As the midday sun begins to sink lower and nights get noticeably longer, it can only mean the reign of summer is coming to an end for the northern half of the world. The autumn equinox arrives at 10:21 a.m. ET (2:20 p.m. UTC) on September 22, officially marking the beginning of fall in the Northern Hemisphere and the start of spring in the Southern Hemisphere.
The word “equinox” comes from Latin and means “equal night,” referring to the roughly 12-hour day and night that occurs only on the two equinox days of the year.
This tidy split in our 24-hour day is linked to the reason Earth has seasons in the first place. The planet spins on an axis that is tilted 23.5 degrees with respect to its orbital plane. That means as Earth travels along its 365-day orbit, different hemispheres tilt closer to or farther from our sun’s warming rays.
An equinox is a geometrical alignment between the sun and Earth in which the sun appears positioned right above our planet’s equator. On these days, both the Northern and Southern Hemispheres experience roughly equal amounts of sunshine. It’s also only on the spring and autumn equinoxes that the sun rises due east and sets due west.
As we head toward December, the Northern Hemisphere will tilt farther away from the sun and receive its rays at a steeper angle, creating longer shadows and cooler conditions indicative of winter. Eventually, the sun will reach its lowest point in the midday sky, marking the December solstice.
Cultures around the world have historically celebrated the dates that represent the changing of the seasons. One notable example is an ancient Maya step pyramid known as El Castillo at Chichén Itzá in Mexico. Exactly at sunset on the spring and autumn equinoxes, sunlight hits the building’s steep staircase at just the right angle to create an eerie snake-like shape that appears to slither along its length.
Other planets also have seasons and equinoxes, although on much more extreme scales. Mars has a very similar tilt to Earth’s and so experiences the same kinds of seasons, but its distance from the sun means that a Martian winter lasts a frigid 154 days.
However, the nightmare planet for anyone with seasonal affective disorder would have to be Uranus. Its axis is tipped nearly 90 degrees, meaning it essentially spins on its side during its 84-year orbit around the sun. This translates to mind-numbing winters that last a whopping 42 years.
For some planets, seasonal variations can even affect our views of these celestial objects. During a Saturn equinox, which rolls around every 15 Earth years, the sun shines edge-on to the planet’s famous rings, casting them in low shadows that can reveal their three-dimensional structure.
Here are some of the other exciting astronomical wonders in store for sky-watchers this week.
Saturn on Display. Saturn will be shining brightly in the low southwestern skies after dusk all week long. Look for the gas giant hovering about 20 degrees above your local horizon on any clear evening. For a beautiful contrast, check out the distinctly fainter star Antares, which will be sitting only six degrees south of Saturn. The brightest star in the constellation Scorpius, Antares has an orange-red hue that will seem to smolder next to Saturn’s creamy yellow color.
Moon in Taurus. Late night on September 20, the waning gibbous moon will be rising in the low eastern horizon, forming a pretty triangular formation with the Hyades and Pleiades star clusters. The V-shaped Hyades, which will be located to the left of the moon, sits 150 light-years from Earth, making it one of the closest such star groupings. The Pleiades, which will be above the moon, is a much younger cluster that lies 440 light-years from Earth. Because the moon shines so brightly, try using binoculars to reveal some of the fainter stellar members in both clusters.
Deneb at Zenith. At the same time that we mark the autumn equinox, there will be a seasonal change in the stars that dominate the overhead evening skies. For observers in mid-northern latitudes like New York and London, look for the bright star Deneb to appear straight overhead, or at the sky’s zenith. Deneb, the most brilliant star in the constellation Cygnus, the swan, is a blue-white giant estimated to be 1,400 light-years away and 110 times as wide as our sun.