Glossary
Parallax & the Parsec

Index Definition, Historical: Solar-system, Stellar

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.

189BCE (Moon)
Hipparchus of Rhodes estimated the distance to the Moon from measurements taken during a solar eclipsein 189BCE. The eclipse was "full" in Hellespont (NW. Turkey), but only partial in Alexandria (Egypt) where 20% of the Sun's disk remained visible. Since 20% of the Sun's disk corresponds to 6~arcmins, then by estimating the distance between Hellespont and Alexandria (derived from their latitude [using say Polaris], and knowing the circumference of the Earth), one can derive the distance of the Moon.
(A geocentric cosmology was the paradigm at the time).

[Image: From Stargazers to Starships]
Hipparchus estimated the distance to be between 4.5 x 10⁸ m and 5.2 x 10⁸ m
(c.f. modern value of 3.8 x 10⁸ m)

c.1600 Telescopes Invented

1672 (Mars)

Giovanni Domenico Cassini derived the distance to Mars when it was in "opposition" (Earth between Sun and planet) using (pseudo) simultaneous observations from Paris & Cayenne (French Guiana; N.coast of S.America). The Copernican system was the paradigm at the time, thus this measurement allowed Cassini to estimate Sun-Earth distance.

He obtained a value of 1.4 x 10¹¹ m
(c.f. modern value of 1.496 x 10¹¹ m).

However his errorbars were huge...

1760s (Venus)
an international campagin to observe the "transit" (planet passes between the Sun & Earth) of Venus in 1761 and 1769 derives a Sun-Earth distance of 1.46 x 10¹¹ m
(c.f. modern value of 1.496 x 10¹¹ m).

1931 (Eros NEA433)

an international campagin to observe the Near Earth Asteroid Eros (NEA433) derives a Sun-Earth distance essentially the same as the modern value (1.496 x 10¹¹ m).

Note in passing.. This object was the goal of NASA's Near Earth Earth Astroid Rendexvous (NEAR) mission (1996-2001), which included a "landing" on the surface of Eros in 2001.
This mission was renamed to NEAR Shoemaker in 2000 as a tribute to the late Gene Shoemaker (died 1997) for his contributions to the study of asteroids.
See the NEAR shoemaker picture archive for a wonderful set of images of Eros, the Earth, and NEA253 Mathilde.

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)