July 24, 2011, 5:56 pm
Herschel and the Orreries
By Henry Woodbury
Self-taught astronomer William Herschel discovered the planet Uranus through a series of observations in the winter and spring of 1781. The discovery was widely published the following year. “Instantly,” writes Richard Holmes in his splendid history The Age of Wonder, “all orreries were out of date” (p. 105).
While Dynamic Diagrams’ digital orrery include Uranus (and Neptune), it is inaccurate in a way common to almost all maps of outer space: that of relative distance. Uranus is more than twice the distance from our sun as Saturn. The distance to the stars is ever more impossible to project. While Herschel was one of the first astronomers to conceive of deep space, not even he guessed at its vastness. In a footnote Holmes writes:
No astronomer yet had the least idea of the enormous distances involved, so huge that they cannot be given in terms of conventional ‘length’ measurements at all, but either in terms of the distance covered by a moving pulse of light in one year (‘light years’), or else as a purely mathematical expression based on parallax and now given inelegantly as ‘parsecs.’ One parsec is 3.6 light years, but this does not seem to help much. One interesting psychological side-effect of this is that the universe became less and less easy to imagine visually. (Holmes’ emphasis, p. 88)
Here is a challenge to champions of visual explanation and yet I fear Holmes is right. An example can be drawn from the use of parallax to measure astronomical distances. In another footnote, Holmes writes:
As with road directions, a diagram is a much better way to explain parallax than a written sentence. But it is interesting to try…. Stellar parallax is a calculation which is obtained by measuring the angle of a star from the earth, and then measuring it again after six months. The earth’s movement during that interval provides a long base line in space for triangulation. (p. 90)
Could Pantheon Books not provide Holmes a designer? Let me try a sketch:
The difference in angles A and B allow a simple trigonometric calculation with a baseline of about 300 million kilometers (left). However, astronomical distances are so great, the actual angles are nearly equivalent (right).
William Herschel and other 18th-century astronomers did not have the instruments to measure that difference. It wasn’t until 1832 that Thomas Henderson used parallax to calculate the distance to our closest star, Alpha Centauri. It wasn’t until the 1920s that Edwin Hubble was able to calculate distances between galaxies using the red-shift method (p. 90).
Holmes describes one other picturesque scene, a “human orrery” played by the poet John Keats as a schoolboy:
Keats did not recall the exact details, but one may imagine seven senior boy-planets running round the central sun, while themselves being circled by smaller sprinting moons (perhaps girls), and the whole frequently disrupted by rebel comets and meteors flying across their orbits. (p. 113)
One must assume that like mechanical orreries and the dD Orrery, the position of the planets was calculated in reference to the sun, not to each other.
Hubble needed a yardstick to calibrate redshift (discovered by Vesto Slipher), which he found in cepheids, leading to those three famous astronomical letters: VAR!
Posted by Eugene on July 26, 2011 at 3:27 pm