# Sidereal Time Calculator

URL: https://speedycalculator.com/calculators/everyday-life/sidereal-time/
Last updated: 2026-10-25

> The sidereal time calculator converts a date, UTC time and longitude into Greenwich Mean Sidereal Time (GMST) and Local Sidereal Time (LST) – the right ascension crossing your meridian – for pointing telescopes.

## Inputs

- Date
- Time (UTC)
- Longitude (east +, west −) (°)

## Results

- Julian date
- GMST (decimal hours)
- LST (decimal hours)
- Greenwich Mean Sidereal Time
- Local Sidereal Time
- Local Sidereal Time (degrees)

## Formula

```
D = Julian date − 2,451,545.0
GMST (hours) = 18.697374558 + 24.06570982441908 × D (mod 24)
LST = GMST + longitude (east positive) ÷ 15
```

## Explanation

A sidereal day – one turn of Earth relative to the stars – is about 23 h 56 min 4 s, so sidereal time gains about 3 minutes 56 seconds a day on clock time. Local sidereal time equals the right ascension of stars on your meridian, which tells you what is best placed to observe.

### How to use the Sidereal Time Calculator

- **Date**
- **Time (UTC)**
- **Longitude (east +, west −)**

Results update as you type, and every field accepts the units shown in its menu.

### How it works

Pick the date and time in UTC and enter your longitude (east positive, west negative).

### Tips

- Check whether the end date should be included – event durations usually include it, elapsed time does not.
- Months have 28–31 days, so "one month" is counted by calendar date, not as 30 days.
- Use 24-hour time or set AM/PM carefully when entering clock times.
- Time zones and daylight-saving changes are not applied unless the calculator says so.

### Common mistakes to avoid

- Counting a 30-day month when a calendar month is meant (or the reverse).
- Forgetting leap years when counting days across February.
- Mixing decimal hours (7.5 h) with clock time (7:30).

### Inputs explained

FieldWhat to enterDateA dateTime (UTC)A clock time (hours and minutes)Longitude (east +, west −)In °

### What each result means

- **Julian date** = [Date] + 2440587.5 + [Time (UTC)]÷24
- **GMST (decimal hours)** = mod(18.697374558 + 24.06570982441908×([Julian date] - 2451545), 24)
- **LST (decimal hours)** = mod([GMST (decimal hours)] + [Longitude (east +, west −)]÷15, 24)
- **Greenwich Mean Sidereal Time** = [GMST (decimal hours)]
- **Local Sidereal Time** = [LST (decimal hours)]
- **Local Sidereal Time (degrees)** = [LST (decimal hours)]×15

## Worked example

Inputs — Date: 2026-10-25; Time (UTC): 22:00; Longitude (east +, west −): -0.1276 °.

Results — Julian date = 2,461,339.41667; GMST (decimal hours) = 0.2868; LST (decimal hours) = 0.2783; Greenwich Mean Sidereal Time = 00:17:12; **Local Sidereal Time = 00:16:42**; Local Sidereal Time (degrees) = 4.174 °.

## FAQ

**What is sidereal time?**
Time measured by Earth's rotation relative to the stars instead of the Sun.

**How long is a sidereal day?**
23 h 56 min 4.09 s.

**Why does LST matter?**
A star is highest in the sky when the local sidereal time equals its right ascension.

**What formula does the Sidereal Time Calculator use?**
D = Julian date − 2,451,545.0.


## Sources

- [U.S. Naval Observatory – Sidereal time](https://aa.usno.navy.mil/faq/GAST)