Around March 20-21 every year, we experience something called the vernal equinox — “spring’s equal night.” Night feels equal to day at this time of year because we are halfway between the longest night, which occurs at the winter solstice, and the longest day, which occurs at the summer solstice.

On a perfect Earth, night and day would indeed be equal at the equinox. But as human calendar makers and timekeepers have learned over the centuries, the Earth refuses to be perfect by our definitions. And that’s the problem: our definitions. They cause day to last longer than night on the equinox.

We define the equinox by what the center of the sun is doing and day and night by what the top of the sun is doing. On the equinox, the center of the sun is above the horizon for 12 hours everywhere on Earth. But because day starts when the top of the sun appears above the horizon, we start counting day a few minutes early.

And day doesn’t end until the top of the sun disappears, so it gets a few extra minutes at the other end too. Therefore the night of the equinox is doubly shortchanged. It comes closest to being equal a few days before the vernal equinox — March 16 or 17 where I live at a latitude of about 45 degrees north.

As if being shortchanged by our human definitions of sunrise and sunset weren’t enough, the night of the equinox loses yet a few more minutes to something called refraction. The Earth’s atmosphere bends the sun’s rays in such a way that it appears to be above the horizon before and after it actually is. At my latitude the total loss is nine or ten minutes.

But the exact lengths of day and night as defined by human beings are less important than what the Earth and sun themselves are doing. At the moment of the vernal equinox, the Earth is at a place in its orbit where it tilts neither toward nor away from the sun. But after the equinox the Northern Hemisphere begins to point ever so slightly back toward the sun.

The sun rises earlier and earlier, travels higher across the sky, and sets later each day. The result is yet longer days and shorter nights, resulting in more sunlight and warming temperatures, all of which combine to accelerate the season we define as spring.