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Astronomy

Sidereal Time and Precession Planner

· Last updated：2026-08-01

Plan observations with local sidereal time, optional nutation, precessed coordinates, and altitude/azimuth slots.

Details

What this tool helps you do

Enter a UTC timestamp, observing location, and equatorial coordinates. The tool computes Julian Date, GMST, optional GAST, IAU 1976/Meeus precession from J2000.0, and horizontal coordinates for each planning slot. Longitude is positive east. The compact nutation model is intended for planning and teaching, not precision astrometry.

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Written by the Elysia Tools team

Tool usage guide

Learn when to use this tool, what it supports, and how real users apply it.

Key facts

Category
Astronomy
Input types
text, number, select, checkbox
Output type
json
Sample coverage
4
API ready
Yes

Overview

Sidereal Time and Precession Planner calculates Julian Date, GMST, optional GAST, precessed equatorial coordinates, and altitude/azimuth for observation slots from a UTC time, location, and target coordinates.

When to use

  • Plan when a target will be visible from a specific latitude and longitude.
  • Compare local sidereal time and target altitude across several observing slots.
  • Apply J2000.0 precession or work with coordinates already reduced to the observation date.

How it works

  1. 1Enter the observation date and time in UTC, using a format such as 2026-08-01T00:00:00Z.
  2. 2Provide latitude, longitude, right ascension, and declination; longitude values use positive numbers east of Greenwich.
  3. 3Select J2000.0 or of-date coordinates and choose whether to apply the optional nutation correction.
  4. 4Set the number and spacing of planning slots to receive a JSON timeline with sidereal and horizontal coordinates.

Use cases

  • Create a short observing schedule showing how a target's altitude and azimuth change over time.
  • Estimate when a right ascension and declination target is favorably placed from a chosen observing site.
  • Teach the relationship between UTC, GMST, GAST, precession, and horizontal coordinates.

Examples

1. Plan a three-slot observation from New York

Amateur astronomer

Background

An observer in New York wants to check a target's position at three points during an evening observing session.

Problem

They need local sidereal time and altitude/azimuth values at 30-minute intervals.

How to use

Enter 2026-08-01T00:00:00Z, latitude 40.7128, longitude -74.006, right ascension 18.615 hours, and declination -24.5 degrees. Select J2000.0, enable nutation, and set 3 slots with a 30-minute interval.

coordinateEpoch: J2000.0; applyNutation: true; slotCount: 3; slotIntervalMinutes: 30

Outcome

The JSON result includes a timeline with local sidereal calculations and horizontal coordinates for all three observation slots.

2. Use of-date coordinates in Shanghai

Astronomy student

Background

A student has coordinates already reduced to the observation date and wants a single planning result without the optional nutation correction.

Problem

They need to preserve the of-date coordinate interpretation and avoid applying the nutation option.

How to use

FAQ

What does this planner calculate?

It calculates Julian Date, GMST, optional GAST, coordinate precession, and altitude/azimuth for each planning slot.

How should I enter longitude?

Enter longitude in degrees, from -180 to 180, with east longitudes represented by positive values.

What is the difference between J2000.0 and of-date coordinates?

J2000.0 coordinates can be precessed from the J2000.0 epoch, while of-date coordinates are already reduced to the observation date.

What does the nutation option change?

It enables the compact nutation correction used for the optional GAST calculation and planning output.

Is this tool suitable for precision astrometry?

No. The compact nutation model is intended for observation planning and teaching, not precision astrometry.

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Enter 2026-08-01T12:00:00Z, latitude 31.2304, longitude 121.4737, right ascension 5.5 hours, and declination 20 degrees. Select of-date coordinates, disable nutation, and set one 60-minute slot.

coordinateEpoch: of-date; applyNutation: false; slotCount: 1; slotIntervalMinutes: 60

Outcome

The JSON output identifies the of-date coordinate setting and returns one horizontal-coordinate planning slot.