If you've already gone back to school, had your first Pumpkin Spice Latte or just noticed a slight nip in the air, you're probably well aware that it's beginning to look a lot like fall. And you'd be right.

The fall season officially began at precisely 10:29 p.m. EDT on Monday, Sept. 22 with the autumnal equinox. That would make today, Tuesday, Sept. 23 the first day of autumn in the northern hemisphere (spring now begins in the southern hemisphere). But other than the obvious drop in temperature and hours of sunlight dwindling down during the day, what's this autumnal equinox all about, and does it actually matter? Sit back, grab a sweater and read on to find out.

What happens during the autumnal equinox?

The autumnal equinox is considered the beginning of fall, astronomically speaking. Meteorologists actually deem Sept. 1 the beginning of autumn, and the general public probably considers the day after Labor Day the start of the new season. The word "equinox" comes from the Latin meaning "equal night." During an equinox, the sun is at its zenith, or its highest point in the sky, which means neither of the Earth's hemispheres is tilted toward the sun. This is said to cause equal amounts of daylight and nighttime everywhere on Earth.

The truth about equinoxes

Though it is a commonly held belief that equinoxes create exactly 12 hours of daytime and 12 hours of nighttime, this actually isn't the case. Daytime is defined as when the sun is above the horizon, and nighttime is when it's below the horizon. However, atmospheric refraction raises the sun's disc, which allows us to see the sun even when it's below the horizon. Since the time of sunrise and sunset are based on when the first bit of the top of the sun is seen or disappears below the horizon, respectively, instead of the center of the sun, this causes the duration of sunlight and nighttime to be close but not entirely equal. 

Equinoxes don't just occur in September

Mark your calendars, folks, because the northern hemisphere will get to experience another equinox in six months. The spring equinox, also known as the vernal equinox, occurs every year in March, and it's scheduled for March 20, 2015. The same thing happens during this day as during the autumnal equinox, giving the earth nearly equal hours of day and night. Since vernal comes from the Latin for "spring," this marks the astronomical beginning of the spring season.

When do the other seasons begin?

You probably know that there are official start dates for winter and summer, too. However, these astronomical events aren't called "equinoxes" because the amount of daylight and darkness on these days is simply not equal. Instead, we refer to these events as the winter and summer solstice, respectively. The winter solstice will occur on Dec. 21, 2014 in the northern hemisphere when the sun is at its southernmost point on the horizon. The earth's axis is tilted so that the planet is the furthest distance away from the sun, which causes the shortest day of the year. Last year, the longest day of the year lasted nine hours and 32 minutes during the winter solstice. The opposite will occur during the summer solstice, which will take place on June 21, 2015, causing the longest day of the year in the northern hemisphere. Since the equinoxes fall roughly between the two solstices, they became known as the start of the other two seasons, fall and spring.

Equinoxes are actually something to celebrate

Aside from the astrological changes taking place during the autumnal equinox, it may just seem like a regular day for many of us. However, it is a day for celebration for many followers of ancient faiths. Modern-day Druids celebrate the equinoxes and solstices at Stonehenge with music, poetry and dance. Pagans also celebrate the autumnal equinox with Mabon, a harvest festival. The ancient Mayans were also said to have built the famous stone pyramid known as "El Castillo" in Chichen-Itza, Mexico to celebrate the equinoxes, intending the shadows cast by the sun to make the steps of the pyramid look like a slithering snake.

Even if you don't follow any of these religions, the official start of fall is something to be excited about. Today's Google Doodle even celebrates the first day of autumn with some colorful foliage filling trees. Now that fall is officially here, apple cider, Halloween and Thanksgiving will all follow shortly after. All of that alone deserves a celebration.