Myth busted: The fall equinox doesn’t mean exactly 12 hours of day and 12 hours of night

This year the fall equinox, when the Sun lines up with the equator, will occur for many of Earth’s inhabitants on Monday 22 September. The name of the astronomical event comes from the Latin word equinoxium, which means “equality between day and night.”

However, that isn’t really the case for anywhere on the planet, although it’s pretty close to 12 hours of light and 12 hours of darkness. Here’s a look at why that is.

The not so “equal” equinox

Thanks to the curvature of the Earth and its atmosphere, the sun appears to rise before it is actually above the horizon and similarly stays in the sky even after it as gone below during a sunset. This is due to the rays of light being bent.

Even though the Sun is aligned with the equator and its rays are seen at both the North and the South Poles, those who live at higher latitudes will actually have a longer day than those at the equator, which experiences nearly equal days and nights all year long.

Instead of the roughly 12 hours and 6 minutes of sunlight at the equator, those near the Artic and Antarctic Circles will have around 12 hours and 22 minutes of sunlight on the equinox. This is because it takes the Sun longer to rise and set the nearer to the poles.

However, while those who live near the equator have days that are always the same length of time, those at higher latitudes experience dramatic changes in the amount of light and darkness they get over the course of a year.

In the Northern Hemisphere, eternal darkness will spread south from the pole with each passing day until the Winter Solstice, while the opposite will occur over Antarctica, being bathed in permanent sunlight. Then the effect will reverse culminating in the Summer Solstice.

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