Coordinates of Planets: An Astronomer's Guide

[t_n_p]

Hi, a novice astronomer here.

Was just looking at some stars/galaxies/planets the other day, and followed up some observations by looking at their locations (on wikipedia).

I noticed that for stars/galaxies there are a range of different coordinate/location systems used, although the celestial coordinate system seems the most common (e.g. 13h 23.7m xxs etc).

My question is thus, what is the convention for denoting the location/coordinates of planets/the moon? Since they are continuously moving, as are we on Earth, is there a dedicated system to describe their location?

Also as an aside, does the same happen with galaxies/stars? Because they are further away does their coordinate location change but by smaller magnitudes? Basically why can't the celestial coordinate system be used to locate planets/the moon?

 

[D H]

I noticed that for stars/galaxies there are a range of different coordinate/location systems used, although the celestial coordinate system seems the most common (e.g. 13h 23.7m xxs etc).

That "13h 23.7m xxs" business is not a coordinate system. It is just the goofy way astronomers represent angle. Instead of a circle being 360 degrees it is 24 hours to astronomers. A minute in this scale is 1/60 of an hour, or 1/4 degree. A second is 1/60 of a minute, or 1/240 degrees, or 1/4 arc seconds.

There are many celestial coordinate systems. Astronomical coordinates are essentially spherical coordinates. Latitude, longitude, altitude are essentially spherical coordinates, for example. Such systems use a directed reference plane and a directed reference line on the plane (the Earth's equator and the intersection of the Greenwich meridian and the equator in case of latitude and longitude). The latitude (or its equivalent) of some point is the angle between the reference plane and the line from the origin to the point; the sign denotes whether the point is above or below the plane. The longitude (or its equivalent) is the angle between some reference line on the plane and the line from the origin to the projection of the point onto the plane.

So it all comes down to choosing an origin, a directed reference plane, and a directed reference line on the reference plane. There are an infinite number of choices. The most commonly used reference planes are the Earth's equator, The Earth's orbital plane, and the galactic plane.

 

[t_n_p]

thanks for the reply.
that helps to clear up that part of the question, but what now of the planets/earth's moon?

 

[twofish-quant]

thanks for the reply.
that helps to clear up that part of the question, but what now of the planets/earth's moon?

If you want the gory details...

http://aa.usno.navy.mil/publications/docs/Circular_179.pdf

What you do is to use observations to distance quasars to define a basic reference system, and then use some algorithms to convert that system to one in which the sun's barycenter is in the middle.

 

[t_n_p]

thanks for the reply.
I found this website: http://www.fourmilab.ch/cgi-bin/Solar
it claims to tell you the right ascension of the planets at any given date you plug in.

Would you say something like this is reliable?

 

[russ_watters]

Probably.

 

[phyzguy]

JPL also has a nice site for generating the coordinates of any planet in a variety of coordinate systems:

http://ssd.jpl.nasa.gov/horizons.cgi