comes just over a week before a full moon graces the skies on October 1, according to The Old Farmer's Almanac.

In North America, the full moon that occurs closest to the fall equinox is usually named "the Harvest Moon"—a moniker that originated with Native American tribes. Normally this name is given to the full moon in September. However, it can also be used to describe an October full moon in cases where a full moon occurs particularly early in September.