Why aren't there exactly 12 hours...

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Why aren’t there exactly 12 hours of daylight and 12 hours of darkness on the spring and fall equinoxes?

On the equinoxes, the very center of the Sun sets just 12 hours after it rose. But the day begins when the upper edge of the Sun reaches the horizon (which happens a bit before the center rises), and it doesn’t end until the entire Sun has set. Not only that, but the Sun is actually visible when it is below the horizon, as Earth’s atmosphere refracts the Sun’s rays and bends them in an arc over the horizon. 

β†’ Learn more about equinoxes and solstices!