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(angle due to different lines of sight)

Parallax is the angle between the apparent location of an object as seen from two different places. It is used to measure their distance to stars. If a very small angle can be measured accurately, a distance to a nearby star can be determined.

For measuring distances to a star, the parallax used is the angle between viewing the star from Earth at two times, half a year apart, when the positions of the Earth differ by 2 AU (the Baseline). The angle cited as parallax (Parallax Angle) is typically half this, the angle from two positions 1 AU apart, e.g., from the Sun and the Earth. A Parsec is the distance of a star with this (1 AU) parallax angle of 1 arcsecond. Typical capabilities of telescopes:

  • Ground-based telescope without Adaptive Optics: 1 arcsecond Angular Resolution: can measure distances to on the order of a parsec.
  • With adaptive optics: 50 milliarcseconds: can measure distances to on the order of 20 parsecs.
  • HST: 50 milliarcseconds.
  • Interferometer: 5 milliarcseconds: can measure distances to on the order of 200 parsecs.

Secular Parallax is using the Sun's motion to gain a longer baseline, but the fact that the target star also has such a motion limits the information that can be gained.


Referenced by:
Astronomical Quantities
Cosmic Distance Ladder
Mass-Luminosity Relation
Gravitational Microlensing
Parsec (pc)
Spectroscopic Parallax
Stellar Distance Determination
Stellar Luminosity Determination
Stellar Parameter Determination
Stellar Radius Determination