Teaching · For Teachers · Earth and space science

From star atlas
to where to point.

List objects by right ascension and declination, then type where and when you are observing. The converter turns each into altitude and azimuth through sidereal time, so a class knows where in the sky to look.

Convert the coordinatesWhat you get

The coordinate converter

The worked example is five bright stars from Boulder at 03:00 UTC on November 11 (8:00 PM the evening before, local time). Nothing you type leaves this browser.

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What you get

Teaching · For TeachersFree

What you finish with
Greenwich and local sidereal time and, for each object, its altitude, azimuth, hour angle and whether it is up. It prints as an observing sheet and downloads as a spreadsheet.
Who it is for
Astronomy and earth science teachers and their students, astronomy clubs and families with a telescope.
Format
Browser calculator for a phone, tablet or computer, by touch or keyboard. Prints a summary; downloads a spreadsheet (.csv); export and import a file.
Access
Free. No account, no sign-in and nothing to install. Everything you type stays in this browser; nothing is sent to Caveat.
Where it lives
The For Teachers space in Caveat Teaching.
Support
Questions, a correction or a request: team@caveatacademic.com.

How to use it

  1. Type the observing place’s latitude and longitude.
  2. Convert your start time to UTC and type the UTC date and time.
  3. List the objects with RA and Dec from an atlas.
  4. Read each altitude and azimuth.
  5. Print the sheet for the observing night.

Convert the coordinates

How RA and Dec become altitude and azimuth

Sidereal time, hour angle, then one spherical triangle.

The worked example: Vega from Boulder, Nov 11, 03:00 UTCTime to GMST95.28°sidereal angle at 03:00 UTAdd longitude350.01°local sidereal timeHour angle70.77°LST − Vega’s RA (279.23°)Alt and az36.8° upbearing 293.2°The worked example: Vega from Boulder, Nov 11, 03:00 UTCTime to GMST95.28°sidereal angle at 03:00 UTAdd longitude350.01°local sidereal timeHour angle70.77°LST − Vega’s RA (279.23°)Alt and az36.8° upbearing 293.2°
Right ascension and declination are fixed on the sky; sidereal time turns the sky to your place and moment.

Two coordinate systems

Right ascension and declination are like longitude and latitude printed on the sky: they stay with the stars. Altitude and azimuth belong to you: the angle above your horizon and the bearing from north. Because Earth turns, the link between the two changes by about one degree every four minutes.

Sidereal time

Sidereal time measures Earth’s turn against the stars rather than the sun, and a sidereal day is about 3 minutes 56 seconds shorter than a solar day. Local sidereal time equals the right ascension now on your meridian; subtract an object’s right ascension and you have its hour angle, which with its declination and your latitude fixes where it is.

Sources

  1. Meeus, J. (1998). Astronomical Algorithms (2nd ed.). Willmann-Bell. Sidereal time at Greenwich; transformation of coordinates.
  2. U.S. Naval Observatory. Approximate sidereal time and the Astronomical Almanac definitions (aa.usno.navy.mil).
  3. SIMBAD Astronomical Database, Centre de Données astronomiques de Strasbourg. J2000 positions of Vega, Altair, Polaris, Capella and Betelgeuse (simbad.cds.unistra.fr).
  4. NASA Science. Night sky network: celestial coordinates (science.nasa.gov).

The worked example uses Boulder’s published coordinates and catalog positions rounded to the second. Your entries stay in this browser. Support: team@caveatacademic.com.