Astronomers determine sidereal time by measuring the position of the Earth relative to the stars, rather than the Sun. Here’s a breakdown of how it works:
- Earth’s Rotation: The Earth rotates once every 24 hours with respect to the Sun, but because the Earth is also orbiting around the Sun, it takes a little more than 24 hours for the same point on Earth to align with the Sun again. This is solar time. However, when measuring time with respect to the stars (which are very far away), the Earth takes approximately 23 hours, 56 minutes, and 4 seconds to complete one full rotation relative to the distant stars.
- Sidereal Day: This 23 hours, 56 minutes, and 4 seconds is the length of a sidereal day, the time it takes for the Earth to complete one full rotation relative to the stars. This is a sidereal time period, as opposed to solar time.
- Sidereal Time Calculation: Sidereal time is calculated based on the position of the vernal equinox (a fixed point in the sky) at a given moment. The position of the vernal equinox moves due to the Earth’s axial precession, but astronomers can track its position precisely. Sidereal time at any location on Earth can be determined by observing where the stars are positioned in the sky relative to a known reference point, such as the vernal equinox.
- Greenwich Sidereal Time (GST): Sidereal time is often expressed in Greenwich Sidereal Time (GST), which is the sidereal time at the Prime Meridian in Greenwich, England. GST is used as a reference, and astronomers can then adjust it for their specific location by considering the local longitude.
- Using Clocks and Computers: In practice, astronomers use precise atomic clocks and computer models to calculate sidereal time, factoring in the Earth’s rotation and orbital movements. These models take into account the Earth’s complex motion in space and can give sidereal time to very fine accuracy.