What this converter does
Astronomers use several clocks at once. Observing logs and variable-star reports use the Julian Date (JD), a running count of days. Catalogues and satellites often use the Modified Julian Date (MJD), which is the JD minus 2,400,000.5, so the numbers are shorter and each day starts at midnight. Telescope mounts need sidereal time, which tracks the stars rather than the Sun. Timing an eclipsing binary or an exoplanet transit needs the barycentric Julian Date (BJD), which removes the minutes that light takes to cross Earth’s orbit. This page shows all of them for one moment, side by side.
How to use it
- Set your place at the top. Local sidereal time and your local clock depend on your longitude and time zone. Pick a city, type your latitude and longitude, or press Use my location. Your location stays on your device.
- Choose the moment. Now gives a live clock that updates every second. Date and time reads your local time (tick This is UTC to type UTC instead). You can also paste a JD, MJD or Unix time to convert it back.
- Read the results. Every value updates as you type.
- Open Heliocentric and barycentric Julian Date and type a star’s J2000 position, or pick an object such as M13, to get the BJD (TDB) and HJD for that star.
- Download the results as an image, PDF or Word file for your notes.
What each result means
- Julian Date (UT): days since noon on 1 January 4713 BC, on the UTC timescale.
- Julian Date (TT): the same count on Terrestrial Time, the smooth timescale that ephemerides use. It runs about 69 seconds ahead of UTC, and that gap is shown as ΔT.
- Greenwich and local sidereal time: the right ascension that is crossing the meridian right now. An object is highest in the sky when the local sidereal time equals its RA.
- BJD (TDB): the time the light would have reached the Solar System’s centre of mass. This is the standard for exoplanet and pulsar timing.
- HJD: the older correction to the Sun’s centre. It is still used in many variable-star papers.
Worked example
Suppose you observe Algol from Toronto at 22:00 local time on 7 November 2026. That is 03:00 UTC on 8 November, JD 2461352.625. Type Algol’s position (03h 08m 10s, +40° 57′ 20″) and the page shows a light-time correction of a few minutes. Add that to the JD (TT) and you get the BJD to report.
Questions
How accurate is it? Sidereal time and Julian Dates are exact to the second. The BJD correction uses the full planetary positions from astronomy-engine and matches professional tools to well under a second. That is plenty for amateur timing work, but check against a research tool for millisecond pulsar work.
Does it need the internet? No. Once the page has loaded, it keeps working offline.
Where the numbers come from
- Sun, Moon and planet positions: astronomy-engine by Don Cross (MIT licence), accurate to about an arcminute.
- City coordinates and time zones: public geographic facts, typed by fibub.
Standard scientific values, such as constants, atomic weights and melting points, are facts found in every textbook; we name where ours were checked. If you are an author or publisher and want a value, a credit or a table changed or removed, tell us and we will act quickly.
Last updated 2026-11-07.