Glossary
Parallax & the Parsec



Index Definition, Historical: Solar-system, Stellar
Parallax is
the apparent change of position of a (closer) object as measured against the positions of more distant object(s) due to the movement of the observer.
(see Bothun, Fig.1.3; also Silk, Fig3.1)

A Parsec is defined as
the distance of an object that exhibits parallax of 1 arcsec
(Easy to remember since the word parsec is a construction from parallax and arcsec)


[Image: EnchantedLeaning.com]

1 parsec = 3.085678 x 1016 m = 3.261633 lyr (light years)

Note: Make sure you understand that Larger parallax means smaller distance


Historical Attempts to Detect Astronomical Parallax


Historically - Solar Sytem Objects

We how know that the measurement of the parallax of objects in the Solar System is hampered by the vast distances (hence small parallax angles) invloved. Furthermore, of course before the first telescopes were invented (c.1600), all observations were performed using the naked-eye.

Historically - The Stars...

We how know that the measurement of the parallax of the stars is hampered by the vast distances (hence small parallax angles) invloved. In addition, there are the effects of
  • "seeing" (the "jiggling of an image due to the Earth's atmosphere),
  • precession (wobble of the oblate Earth's axis due to torquing by the Moon)
  • aberrration (motion of the Earth in its orbit & the finite speed of light), nutation (wobble of the Earth's axis due to torques by the other objects in the Solar system), and
  • proper motion (apparent motion of the star due to the relative motion between the Star and the Sun)
that need to be taken into account. Furthermore, of course before the first telescopes were invented (c.1600), all observations were performed using the naked-eye.

However equally important to appreciate is that the search for parallax was key to put a scale on the Copernican system (c.1541) of the universe (still is some doubt in the early 1600s).

  • Numerous attempts/claims
    • Tycho Brahe
    NONE CONVINCING


    c.1600 Telescopes Invented/Used for Astronomy
  • Numerous suggestions/attempts/claims
    • Galileo Galilei (c.1630)
    NONE CONVINCING

  • 1838 (61 Cyg)

    Friedrich Bessel (convincingly) measures the parallax to 61 Cyg, deriving a value of 0.314 arcsec (c.f modern value of 0.287 arcsec). This result is followed soon after by the measurements of

    • 1839-- Thomas Henderson (Alpha Cen) and
    • 1839-- Wilhelm Struve (Vega)


For More Information

Nice WWW link with illustration of Parallax (and Retrograde motion, and much much more)
(Univ.Kentucky - Astronomy 161)

A popular book on the historical search & detection of Parallax see:
  • "Parallax: The Race to Measure the Cosmos"
    Author(s): Alan W. Hirshfeld
    Publisher: W.H.Freemanand Company
    ISBN 0-7167-3711-6 (hardback)