Remind me: What's the autumnal equinox?

It's officially the first day of fall in the northern hemisphere and Google is helping everyone celebrate. Today marks the autumnal equinox. Right, what is an equinox again?

An equinox occurs when the sun shines directly on the equator. This happens only twice a year, in September and March, and effectively means that the length of day and night are equal.

But contrary to popular belief, the seasons aren't determined by the changing proximity to the sun.

“In the northern winter – in December – the sun is actually closest to the Earth by a small amount, and in the summer it’s actually farther away,” explained Jay Holberg, a senior research scientist at the lunar and planetary lab at the University of Arizona in an interview with National Geographic.

It's actually Earth's tilt that affects the seasons. When the northern hemisphere is tilted towards the sun, we experience summer because sunlight hits the hemisphere directly. On the other hand, the southern hemisphere is then tilted away from the sun, causing the cooler temperatures.

So while the autumnal equinox signals the coming of scarves, pumpkins, and hot drinks in the northern hemisphere, the southern hemisphere is celebrating the vernal or spring equinox and prepping for beach season. That's why this Google Doodle doesn't appear, for example, for folks doing web searches in Australia. 

Humans have long been interested in the sun's movements, building physical monuments to measure and commemorate the giant star.

Many historians believe Stonehenge was built as an astronomical calendar. "The great sandstone monoliths at Stonehenge in southern England are said to mark the autumn and spring equinoxes, as well as the summer and winter solstices, the longest and shortest days of the year respectively," wrote The Christian Science Monitor's Noelle Swan.

The ancient Mayan civilization also built a celestial calendar: a 79-foot stone pyramid known as El Castillo.

"More than 1,000 years after its construction, thousands of people still gather around El Castillo during the equinox to witness a mesmerizing trick of light and shadow. As the sun sets, a series of triangular shadows align in such a way that a diamond-backed snake appears to slither down the stairway of the pyramid," wrote Ms. Swan.

Though you may not have a stone monument to assist you in your celebration, just remember that you won't see equal day and night again until March, so enjoy it while it lasts.