The Right Angle 
If you want to know how far it is to the front door of a neighbor’s house, you can pace it off by foot, or to be more precise you can use a tape measure.
Even if you couldn’t leave your own yard, though, you could still get a good measurement by using geometry. You’d note the direction to the door from a particular spot in your yard, then move a few feet to either side and plot the direction again. From the angle between those two directions, you can calculate the distance to the other house.
That’s not very efficient for measuring the distance to a neighboring house. But it’s the best method for measuring the distances to neighboring stars.
Consider Denebola, the star at the “tail” of Leo. It’s in good view on March nights, springing high across the sky.
The best measurement of Denebola’s distance came from a European spacecraft in orbit. It measured the direction to Denebola at six-month intervals, when Earth was on opposite sides of the Sun. From the angle between those two directions, astronomers calculated the star’s distance: just 35.9 light-years -- practically next door on the astronomical distance scale.
The measurement is accurate to within two-tenths of a light-year, which is quite a small margin of error. But when you consider the vastness of a light-year, it means the error could be more than a trillion miles. So measuring distances to the stars is a tricky business -- even for our closest neighbors.
Script by Damond Benningfield, Copyright 2012