This Sunday, the autumnal equinox is upon us, and with it, a shift to shorter days, longer nights and the astronomical start of fall in the Northern Hemisphere.
Equinoxes occur when Earth reaches a point in its orbit where the sun shines directly on the Equator. This happens twice a year, in March and in September, and is a result of Earth’s spinning on an axis that is tilted 23.5 degrees from the plane of its orbit. During an equinox, places around the globe experience nearly equal periods of day and night.
Here are five interesting facts about the fall equinox.
Its timing can change from year to year.
The autumnal equinox always occurs in September, but its date can shift each year. That’s because Earth takes slightly longer than a calendar year — 365 days, 6 hours and 9 minutes, to be exact — to complete one full orbit around the sun.
Last year, the autumnal equinox was on Sept. 23. But this year, according to the National Weather Service, it falls on Sept. 22, at 8:44 a.m. Eastern time.
The astronomical start of autumn differs from its meteorological start.
The ever-changing dates of equinoxes and similar events cause the timing and length of the astronomical seasons to vary, which means they are not useful for scientists analyzing climate trends from year to year.
Instead, meteorologists rely on a separate system that establishes seasons by the months of the calendar. Meteorological fall began on Sept. 1 and will conclude at the end of November.
Astronomical fall, on the other hand, ends at the winter solstice, which this year happens on Dec. 21.
Earth’s ‘twilight zone’ runs exactly north to south during the equinox.
The line dividing day and night on a planet’s surface is called a terminator, and on most days, Earth’s is slanted when viewed from space. But on both of the equinoxes, the terminator aligns straight up and down, running from pole to pole.
Seen from space, this effect is stark. Meteosat-9, a European weather satellite, has in the past captured a snapshot of Earth’s terminator during the vernal equinox, which denotes the astronomical start of spring.
The fall equinox will be the first sunrise in months at the South Pole.
Most people on Earth will experience around 12 hours of day and 12 hours of night during September’s equinox. But scientists and staff members “wintering over” at Antarctic research stations near the South Pole will see the sun peek over the horizon for the first time in six months. That isn’t many people — the Amundsen-Scott South Pole Station houses up to 42 staff members and scientists during the southern winter.
The opposite is true at the North Pole: Its last sunset happens on the autumnal equinox, bathing the site in darkness until the vernal equinox in March.
Other worlds also have equinoxes.
Any planet with an axial tilt will experience an equinox, but how often they occur depends on the planet’s orbital period around the sun.
Jupiter has an equinox every six years, twice in its nearly 12-year orbit.
On Saturn, an equinox occurs once every 15 years or so, during which its rings orient edge-on to our line of sight from Earth and appear to vanish. The last time this happened, in 2009, NASA’s Cassini orbiter used the opportunity to study the planet’s rings in low light.
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