How can triangulation be used to calculate the approximate distance to very distant celestial bodies like stars, globular clusters, etc.? And can it be used to measure the distance to a Black Hole? (Can someone please help me with tags?)
Measure the angular distance between a star and the distant background stars.
If you know the length of the baseline (the Earth's orbit) and the angle then you know the distance to the star. In fact we define the distance to stars in terms of this angle and the Earth's orbit - see http://en.wikipedia.org/wiki/Parsec
Because of the blurring effects of the atmosphere it's difficult to measure angles much less than 1 arcsec, and so determine the distance to stars more than a few parsecs away directly by this method.
The hipparcos satelite was able to make much more accurate measurements ( less than 1 milli-arcsec) and so measured distances 1000x further