Parallax
The apparent shift of a nearby object against a distant background when seen from two places; the size of the shift gives its distance.
Hold a finger at arm's length and look at it with one eye, then the other. It seems to jump against the background. That shift is parallax, and it is one of the basic ways of measuring distance: the farther away something is, the smaller its shift for the same separation between viewpoints. Know the separation, measure the angle, and simple trigonometry gives the distance.
In the 18th century the great unknown in astronomy was the distance from Earth to the Sun, the astronomical unit. Kepler's laws gave the relative sizes of the planets' orbits, but not their true scale. In 1716 Edmond Halley proposed a solution. During a rare transit of Venus, when Venus crosses the face of the Sun, observers far apart on Earth would see it follow slightly different paths and cross at slightly different times. Comparing their timings would give Venus's parallax, and from it the Sun's distance.
Expeditions travelled the world for the transits of 1761 and 1769, including James Cook's voyage to Tahiti. Weather, war and the blurring "black drop" effect made precise timing difficult, but the results began to converge on today's value of about 149.6 million kilometres. The Sun's parallax, the angle that Earth's radius would span as seen from the Sun, is only about 8.79 arcseconds.
For stars the shifts are smaller still, and astronomers measure them using the whole of Earth's orbit as the baseline.
Coming up in future stories
- The Astronomer Who Came Home Dead 1760–1771 · Mauritius, Pondicherry (India) and Paris