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. This year, the autumnal equinox arrives precisely at 10:21 a.m. (EDT) on Thursday, September 22. Unlike an event like New Year’s midnight that follows the clock around the time zones, equinoxes happen at the same moment everywhere.
2. There are two equinoxes annually, vernal and autumnal, marking the beginning of spring and fall. They are opposite for the northern and southern hemispheres.
3. The 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. From hereon, the days get shorter until the winter solstice in 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. Hence, the four season, as illustrated below.
photo credit:Wikimedia Commons/CC BY 2.05. Because 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.
6. “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 exactly the case.
7. This year, the sun will rise at 6:44 a.m. EDT on the equinox and will set at 6:52 p.m., giving us 8 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. Exactly equal day and night won’t happen until September 25 with sunrise as 6:47 a.m. EDT and sunset at 6:47 p.m. This day is known as the equilux! ("Lux" being Latin for light, isn't that pretty?)
9. For the astrology-minded, the morning of the autumnal equinox is when the sun enters Libra ... the sign of balanced scales. Equal day and night, balanced scales, seeing a connection here...
11. In China the September equinox is celebrated during the Mid-Autumn Festival, also known as the Moon Festival. The bounty of summer’s harvest is celebrated and the festivities are rampant with moon cakes, round pastries made from bean paste and other sweet and/or savory ingredients.
credit photo:Wee Keat Chin/Flickr/CC BY 2.012. This 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 22, no matter where you are, you can see where that point it 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.
This updated article for 2016 was originally published in 2015.