Today is the first day of spring in the northern half of the world. If you live in Australia, it’s the first day of autumn—the vernal equinox in the northern hemisphere and the autumnal equinox in the southern hemisphere. Today, the sun shines directly on the equator, and the hours of daylight and nighttime are nearly equal around the world.

New seasons are important for many people. Some count down the day because of what they hope the new season will bring. Perhaps you’ve been marking off a calendar for spring in Wisconsin to signal the end of another winter. Or…