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The Parallax
Method
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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.
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The parallax angle p
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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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