Why positions come in so many forms
The same star can be written several ways. Catalogues and papers use J2000 right ascension and declination, which are fixed to the sky as it was at the start of 2000. Many telescope mounts point using JNow, the position “of date”. JNow drifts from J2000 by about 50 arcseconds a year because Earth’s axis slowly wobbles (precession). Radio astronomers and anyone mapping the Milky Way use galactic longitude and latitude. Planetary scientists use ecliptic coordinates, measured from the plane of Earth’s orbit. And at the eyepiece, what matters is altitude and azimuth: how high the object is and in which direction. This converter shows all of them together.
How to use it
- Set your place at the top. Your place is only needed for altitude, azimuth and hour angle.
- Type an object such as M42, NGC 7000, Andromeda or Jupiter, and its position fills in. Or choose which system your numbers are in and type them yourself.
- Angles can be typed any common way:
05 35 17.3,5h35m17s,-5:23:28,-05° 23′ 28″, or decimal degrees such as83.82. A single decimal number for right ascension is read as degrees, the way catalogues such as SIMBAD print it. - At sets the moment for JNow, altitude and azimuth. It starts at the current time.
- For many targets, paste a list into A whole list, one per line as
name, RA, Dec, and press Convert the list. You can also type just a catalogue name (such asM13) on a line. Download CSV gives a spreadsheet with every system as columns.
What each result means
- RA, Dec J2000: catalogue position, shown in hours and degrees and in decimal degrees.
- RA, Dec of date (JNow): precessed and nutated to the moment you chose. Use it if your mount or app asks for JNow.
- Galactic l, b: l = 0 points to the centre of the Milky Way in Sagittarius, and b = 0 is the galaxy’s mid-plane.
- Ecliptic λ, β: measured along and from the plane of the planets’ orbits, using the J2000 ecliptic.
- Altitude, azimuth and airmass: altitude includes normal atmospheric refraction. Azimuth is measured from north through east. Airmass is how much air you look through (1.0 overhead, about 2 at 30°).
- Hour angle: time since the object crossed your meridian. A negative value means it is still rising.
Worked example
Type M42. It fills in as RA 05h 35m 16.5s, Dec −05° 23′ 23″ (J2000). In November 2026 the JNow position is about 05h 36m 36s, −05° 22′ 19″. The galactic coordinates are about l = 209.01°, b = −19.38°, which puts the nebula well below the plane of the Milky Way, toward the galaxy’s outer edge.
Questions
How accurate is it? The conversions use the IAU rotation matrices in astronomy-engine and agree with professional tools to well under an arcsecond. That’s more than enough for pointing and planning.
Is my list uploaded? No. Everything is worked out on your device, and the page keeps working offline once it has loaded.
Last updated 2026-11-07.