# Sidereal Time and Precession Planner Plan observations with local sidereal time, optional nutation, precessed coordinates, and altitude/azimuth slots. > Canonical page: https://elysiatools.com/en/tools/astronomical-sidereal-time-precession-planner - **Category:** Astronomy - **Keywords:** sidereal time, GMST, GAST, precession, nutation, observing planner, RA Dec ## Overview 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. ## Inputs - **Date/Time (UTC)** (text): 2026-08-01T00:00:00Z - **Latitude** (number) - **Longitude** (number) - **Right Ascension (hours)** (number) - **Declination (degrees)** (number) - **Coordinate Epoch** (select) - **Apply Nutation** (checkbox) - **Slot Count** (number) - **Slot Interval (minutes)** (number) ## 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 - Enter the observation date and time in UTC, using a format such as 2026-08-01T00:00:00Z. - Provide latitude, longitude, right ascension, and declination; longitude values use positive numbers east of Greenwich. - Select J2000.0 or of-date coordinates and choose whether to apply the optional nutation correction. - Set 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. ## Frequently asked questions ### 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? 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