2021 fall equinox: 8 things to know about the start of fall

The fall equinox is upon us: on Wednesday, September 22, the northern and southern hemispheres will experience an equal amount of daylight.

For those in the northern hemisphere, this marks the start of the astronomical fall, with daylight hours continuing to shorten until the winter solstice in December. For those south of the equator, it’s the start of spring. This week’s full moon – closest to the fall equinox and this year the last full moon of summer – occurs on Tuesday and is known as the harvest moon.

Technically speaking, the equinox occurs when the sun is directly aligned with the equator. This will happen at 3:21 p.m. EST on Wednesday (for some parts of the world it will be Thursday morning at this time).

Below is a little science guide to the season’s most even day and night.

1) Why do we have equinoxes?

The fall and spring equinoxes, the seasons, and the varying length of daylight hours throughout the year are all due to one fact: the Earth rotates on an inclined axis.

The tilt – likely caused by a massive object hitting Earth billions of years ago – means that for half of the year the North Pole is pointed toward the sun (as in the image below) . For the other half of the year, the South Pole receives more light. This is why we have seasons.

Nasa

Here is a time-lapse demonstration of the phenomenon filmed over the course of an entire year from space. In the video, you can see how the line separating day from night (called the terminator) oscillates between the poles throughout the year.

NASA / Meteosat / Robert Simmon

A fun fact about the terminator: it is not the case that it divides the Earth into two halves of light and dark. This is because our atmosphere bends the sunlight a bit, essentially extending it over a slightly larger area. This “causes the land covered by sunlight to have a larger area than the land covered by darkness,” says the National Oceanic and Atmospheric Administration (NOAA).

So hold on to this simple, sweet thought: there is always a little more daylight than darkness on Earth, as a whole.

And here’s yet another cool way to visualize the seasons. In 2013, a resident of Alberta, Canada took this pinhole photograph of the sun’s path throughout the year and shared it with the EarthSky astronomy website. You can see the dramatic change in the sun’s arc from December to June.

(You can easily create a similar picture at home. All you need is a can, photo paper, tape, and a pin. Instructions here.)

2) Is the equinox really the first day of autumn?

Well it depends: do you ask a meteorologist or an astronomer?

From a meteorological point of view, summer is defined as the three hottest months of the year, winter the three coldest months and–between the months are spring and fall.

Here’s how NOAA breaks it down:

The meteorological spring includes March, April and May; the meteorological summer includes June, July and August; the meteorological autumn includes September, October and November; and the meteorological winter includes December, January and February.

Astronomically speaking, yes, fall begins when the southern hemisphere begins to receive more sunlight than the northern hemisphere. And it begins at the fall equinox.

3) How many hours of day will I have at the equinox?

Equinox literally means “equal night”. And during the equinox, most places on Earth will see about 12 hours of daylight and 12 hours of darkness.

But not all places will experience exactly the same amount of daylight. For example, Monday, Fairbanks, Alaska will see 12 hours and 14 minutes of daylight. Key West, Florida will see 12 hours and seven minutes. The differences are due to the way sunlight is refracted (bent) as it enters Earth’s atmosphere at different latitudes.

You may also notice that these two locations have daylight hours greater than 12 hours. Aren’t day and night meant to be equal? The time of day is slightly longer than the night on the equinox because of how we measure the length of a day: from the first sign of the sun rising on the horizon in the morning to the very last glimpse. before it falls below the horizon in the evening. Because the sun takes a while to rise and set, it adds a few more minutes of daylight.

Check out TimeAndDate.com to see how many hours of sunshine you will have during the equinox.

4) Can I really balance an egg on its tip during the equinox?

AFP / Getty Images

You might have been told when you were a kid that on the equinox it’s easier to balance an egg vertically on a flat surface than on other days of the year.

The practice originated in China as a tradition on the first day of spring in the Chinese lunar calendar in early February. According to the South China Morning Post, “The theory is that at this time of year the moon and earth are in exactly the right alignment, with the celestial bodies generating the perfect balance of forces necessary to make this possible.

It is a myth. The amount of sunlight we receive during the day has no power over Earth’s gravitational pull or our ability to balance things out. You can balance an egg at its end any day of the year (if you are good at balancing things).

5) When do the leaves start to change color?

Karen Bleier / AFP / Getty Images

As the days start to get shorter, deciduous (green leafed) trees begin to signal their leaves to stop producing chlorophyll, the green pigment responsible for leaf color and photosynthesis.

Since the color change is more dependent on light than temperature, it occurs at virtually the same time from year to year, according to the United States National Arboretum.

Temperature and weather conditions, however, can have an impact on the intensity of fall colors and their shelf life. They can also subtly affect when the leaves start to change. And drought can change the speed at which leaves turn.

Due to all the variables involved, it can be difficult to accurately predict when fall colors will peak and how long they will last in a particular area. But here’s an admirable effort: The SmokyMountains.com website (a site promoting tourism in the Smoky Mountains) created this interactive map (click the link to play with) to determine the maximum fall colors across the mountains. United States by County.

6) What is really in “pumpkin spice”?

“Pumpkin Spice” is not a single spice, but a mixture of them. And it doesn’t contain a pumpkin.

This recipe from Epicurious includes cinnamon, ginger, allspice, nutmeg and cloves. It’s fall – go ahead and sprinkle whatever you want with it.

7) Is there an ancient monument that does something cool during the equinox?

During the winter and summer solstices, crowds flock to Stonehenge in the UK. During the solstices, the sun rises or sets depending on the layout of the 5,000-year-old monument. And while some also flock to Stonehenge for the fall equinox, the real place to go is Mexico.

Indeed, at the equinox, the pyramid of Chichen Itza, in the Yucatan peninsula, offers a marvelous spectacle. Built by the Mayans around 1,000 years ago, the pyramid is designed to cast a shadow over the equinox depicting the body of Kukulkan, a feathered serpent god. A serpent-headed statue is located at the bottom of the pyramid, and when the sun sets on the day of the equinox, the sunlight and shadow show the body of the serpent joining the head.

It’s easier to see in a video. Find out below.

8) Are there equinoxes on other planets?

Yes! All the planets in our solar system rotate on an inclined axis and therefore have seasons. Some of these tilts are minor (like Mercury, which is tilted 2.11 degrees). But others are more Earth-like (23.5 degrees) or even more extreme (Uranus is tilted 98 degrees!).

Below, see a beautiful composite image of Saturn on its equinox captured by the Cassini spacecraft (RIP) in 2009. The gas giant is tilted 27 degrees to the sun, and the equinoxes on the planet are less frequent than on the planet. Earth. Saturn only sees an equinox about once every 15 years (because it takes 29 years for Saturn to orbit the sun).

Cassini / NASA Imaging Team

During Saturn’s equinox, its rings become unusually dark. This is because these rings are only about 9 meters thick and when the light hits them full force, there is not much surface to reflect.