# Local Sidereal Time – Definition & Detailed Explanation – Astronomical Units & Measurements Glossary

## I. What is Local Sidereal Time?

Local Sidereal Time (LST) is a measure of the hour angle of the vernal equinox on a specific location on Earth. It is a timekeeping system that is based on the rotation of the Earth with respect to the stars, rather than the rotation of the Earth with respect to the Sun like our standard timekeeping system. LST is used in astronomy to determine the position of celestial objects in the sky at a given time and location.

## II. How is Local Sidereal Time Calculated?

Local Sidereal Time is calculated using the formula:

LST = GMST + Longitude

Where GMST is the Greenwich Mean Sidereal Time, which is the hour angle of the vernal equinox at the prime meridian (0 degrees longitude). The longitude is the observer’s east longitude, measured in degrees. By adding the observer’s longitude to the GMST, we can calculate the LST for a specific location on Earth.

## III. Why is Local Sidereal Time Important in Astronomy?

Local Sidereal Time is important in astronomy because it allows astronomers to accurately locate and track celestial objects in the sky. By knowing the LST at a specific location, astronomers can determine when a particular object will be visible in the sky and where it will be located. This is crucial for observational astronomy, as it helps astronomers plan their observations and make accurate measurements.

## IV. What is the Relationship Between Local Sidereal Time and Right Ascension?

Right Ascension (RA) is a celestial coordinate that is similar to longitude on Earth. It is measured in hours, minutes, and seconds and is used to pinpoint the east-west position of a celestial object in the sky. The relationship between Local Sidereal Time and Right Ascension is that the LST at a specific location is equal to the RA of the celestial meridian that is currently passing through the observer’s location. This means that the LST can be used to determine the RA of a celestial object at a given time and location.

## V. How Does Local Sidereal Time Differ from Universal Time?

Local Sidereal Time differs from Universal Time (UT) in that LST is based on the rotation of the Earth with respect to the stars, while UT is based on the rotation of the Earth with respect to the Sun. UT is the standard timekeeping system used by most people around the world, while LST is primarily used in astronomy. LST is measured in hours, minutes, and seconds, while UT is measured in hours, minutes, and seconds as well, but is adjusted for leap seconds to keep it in sync with the rotation of the Earth.

## VI. How Can Local Sidereal Time be Used in Observational Astronomy?

Local Sidereal Time can be used in observational astronomy to determine when and where celestial objects will be visible in the sky. By knowing the LST at a specific location, astronomers can plan their observations to capture specific objects at the optimal time. LST can also be used to calculate the rise and set times of celestial objects, as well as their culmination times when they are at their highest point in the sky. Overall, LST is a valuable tool for astronomers to accurately track and study celestial objects in the night sky.