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.

Elysia Tools

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Astronomy

Sidereal Time and Precession Planner

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.

Execution

Run this tool

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Prepared example runs

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2 examples

Plan a three-slot observation from New York

Compute local sidereal time, J2000 precession, and altitude/azimuth for a target across three 30-minute slots.

{"result":{"timeline":[...]}}

Use of-date coordinates without nutation

Keep coordinates already reduced to the observation date and omit the equation-of-equinoxes correction.

{"result":{"coordinateEpoch":"of-date","timeline":[...]}}

Inputs

Options are grouped so long tools stay easier to scan.

9 options

ContentPaste or type the main input values.1

Date/Time (UTC)textRequired

SettingsAdjust formats, ranges, numbers, and modes.7

LatitudenumberRequiredLongitudenumberRequiredRight Ascension (hours)numberRequiredDeclination (degrees)numberRequiredCoordinate EpochselectRequiredSlot CountnumberRequiredSlot Interval (minutes)numberRequired

TogglesEnable or disable optional behavior.1

Apply NutationcheckboxOptionalEnabled when checked

Result

Ready for a run

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Samples

Sidereal Time and Precession Planner

What this tool helps you do

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Prepared example runs

Inputs

Result

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Prepared example runs

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Key facts

Overview

When to use

How it works

Use cases

Examples

1. Plan a three-slot observation from New York

2. Use of-date coordinates in Shanghai

FAQ

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