Magnification is the ability of a telescope to make a small, distant
object large enough to examine in detail. If you look at the Moon
(250,000 miles away) with a 125 power (125x) telescope, it's essentially
the same as looking at it with your bare eyes from 2000 miles away
(250,000 ÷ 125 = 2000). The same telescope used terrestrially will make
an object one mile away appear to be only 42 feet away (5280 feet ÷ 125 =
42).
The magnification of a telescope is determined by dividing the
focal length of the telescope (usually in millimeters) by the focal
length of the eyepiece used (again, usually in millimeters; but in all
cases by the same unit of measurement used for the telescope focal
length). For example, a 2000mm focal length telescope and a 10mm focal
length eyepiece will give you a magnification of 200 power (2000 ÷ 10 =
200). The same 2000mm telescope with a 20mm eyepiece will give you 100x
(2000 ÷ 20 = 100).