The fall equinox (also known as the autumnal equinox) is one of two times in the year when the Sun is over the equator. The equator is an imaginary line that divides the Earth at the center, like cutting a colossal ball in half. If you visited the equator, you wouldn't see a line on the Earth, though it would be pretty warm there! For the Northern Hemisphere (the part of the Earth to the North of the equator), the fall equinox is around September 22nd or 23rd, but for the Southern Hemisphere (the part of the Earth to the South of the equator), the equinox is on March 20th or 21st.
Why Does the Fall Equinox End up on Different Days?
The fall equinox happens at about the same time every year, but the days seem to change because of how we tell time. The calendar we use is called a Gregorian Calendar, and it has 365 days in a typical year and 366 days during a leap year. Because the Earth takes just over 365 days to orbit the Sun, we have a leap year every four years or so to adjust the calendar again.
Because the Earth’s axis has a bit of a “wobble” to it, the solstice may also fall sooner or later than the year before, but since this all requires a lot of precise measuring to track, we celebrate the solstice on the closest day to the event. While scientists can follow precisely when the Sun hits the Earth's two hemispheres equally, celebrating the fall equinox doesn't have to be precisely timed with the Sun; otherwise, we'd stay awake for a full day celebrating the event.
What Does the Fall Equinox Look Like in Space?
First, imagine the Earth in space. The Earth rotates on its axis counterclockwise. During the equinox, the Earth is turned, so the equator is lined up with the Sun, meaning that both the Northern and Southern Hemispheres get the same amount of light.
Think about it like this: if you had a soccer ball and put a ribbon around the exact middle of it, that would be the Earth. Next, if you took a flashlight and shone it at the ball, you would get different shadows depending on where you aimed at the soccer ball. This is because the equinox happens when you shine the flashlight directly at the equator. Because of how the Earth turns, the amount of daylight and nighttime the Earth experiences is precisely even on the fall equinox. Cool, right?
Earth isn’t the only planet that has an equinox. Saturn is another planet in our solar system that experiences equinoxes, but since Saturn takes 30 Earth years to orbit the Sun, its equinoxes happen every 15 years. Wouldn't it be weird if fall only happened once every 15 years? You might only get to see fall five or six times in your entire life if it took 15 years for the equinox to come around!
Is the Fall Equinox the First Day of Fall?
Technically speaking, yes, the fall equinox is the first day of fall in the Northern Hemisphere. However, what day fall starts as far as weather conditions are concerned depends on location. Most meteorological (meat-ee-ore-oh-loj-ick-al) standards start the fall season on the 1st of September, though this varies depending on where you are.
Because the Earth is tilted, fall in the Southern Hemisphere starts when it's spring in the Northern Hemisphere. So that means that the Southern Hemisphere is just heading into fall during the spring equinox in March for the Northern Hemisphere. Cool, right?
Besides tracking the weather, there are plenty of other ways to determine if fall is on the way. By watching for signs in the natural world, you can tell if it is almost fall. These are called phenological (fen-oh-loj-ick-al) indicators. You've probably seen some phenological indicators of the seasons changing before like flowers blooming or birds singing. For fall, signs like leaves falling off trees, birds migrating for the season, and animals storing food for the winter can all be signs that fall is on the way.
What is the Harvest Moon?
The harvest moon is the first full moon closest to the equinox. However, there isn't always a full moon on the equinox because of how our calendar works and how the moon rotates. That means that the closest full moon to the equinox gets called the harvest moon, as many people harvested their crops in the fall as it got colder.
While there's nothing special about how the harvest moon looks, people used to think that the full moon's light near harvest time let farmers bring in more produce since it was bright out longer. While this may or may not be true, it’s a cool idea.
We're pretty lucky here on Earth to be able to experience 4 seasons each year! Which season is your favorite?
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