Observational Techniques: Units and Coordinates Notes
Celestial Coordinates
The aim of this section of the course is to familiarise you with celestial Coordinates. This
is necessary so you can work out what is observable at a given time on a given night, from a
given observatory.
I found some good notes explaining the celestial coordinate system on the web:
If studying solar system objects, you will also need to know about orbital elements: check out the
following web page:
When trying to work out what is observable on a given night, it is often heelpful to know the siderial
time at midnight. Here is a table listing this for various dates through the year (generated by the IRAF
astutils, asttime program):
- 1st Jan: 06hr, 02min
- 1st Feb: 08hr, 05min
- 1st March: 09hr, 55min
- 1st April: 11hr, 58min
- 1st May: 13hr, 56min
- 1st June: 15hr, 58min
- 1st July: 17hr, 56min
- 1st August: 19hr, 59min
- 1st Sept: 22hr, 01min
- 1st Oct: 23hr, 59min
- 1st Nov: 02hr, 01min
- 1st Dec: 04hr, 00min
In addition to the equatorial coordinates described in the notes above, astronomers also use
(at least) two other coordinate systes: galactic coordinates and ecliptic coordinates. In
galactic coordinates, the galactic plane is the equator, and galactic longitude 0 is the
direction towards the centre of our galaxy. Ecliptic coordinates use the ecliptic
(the Earth's path around the sun) as the equator, and are used for solar system work.
Brightness Units
Most astronomical units (eg. parsecs, solar luminosities) can be straightforwardly looked up and
present no particular problems
(e.g. this web page). The exception is units measuring the apparent brightness of an
astronomical source. Unfortunately, we use a terrible collection of mutually inconsistent,
barbarically primitve units, some from ancient history. You will, alas, have to learn how to master
these units and convert between them. The following notes will show you how: