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September Equinox: 10 Facts About the First Day of Fall

On September 22/23, 2017, the day and night will be almost equal in most locations. Here are 10 facts about the first day of astronomical fall (autumn) in the Northern Hemisphere.

A pathway in a park during fall (autumn).

September is North's fall equinox.

September equinox is the vernal equinox in the Southern Hemisphere, where it marks the beginning of spring.

©iStockphoto.com/Givaga

1. Second Equinox of the Year

The September equinox is on or around September 22, while the first equinox of the year, the March Equinox, takes place on or around March 21 every year.

2. Say Goodbye to Summer...

...in the Northern Hemisphere. Astronomically, the September equinox is the autumnal, or fall, equinox marking the end of summer and the beginning of fall (autumn). The fall season ends on December Solstice, when astronomical winter begins.

For meteorologists, on the other hand, fall in the Northern Hemisphere begins about 3 weeks before the September equinox on September 1 and ends on November 30.

Green plant sprouting through snow.
Spring is the season of new beginnings.
Spring is the season of new beginnings.
©iStockphoto.com/marlenka

3. Also Welcomes Spring

In the Southern Hemisphere, the September equinoxit is the vernal (spring) equinox.

The September equinox is also known as the vernal or spring equinox in the Southern Hemisphere and is considered by astronomers as the first day of spring there.

4. A Specific Moment in Time

Illustration image
Equinox at Chichen Itza, Mexico.
The starcaises at the main Maya pyramid, El Castillo, at Chichen Itza, Mexico are built at a carefully calculated angle which makes it look like a snake of sunlight slithers down the stairs the moment the equinox occurs.
©bigstockphoto.com/Borna

Equinoxes are not day-long events, even though many choose to celebrate all day. Instead, they occur at the exact moment the Sun crosses the celestial equator – the imaginary line in the sky above Earth’s Equator.

At this instant, Earth's rotational axis is neither tilted away from nor towards the Sun.

In 2017, the Sun's crosses the celestial equator from north to south on September 22, at 20:02 UTC. Because of time zone differences, the equinox will take place on September 23, 2017 at locations that are at least 10 hours ahead UTC. These include cities in eastern Australia, eastern Russia, Guam and New Zealand.

5. The Date Varies

Statue of Pope Gregory XIII Bologna, Italy.
Statue of Pope Gregory XIII Bologna, Italy.
Statue of Pope Gregory XIII, who introduced our modern-day calendar.
©iStockphoto.com/ppl58

While the September equinox usually occurs on September 22 or 23, it can very rarely fall on September 21 or September 24. A September 21 equinox has not happened since 1000 CE.

However, in the 21st century, it will happen twice – in 2092 and 2096. The last September 24 equinox occurred in 1931, the next one will take place in 2303.

The equinox dates vary because of the difference between how the Gregorian calendar defines a year (365 days) and the time it actually takes for Earth to complete its orbit around the Sun (about 365 and 1/4 days).

This means that each September equinox occurs about 6 hours later than the previous year's September Equinox. This eventually moves the date by a day.

Note: These dates are based on the time of the equinox in UTC. Due to time zone differences, locations ahead of UTC may celebrate the September equinox a day later and locations behind UTC may celebrate it a day earlier.

6. Equal Day and Night...

Day and night landscape in Hong Kong, China.
Most locations on Earth do not experience equal day and night on September equinox.
Most locations on Earth do not experience equal day and night on September equinox.
©iStockphoto.com/YiuCheung

The term equinox comes from the Latin words aequus, meaning equal and nox, meaning night. This has led to the common misconseption that everybody on Earth experiences equal day and night – 12 hours of daylight and 12 hours of night time – on the day of the September equinox.

7. ...But Not Quite

In reality, most places on Earth enjoy more than 12 hours of daylight on this day. This is because of 2 reasons: the way sunrise and sunset are defined and atmospheric refraction of sunlight.

Locations that are not on the equator do get to experience equal day and night twice a year, usually a few days before or after the equinoxes. The dates for this event, which is also known as equilux, depend on a location's latitude – those closer to the Equator celebrate it a few days before the equinox.

8. Shorter Time Between Moonrises

Harvest Moon is the Northern Hemisphere's first fall (autumn) full Moon.
The Harvest Moon is also sometimes called Corn Moon, after the corn harvest in the months of fall.
The Harvest Moon is also sometimes called Corn Moon, after the corn harvest in the months of fall.
©iStockphoto.com/klagyivik

The full Moon closest to the September equinox, the Harvest Moon, is astronomically special. This is because the time between 1 moonrise to another around this period becomes shorter.

On average, the Moon rises about 50 minutes later every day in a lunar month – the time period between 2 full Moons or 2 new Moons. Around the Harvest Moon, the time difference between 2 successive moonrises decreases to about 30-40 minutes for a few days.

Known as the Harvest Moon Effect, this phenomenon occurs due to the low angle the Moon's orbit around Earth makes with the horizon during this time of year.

The reverse effect occurs in the Southern Hemisphere, where the Moon rises more than 50 minutes later than on the previous day.

In the old days, the early moonrise for a few days around the equinox in the Northern Hemisphere meant that farmers could work and harvest their crops for a longer time in the evenings.

Colorful northern lights in Alberta, Canada.
The Aurora Borealis can come in many colors.
The Aurora Borealis can come in many colors.
©iStockphoto.com/Nata-K

9. Prepare for Northern Lights

As the September equinox rolls by, the chances to see the aurora borealis display increases for those located at high Northern Hemisphere latitudes. According to NASA, the equinoxes are prime time for Northern Lights – geomagnetic activities are twice more likely to take place in the spring and fall time, than in the summer or winter.

10. Celebrated Around the World

Many cultures around the world hold feasts and celebrate festivals and holidays to mark the September equinox.

Topics: Astronomy, Seasons, September, Equinox, Sun

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Vernal & Autumnal Equinox

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  2. Autumnal (Fall) Equinox
  3. March Equinox
  4. 10 Facts: March Equinox
  5. March Equinox Celebrations
  6. September Equinox
  7. 10 Facts: September Equinox
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