The Parallax
Method

We have previously discussed the most reliable way to determine distances in astronomy: the method of parallax.

Measuring the Parallax Angle

The parallax angle p is illustrated in the following figure.

The parallax angle p


If this angle, which corresponds to a small shift in apparent position on the celestial sphere because of the differing vantage points as the Earth moves around its orbit, can be measured accurately, the distance can then be determined from simple trigonometry.

Limitations on Parallax Measurements

Unfortunately, the parallax angle is very small because of the great distances to stars. Thus, only for the more nearby stars can it be measured reliably. Roughly speaking, ground-based telescopes can only measure parallax reliably for stars that are within a few hundred light years from us (the light year is defined below). Telescopes above the atomosphere such as the Hubble Telescope can measure smaller parallax shifts and thus larger distance, but even in that case the most distant objects for which distance can be determined by parallax of a few thousand light years away.

For more distant objects the parallax is simply too small to measure and we must turn to other less direct methods to determine distance.


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