equal night of spring

For those of us in the United States, the vernal equinox arrives the evening of March 20

Twice a year the axial skewer tips are pointing neither toward nor away from the Sun, but instead are positioned exactly off to the side. If you could imagine being on the Sun and looking out toward our imaginary-skewered Earth, it would be like looking at a ball of yarn with a knitting needle stuck through it, in perfect profile. These are the times of the equinox, when the linked geometry of Earth’s rotational and orbital planes together bestow a day of equal parts light and night across the entire globe. And while the equinox is formally calculated based on the moment when Earth first enters its profile position, the Sun is so comparatively huge that it takes us time to pass any point of it, and equinoctial conditions will effectively persist for several days.