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Earth and Space Sciences · Glossary

What is Annual parallax, parsec?

Also known as: annual parallax · parsec

Definition 2.14 University Earth and Space Sciences — Year 1 · Chapter 2 — Telescopes, Detectors and Photometry

The annual parallax pp of a star is the angle subtended at the star by the radius of the Earth’s orbit, one astronomical unit, seen perpendicular to the line of sight; it is the semi-major axis of the yearly ellipse the star appears to trace. The parsec (pc\mathrm{pc}) is the distance at which one astronomical unit subtends one second of arc:

1 pc=1 autan⁡1′′=206 265 au=3.0857×1016 m=3.2616 ly.1\,\mathrm{pc} = \frac{1\,\mathrm{au}}{\tan1{}^{\prime\prime}} = 206\,265\,\mathrm{au} = 3.0857 \times 10^{16}\,\mathrm{m} = 3.2616\,\mathrm{ly} .
Annual parallax (not to scale): seen from the star, the radius of the Earth’s orbit subtends the angle p. Six months apart, the star’s direction differs by 2p. The real angle for the nearest star is below one second of arc.
Annual parallax (not to scale): seen from the star, the radius of the Earth’s orbit subtends the angle pp. Six months apart, the star’s direction differs by 2p2p. The real angle for the nearest star is below one second of arc.
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