Sidereal Time, Civil Time and Solar TimeSidereal time is a method astronomers use to locate objects. Using sidereal time its possible to point a telescope to an object in the sky.
This article is part of a series of articles. Please use the links below to navigate between the articles.
- Astronomy for Beginners - Complete Guide
- What are Right Ascension (RA) and Declination (Dec)?
- What is Angular Size in Astronomy?
- Sidereal Time, Civil Time and Solar Time
- Magnitude Scale and Distance Modulus in Astronomy
- What Are The Equinoxes and Solstices About?
- How Do We Measure Distance in Space Using Parallax and Parsecs
- Brightness, Luminosity and Flux of Stars Explained
- The Solar System and Planets Guide and Factsheet
- Kepler's Laws of Planetary Motion Explained
- What Are Lagrange Points?
- Gravitational Forces and Common Equations
- List of Astronomy Equations with Workings
- Glossary of Astronomy & Photographic Terms
- Astronomical Constants - Useful Constants for Astronomy
Civil time is based on the position of the Sun in the sky. The 24 hours on a clock represent an apparent solar day. The apparent solar day is the time between successive transits of the Sun across an observer's meridian.
Apparent solar time, as measured by clocks, is not a very accurate system for measuring actual solar time. This is because the Earth's orbit is elliptical, and the Earth's rotation axis is inclined by 23.5° from the orbital plane. This system would require clocks to run faster on some days and slower on others. To get around this, civil time uses an average solar time, meaning that a solar day is an average of 24 hours, which is constant throughout the year. The difference between average solar time and apparent solar time is called the equation of time and varies by up to 16.25 minutes throughout the year. This average time gives rise to the Greenwich Mean Time.
Sidereal Time for Astronomers
During a year, the Sun appears to move around the sky along the ecliptic, and if we look at the night sky each month at midnight, we will see that the constellations also appear to move around the sky (albeit much slower). Consequently, the time measured by the distant stars and the solar time measured by the Sun cannot be equal.
Astronomers' time is best measured concerning the background stars and is referred to as sidereal time.
The normal definition of a sidereal day is "the time between successive transits of the First Point of Aries, whereas the mean solar day is defined as the average time between transits of the Sun.
The Earth has a normal rotation rate of 365 times per year, so we expect the Sun to transit 365 times yearly, compared with 366 transits of our chosen star. The extra transit occurs after the final transit of the Sun. This accounts for the difference between solar time and sidereal time.
By dividing the "extra" day into 365 parts, we can calculate that a sidereal day is about 4 minutes shorter than a solar day. Consequently, stars appear to rise 4 minutes earlier each evening.
The zero point used for sidereal time is the transit of the First Point of Aries at Greenwich, England, also known as Greenwich Sidereal Time (GST). It is 0 hours at solar noon at the March equinox (approximately March 21). It is also 0h at midnight at the time of the September equinox (approximately September 21) in Greenwich.
Greenwich Sidereal Time is the local sidereal time for observers at Greenwich. Observers at other longitudes will also have their own local sidereal time (LST). This is calculated from GST using the following formula.
Equation 14 - Local Sidereal Time
Where λ is the observer's longitude expressed in hours:minutes.
Worked Example
Assume Greenwich Sidereal time is 12:30. What is the local sidereal time at an observatory at +20° (or 20° E)?
Longitude needs to be converted into hours and minutes. Since 24h = 360°, 20° corresponds to:
Equation 15 - Calculate Local Sidereal Time
1.33 hours is the decimal form of 1h 20 minutes.
Equation 16 - Local Sidereal Time Calculation
Hour Angle, Sidereal Time and Right Ascension
A celestial body's hour angle (HA) is defined as the angle measured westwards in units of time along the celestial equator from the observer's meridian to the hour circle passing through the celestial body. It, therefore, represents the sidereal time that has elapsed since that object was on the meridian.
From this definition, the LST is the hour angle of the First Point of Aries.
The following equations illustrate the relationships between Local Sidereal Time, Right Ascension and Hour Angle.
Equation 17 - Relationship between LST, HA and RA
