For those of us who approach things by the Goldilocks Principle (not too little, not too much), spring equinox is practically perfect. Along with the fall equinox, this is when we get that glorious balance of daytime and nighttime all over the world — from the steamy equator to the frigid poles — with just about 12 hours of each.

Thursday is the first day of spring, or the vernal equinox, in the Northern Hemisphere. During an equinox, the center of the sun is on the same plane as the equator. After the vernal equinox in March, the Northern Hemisphere tilts toward the sun until the June solstice, when the sun begins to migrate back south.

The vernal equinox arrives on Thursday, marking the start of the spring season for the Northern Hemisphere and the fall in the Southern Hemisphere. On the equator, the sun will be directly overhead at noon. Equinoxes are the only time when both the north and south poles are lit by sunshine at the same time.