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Observational Techniques: Units and Coordinates Notes

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:

Celestial Coordinate Notes

If studying solar system objects, you will also need to know about orbital elements: check out the following web page:

Orbital Elements Notes

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):

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:

Brightness Unit Notes (pdf)