Parallax

:This article is about parallax, the perspective shift. For the DC comic book character see Parallax (comics).

Solar parallax

After Johannes Kepler discovered his Third Law, it was possible to build a scale model of the whole solar system, but without the scale. To fix the scale, it suffices to measure one distance within the solar system, e.g., the mean distance from the Earth to the Sun or astronomical unit (AU). When done by triangulation, this is referred to as the solar parallax, the difference in position of the Sun as seen from the Earth's centre and a point one Earth radius away, i.e., the angle subtended at the Sun by the Earth's mean radius. Knowing the solar parallax and the mean Earth radius allows one to calculate the AU, the first, small step on the long road of establishing the size — and thus the minimum age — of the visible Universe.

Related Topics:
Johannes Kepler - Third Law - Sun - Astronomical unit - Triangulation

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

A primitive way of determining the distance to the Sun in terms of the distance to the Moon was already proposed by Aristarchus: if the Sun is relatively close by, the first and last quarters of the Moon will not happen in time precisely in the middle between new and full moon. Unfortunately the method (which unrealistically assumes regular circular motion for the Moon) becomes progressively imprecise for solar distances much larger than the distance of the Moon, and Aristarchus obtained a nonsensical result. It is, however, in essence a parallax method.

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

It was proposed by Edmund Halley in 1716, that the transit of Venus over the solar disc be used to derive the solar parallax. And so it was done in 1761 and 1769. There is the famous story of the French astronomer Guillaume Le Gentil, who travelled to India to observe the 1761 event, but didn't reach his destination in time due to war. He stayed on for the 1769 event, but then there were clouds blocking the Sun...

Related Topics:
Edmund Halley - 1716 - Transit of Venus - 1761 - 1769 - Guillaume Le Gentil - India

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

The use of Venus transits was less successful than had been hoped due to the black drop effect.

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

Much later, the solar system was 'scaled' using the parallax of asteroids, some of which, like Eros, pass much

Related Topics:
Asteroid - Eros

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

closer to Earth than Venus. In a favourable opposition, Eros can approach the Earth to within 22 million kilometres. Both the

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

opposition of 1901 and that of 1930/1931 were used for this purpose, the calculations of the latter determination being completed by

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

Astronomer Royal Sir Harold Spencer Jones.

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

Also radar reflections, both off Venus (1958) and off asteroids, like Icarus, have been used for solar parallax

Related Topics:
Radar - Icarus

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

determination. Today, use of spacecraft telemetry links has solved this old problem completely.

Related Topics:
Spacecraft - Telemetry

~ ~ ~ ~ ~ ~ ~ ~ ~ ~