Some Terms You Should Know: Astronomical Distances

These are some terms you will come across in your lab class dealing with parallax and the measurement of astronomical distances.
Parallax
The apparent change in position of an object against a distant background due to a change in the observer's position.
Parallax Angle
One half the angle a star appears to move due to parallax.
Parsec
A distance equal to 3.26 light years. An object one parsec away will have a parallax angle of 1 second of arc when viewed from a baseline of 1 AU.
Plate Scale
A conversion factor, for a specific photograph, of linear distance to angular distance. (E.g., from cm to degrees.)
Apogee
The point in its orbit when an object orbiting the Earth is farthest from the Earth.
Perigee
The point in its orbit when an object orbiting the Earth is closest to Earth.

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This page was last updated on 30 March 1998
and is © 1998 by Jim McDonald.