how astronomer measure the disstance of planets, stars aand galaxies?

Answer 1

http://answers.yahoo.com/question/index?qid=20070707094552AA76uTF

Answer 2

There are many ways, but there are a few fundamental ways:

1. Parallax. Used to measure closest objects. The postion in the sky (against the distant star background) will be different when the earth is at different points in it's orbit. Measurements are made 6-months apart, and distance computed geometrically.

2. Cepheid variables. Variable stars whose brightness is cyclic. The period of the cycle is related to absolute brightness. Calibrated with nearby cepheids, when ones in distant galaxies are found, their absolute brightness is computed from the cycle period, and distance computed from apparent brightness according to the inverse-square law.

3. Supernova brightness. Absolute brightness of supernovas fall withing a limited range. Since they are very bright, they can be used to estimate the distance of very distant galaxies when a supernova is observed in them.

4. Red shift. The most distant galaxies are measured using the red shift due to universe expansion. The more distant the galaxy, the greater the red shift.

See http://en.wikipedia.org/wiki/Cosmic_distance_ladder

Answer 3

Planets and the closest stars by triangulation.
Galaxies by noting the apparent brightness of pulsars.

Answer 4

In light years. Light travels 299,792,458 metres per second. So if something is 4.22 light years away (Proxima Centauri), that's how long it takes for light to travel from there to here. There are 3,556,926 seconds in a year.