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Formula & Calculator

Local Sidereal Time (Approx.)

Gives the local sidereal time, which tells an observer which stars and constellations are currently overhead or crossing the meridian.

Written by the Vorzep Team

AstronomyTimekeepingTelescope Use

Local Sidereal Time Calculator

LST = GST + Longitude / 15
Select the variable to solve for, then enter the other two values
LSTGSTLongitude
Solve for:
City Presets
Select city: Set Longitude
Enter the other two values
h
h
°
LST = Local Sidereal Time (hours) GST = Greenwich Sidereal Time (hours) Longitude = degrees East (+) or West (-) 1 hour = 15° of longitude

Variables

SymbolQuantityUnit
LSTLocal sidereal timehours
GSTGreenwich sidereal timehours
LongitudeObserver's longitude (east positive)deg

What it means

Local Sidereal Time (LST) is the hour angle of the vernal equinox at a given location. It is calculated by adding the observer’s longitude (in degrees east, divided by 15 to convert to hours) to Greenwich Sidereal Time (GST). LST is used to determine which celestial objects are visible: an object with right ascension (RA) equal to LST is on the meridian. This is fundamental for planning astronomical observations, for telescope pointing, and for understanding the rotation of the Earth relative to the stars. Understanding LST is essential for astronomers and astrophotographers to schedule observations and to navigate the night sky accurately.

Worked example

Local Sidereal Time – Two Detailed Examples

Real‑World
Scenario: An observer in New York City (longitude −74.0°) wants to know the local sidereal time (LST) when the Greenwich sidereal time (GST) is 10.5 hours. They use LST = GST + Longitude/15. This helps them point their telescope accurately to a celestial object whose right ascension is known. The conversion is essential for setting the telescope's hour angle.
ParameterValue
GST (h)10.5
Longitude (deg)-74.0
1LST = 10.5 + (-74.0/15) = 10.5 - 4.933 = 5.567 h
Result 5.57 h ✓ Local sidereal time
Scenario: An astronomer in Sydney, Australia (longitude 151.2° E) is observing at a time when GST is 3.1 hours. They want to synchronise their telescope control software with the local sidereal time. Using the formula, they convert longitude to hours (151.2/15 = 10.08 h) and add to GST, obtaining LST ≈ 13.18 h. This ensures that the telescope points to the correct target coordinates.
ParameterValue
GST3.1
Longitude151.2
1LST = 3.1 + 151.2/15 = 3.1 + 10.08 = 13.18 h
Result 13.18 h ✓ Local sidereal time
Insight: Sidereal time is the hour angle of the vernal equinox. LST = GST + longitude (in hours). It allows astronomers to relate celestial coordinates to their local meridian.

Common mistakes

  • LST: Local Sidereal Time – the hour angle of the vernal equinox.
  • GST: Greenwich Sidereal Time – at 0° longitude.
  • Longitude: East longitude is positive, West is negative – and convert to hours (÷15).
  • Units: LST in hours – keep 0‑24 range (mod 24).
  • Approximation: This is an approximate formula – exact calculation requires Julian Date and UT.

Applications

Local sidereal time (LST) is the hour angle of the vernal equinox at a given longitude, providing a measure of which right ascension is currently on the meridian. It is essential for pointing telescopes accurately. The formula LST = GST + Longitude/15 converts Greenwich sidereal time to local time. Amateur and professional astronomers use LST to know when a celestial object is visible at its highest point, simplifying observation planning. By calculating LST, one can align telescopes to track objects as they rotate across the sky. This concept is also used in satellite tracking and in the design of observatory control systems. Understanding LST is crucial for any observational astronomy work.

  • Telescope pointing and object tracking
  • Planning of observing sessions and target selection
  • Interpretation of astronomical catalogs (coordinates)
  • Satellite and spacecraft navigation using stellar references
  • Educational demonstration of Earth's rotation and sky motion