Just 800 years ago, perihelion and the winter solstice aligned. Due to the precession of Earth's orbit, they are slowly drifting apart, completing a full cycle every 21,000 years. 5,000 years from now, the spring equinox and the Earth's closest approach to the Sun will coincide. Note that, at all times, the tilt of the Earth during the equinox is perpendicular to the imaginary line made by connecting the Sun to the Earth.GREG BENSON AT WIKIMEDIA COMMONS

Although equinox literally means “equal night,” it isn’t exactly true that there are 12 hours of day and 12 hours of night everywhere on Earth during the equinox. Nor is the equinox defined by a specific calendar day; rather, it’s a specific moment in time that corresponds to planet Earth passing through a special point on its orbit.

If you were to draw an imaginary line through the Earth, from North Pole to South Pole, you’d find the line that represented Earth’s rotational axis. If you drew a second imaginary line, connecting the center of the Earth to the center of the Sun, you’d see that these two lines made an angle with one another. During the solstices, the Earth’s axis tilts maximally towards or away from the Sun. But during the moment of equinox, you make a right (90°) angle, which means that something very special occurs.