Learn when the autumn equinox happens and what to expect as we bid farewell to summer.
Well hello, fall! Even though it happens year after year, the arrival of autumn is always a little surprising. Almost as if on a switch, one day late in the summer you feel it – a subtle crispness in the air. And before you know it, it’s pumpkin-spice-everything everywhere. We are suddenly swathed in sweaters and wearing boots and bombarded by shades of orange, often even before the thermometer warrants it. After slogging through a long hot August, it's exciting.
We can thank the autumnal equinox for this shift from sultry summer to cozy fall. And while most of us are aware of when the first day of autumn lands on the calendar, there’s more to the equinox than meets the eye. Consider the following.
1. When is the 2019 equinox?This year, the autumnal equinox arrives precisely at 07:50 UTC (3:50 a.m. EDT) on Monday, September 23. Unlike an event such as New Year’s midnight that follows the clock around the time zones, equinoxes happen at the same moment everywhere.
2. It's fall, it's spring!There are two equinoxes annually, vernal and autumnal, marking the beginning of spring and fall. They are opposite for the northern and southern hemispheres – so for those of you in the south, happy spring!
3. It's all about the celestial equatorThe autumnal equinox happens the moment the sun crosses the celestial equator, which is an imaginary line in the sky that corresponds to Earth’s equator. (Old Farmer's Almanac describes it as a plane of Earth’s equator projected out onto the sphere.) Every year this occurs on September 22, 23, or 24 in the northern hemisphere.
4. The leap year plays a partBecause it takes the Earth around 365.25 days to orbit the Sun – and why we have a leap year every 4 years – the precise time of the equinoxes varies from year to year, usually happening around six hours later on successive years. On leap years, the date jumps back an entire day.
5. It gives us longer nightsFrom hereon, nights are longer than days and days continue to get shorter until December, when the light will begin its slow climb back to long summer days. Winter solstice is technically the shortest day of the year, while the summer solstice in June boasts the most sunlight.
6. The meaning of "equinox"“Equinox” comes from the Latin words “equi” meaning “equal” and “nox” meaning “night.” This implies that there will be equal amounts of daylight and darkness, however such is not precisely the case.
7. The equinox is not exactly equalThis year, the sun will rise at 6:44 a.m. EDT on the equinox and will set at 6:51 p.m., giving us ~7 minutes of day over night. Although the sun is perfectly over the equator, we mark sunrises and sunsets at the first and last minute the tip of the disk appears. Also, because of atmospheric refraction, light is bent which makes it appear like the sun is rising or setting earlier.
8. What is the equilux?Despite the equinox's name, equal day and night doesn't happen until sunrise and sunset occur precisely 12 hours apart, which depends on a location's latitude; the closer to the equator, the closer it is to the equinox. This day is known as the equilux – from the Latin “equi” for “equal” and “lux” for “light.”
9. The sun signs play along as wellFor the astrology-minded, the morning of the autumnal equinox is when the sun departs Virgo and enters Libra; the scales, how appropriate! According to astrologists, this is a good time for balance and harmony.
10. It determines the Harvest MoonAs for the other celestial orb we obsess on, the full moon nearest to the autumnal equinox is called the Harvest Moon for the luminosity that affords farmers the ability to work late. It's also been called the Full Corn Moon (see: Full moon names and what they mean). The Harvest Moon is usually associated with the September full moon, although if the October full moon happens to fall closer to the date, she takes the title. This year's Harvest Moon happened on Friday, September 13.
11. The Northern LIghts will be extra visibleWith more nighttime darkness, there is simply more hours for viewing; if you are close to the Arctic Circle in the summer, there is too much daylight. But the aurora is also stronger around the equinox because of the planet's 23.5° tilt and the magnetic field of the solar wind ... a magical combination that
12. It's the perfect time to get your bearingsThis year on the equinox, as happens every year, the sun will rise precisely due East and will set precisely due West. Everywhere on Earth, except at the North and South Poles, there is a due east and due west point on the horizon; by observing the sun as it travels along this path on September 23, no matter where you are, you can see where that point is for your location. Pick a landmark, make a mental note, and enjoy the knowledge that while so much in this world is in flux, the sun is constant and will return to its perfect East and West on the days of equinox.
Updated September 21, 2019.