Star Time: Sidereal vs Solar Timekeeping

Do you know the difference between solar time and sidereal time? The clocks we use in day-to-day life are based on solar time, which is measured with respect to the rotation of the earth/ sun's apparent motion across the sky. But when astronomers don't want to worry about just where in its orbit the earth is, they use sidereal time to figure out where to point their telescopes. 'Sidereal' comes from the Latin 'sidus', which means 'star'. Instead of the sun, sidereal time tracks the Earth's rotation relative to the stars, and is more accurate.

This clock shows both solar and sidereal time.

A solar day is 24 hours, the time it takes the sun to reappear in its highest point in the sky. However, in 24 hours the Earth has gone a bit past a full rotation around its axis. SInce the Earth is also moving along an orbit as well as spinning, it it only takes 23 hours, 56 minutes and 4.1 seconds for a point on the earth to be directly facing the sun again. So with a ~23.56 sidereal day, the stars are always in the same place in the sky at the same time each day.

You can see this by noticing that star rise about 4 minutes earlier every night, in standard (solar) time. With sidereal time, however, a star or astronomical feature will be in the same spot each night from a specific point on the earth, and scientists can point their instruments to the exact coordinates they need. After one year, 366 sidereal days have passed, one more than solar days.

Position 1 = noon.Position 2 = one sidereal day later (earth has moved along in its orbit).Position 3 = noon (additional 4 minutes of rotation).

In the first moment of spring in the northern hemisphere, the place where the sun's position in the sky is called the vernal equinox. Sidereal noon is when the vernal equinox is directly overhead. Specifically, sidereal time is the angle from the observer's meridian to the circle that passes through the vernal equinox and both celestial poles.

However, even the sidereal day is not entirely accurate, due to the vernal (or March) equinox precessing slowly westward over a 26,000-year period relative to the fixed stars. A true sidereal period is 0.0084 seconds longer than the common sidereal day, and is measured as the stellar angle, or the Earth Rotation Angle, 360° of which makes a full Earth rotation.