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Fall equinox 2020: What to know about change of seasons
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Fall equinox 2020: What to know about change of seasons

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Twice a year, everyone on Earth is seemingly on equal footing — at least when it comes to the distribution of light and dark.

Tuesday marks the second and final equinox of 2020. If you reside in the Northern Hemisphere, you know it as the fall equinox (or autumnal equinox). For people south of the equator, this equinox signals the coming of spring.

People along the equator have roughly 12-hour days and 12-hour nights all year long. But people close to the poles, in destinations such as the northern parts of Canada, Norway and Russia, go through wild swings in the day/night ratio each year with long, dark winters and summers where night barely intrudes.

During equinoxes, everyone from pole to pole gets a 12/12 split of day and night. But it isn't as perfectly "equal" as you may have thought. More on that below.

Here are the answers to your fall equinox questions:

Where does the word 'equinox' come from?

From our CNN Fast Facts file: The term equinox comes from the Latin word equinoxium, meaning "equality between day and night."

Precisely when does the fall equinox happen?

The equinox arrives at 13:31 UTC (Coordinated Universal Time) Sept. 22. For people in places such as Toronto and Miami, that's 9:31 a.m. local time. In Los Angeles and Vancouver, that means it arrived at 6:31 a.m.

In Madrid, Berlin and Cairo, it comes precisely at 3:31 p.m. Going farther east, Dubai marks the exact event at 5:31 p.m. For residents of Bangkok, it's 8:31 p.m. while Singapore and Hong Kong clock in at 9:31 p.m. You can click here to see more cities (rounded down by one minute).

Why does fall equinox happen?

The Earth rotates along an imaginary line that runs from North Pole to South Pole. It's called the axis, and this rotation is what gives us day and night.

However, the axis tilts at 23.5 degrees, as NASA explains. That positions one hemisphere of the planet to get more sunlight than the other for half of the year's orbit around the sun. This discrepancy in sunlight is what triggers the seasons.

The effect is at its maximum in late June and late December. Those are the solstices, and they have the most extreme differences between day and night, especially near the poles. (That's why it stays light for so long each day during the summer in places such as Scandinavia and Alaska.)

Are the Northern Lights more active at the equinoxes?

Yes, they often put on more of a show this time of year. It turns out the autumnal equinox and spring (or vernal equinox) usually coincide with peak activity with the aurora borealis.

These geomagnetic storms tend to be most active in March and April and then again in September and October, according to 75 years of historical records analyzed by solar physicist David Hathaway of the Marshall Space Flight Center in Huntsville, Alabama.

So why isn't the equinox exactly equal?

It turns out you actually get a little more daylight than darkness on the equinox, depending on where you are on the planet. How does that happen?

As the US National Weather Service explains, the "nearly" equal hours of day and night are because of the complex way a sunrise is measured and the refraction of sunlight in our atmosphere.

This bending of light rays "causes the sun to appear above the horizon when the actual position of the sun is below the horizon." The day is a bit longer at higher latitudes than at the equator because it takes the sun longer to rise and set the closer you get to the poles.

So on fall equinox, the length of day will vary a little depending on where you are. For the truly equal day/night split, you have to wait a day after the official equinox. That day is called the equilux, and it's on Wednesday, Sept. 23, this year.

Here are some fall equinox scenes over the years:


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