Local Sidereal Time Calculator

Calculation mode

Use the second mode if you already know RA and hour angle.

Date

Time

Input time is

UTC offset

Used only when input time is local.

ΔUT1 (seconds)

Optional. Typical range is about ±1 second.

Longitude

East positive, West negative. Enter 0–180.

Target right ascension (optional)

If provided, the hour angle is computed.

What you get

Julian Date (UT1 approximation) for the chosen moment.
Greenwich mean sidereal time and local sidereal time.
Hour angle for a target if you enter its right ascension.

Right ascension (hours)

Hour angle (hours)

Relation

LST = RA + HA

This mode skips calendar time and longitude.

Results appear above this form after submission.

This calculator uses a standard mean-sidereal-time model based on the Julian Date. First, it computes the Julian Date (JD) from the chosen moment (using UTC adjusted by ΔUT1 when provided).

T = (JD − 2451545.0) / 36525

Greenwich mean sidereal time (GMST) in degrees is then estimated by:

GMST = 280.46061837 + 360.98564736629·(JD − 2451545.0) + 0.000387933·T² − T³/38710000

Local sidereal time (LST) follows from longitude (east positive):

LST = GMST + longitude

Angles are wrapped to 0–360°, and converted to hours using 15° per hour.

Select Date/Time + Longitude for a full sky-time calculation.
Enter the date and time. Choose whether it is UTC or local time.
If using local time, provide the UTC offset like +05:00.
Enter longitude in degrees and choose east or west.
Optionally enter a target right ascension to get its hour angle.
Press Calculate. Use the download buttons to export your results.

Date (UTC)	Time (UTC)	Longitude	Expected LST (approx)
2026-01-12	00:00:00	0° E	07:26:00.72
2026-01-12	00:00:00	67.0099° E	11:54:03.09
2026-06-01	12:00:00	70° W	23:59:56.74

These values use the same GMST model as the calculator and may differ slightly from high-precision almanacs.

1) What local sidereal time represents

Local sidereal time (LST) is the right ascension currently crossing your local meridian. It is a sky-based clock: when LST equals a target’s right ascension, that target is transiting and reaches its highest altitude for the night.

2) Why LST drifts relative to civil time

A sidereal day is about 23h 56m 4s, roughly four minutes shorter than a solar day. Because Earth rotates once relative to the stars slightly faster, the LST at a fixed civil time increases by about 3m 56s each day, shifting transit times.

3) Using longitude to shift GMST into LST

Greenwich mean sidereal time (GMST) is defined at 0° longitude. Your site’s longitude shifts that reference. East longitudes add time and west longitudes subtract time. A 15° change in longitude corresponds to 1 sidereal hour.

4) Julian Date as the time backbone

The calculator converts your chosen moment into a Julian Date (JD), a continuous day count used in astronomy. JD avoids month-length and leap-year complications, enabling smooth sidereal-time formulas and consistent interpolation between seconds and days.

5) Accuracy notes and ΔUT1

The model uses a widely used mean-sidereal-time approximation and can differ slightly from high-precision almanacs. If you know ΔUT1 (UT1 − UTC), entering it refines the Earth-rotation time used for JD and can reduce small timing offsets.

6) Planning observations with hour angle

Hour angle (HA) links LST to right ascension: HA = LST − RA. Negative HA means the target is east of the meridian (it will transit later), while positive HA means it has already transited. Many mounts and pointing models use HA directly.

7) Practical examples for imaging and outreach

For imaging, schedule targets near transit to minimize airmass and improve seeing stability. For outreach, LST helps build a “what’s up now” list: choose objects whose RA is within about ±2 hours of current LST for comfortable sky positions.

8) Data handling and exports

After computing, export results as CSV for logs and spreadsheets, or as a PDF snapshot for field notes. Keeping JD, GMST, LST, and longitude together is helpful when comparing sessions, verifying mount timing, or recreating a pointing plan later.

1) What is the difference between GMST and LST?

GMST is sidereal time at Greenwich (0° longitude). LST is GMST shifted by your longitude, giving the sidereal time on your local meridian for the same moment.

2) Why does LST change about four minutes per day?

Earth must rotate a little more than 360° each day to face the Sun again. Relative to the stars, one rotation is shorter, so sidereal time advances roughly 3m 56s per solar day.

3) Should I use UTC or local time inputs?

Either works. If you enter local time, include the correct UTC offset so the calculator can convert to UTC internally. Using UTC avoids daylight-saving confusion and is preferred for logs.

4) What longitude sign convention is used here?

East longitudes are positive and west longitudes are negative. The form asks for degrees plus an E/W selector, then converts to the signed value before computing LST.

5) How accurate is the result for telescope pointing?

For most amateur planning and pointing, the mean model is adequate. Small differences from almanac values can occur due to approximations and Earth-rotation details, especially over long timescales.

6) What formats can I use for right ascension and hour angle?

You can enter decimal hours (like 5.25) or a clock format (like 05:15 or 05:15:00). Hour angle may be negative to represent an object east of the meridian.

7) Why does the calculator mention ΔUT1?

Sidereal time is tied to Earth rotation, which is tracked by UT1. ΔUT1 provides a small correction between UT1 and UTC. If you don’t know it, leaving 0 seconds is commonly fine.