Measuring Distances in Space

       The system called trigonometric parallax,or parallax, is a variation on a technique that has been used by surveyors for hundreds of years.  Parallax is the apparent motion of a nearby star against the background of more distant non-moving

stars. When measuring parallax, astronomers use a unit of distance called a parsec, which is equal to 3.26 light years. The first use of parallax to measure the distance to a star was in 1838. Astronomers also use a baseline to measure objects by parallax. A baseline is an imaginary line from which the distance to an object is measured; the length of the baseline is known.

 

       If surveyors need to determine the distance to another object, they begin by marking out a baseline that is easy to work with, such as 30 m. They form

a right-angled triangle using the baseline and the object. From the other end of the baseline, they measure the angle to the object. Then using a

branch of mathematics called trigonometry, they determine the angle at the object and the distance back to the baseline. In astronomy, the distances to the stars are so large that very long baselines

are needed to increase the accuracy of the calculations.  The longest baseline astronomers can use is the diameter of Earth’s orbit. To achieve

this long baseline, astronomers must measure the angles to a star six months apart, when Earth is at opposite ends of its orbit.  

 

       The Parallax method works accurately to about 200 lightyears.  At greater distances, the angles involved are so small that there are too many observation errors for calculations to be valid.  As a result, for the more distant stars and galaxies astronomers must rely on other methods to calculate distances.