Why does Sidereal time in the American Ephemeris reset to 00:00 every Autumnal Equinox?

By Emily Layng

A sidereal day is 23hr 56’ and 4” seconds, which is how long it takes the Earth to rotate 360° as measured from a fixed star. Our Solar day on Earth is 24:00 hrs because as the Earth rotates, it is also orbiting the Sun, so it has to go a bit further than 360° to “catch up” to where it was when measured at the beginning of its rotation.

] That difference is roughly 3' 56" each day, which accumulates over the course of a year until it equals 24 hours, and then resets to 00:00.

Because Sidereal time isn’t associated with our annual calendar, astronomers needed a starting point, basically just to keep the number down to something relatable. By convention, that starting point is tied to the equinox, which is the intersection of the celestial equator and the ecliptic. This keeps sidereal numbers within reason and in the American Ephemeris, this reset  results in the annual return of listed sidereal time to 00:00 on or around September 20th. Like our calendar year, there is no reason astronomically; just as our year doesn’t suddenly "start over" at midnight on January 1. By convention, we’ve agreed that this is where we begin counting the new cycle.

Sidereal time is based on the Earth's orientation to a fixed star while Solar time is based on Earth’s rotation and orbit around the Sun.