History of Astronomy Example Exam Question Hints and Suggestions

The facts essentially fall into three categories: what happened, who did it and when it happened. The first of these is probably the easiest, since there is some logical ordering to help fill out your memory; the second and third are purely memory-based.

This gives a basic skeleton. You need to flesh it out with details (what are the advantages of the convex eyepiece, or of the achromatic lens? What were the engineering improvements of the 19^th century?), and there are things you could add, such as the introduction of photography in the mid-19^th century and the consequent need to consider the focus for blue light (which even in an achromatic objective is not the same as the focus for red/yellow/green).

• Around 1700, refracting telescopes used Keplerian optices with simple lenses. They therefore had to be very long and cumbersome to minimise chromatic aberration. They were, however, established and understood technology. Reflecting telescopes were newer (mid 17^th century), and very much more difficult to make (they relied on casting a curved metal mirror, which was challenging for the technology of the day) and maintain (speculum metal tarnishes and has to be reground); the ligh gathering was also poorer for the same size of objective (speculum's reflectivity is not as good as modern mirrors). Consequently, although reflectors were known to be immune to chromatic aberration, they were not much used.
• Around 1800, refracting telescopes were relatively small instruments, but capable of high precision and installed on good mountings. Reflectors were much bigger, so with better light-gathering power, but much more crudely mounted (usually on alt-az mounts) because of their great weight, and had some optical problems because speculum deforms easily. Therefore, at this time refractors are better precision instruments; reflectors are better for faint objects.
• Around 1900, reflecting telescopes were beginning to become established as front-line research instruments: they were still larger than refractors, but now equally precisely mounted and with equally good-quality optics. Refractors had become very long again, owing to the desire for larger objectives, and were once more very unwieldy and difficult to use. At this point refractors had few remaining advantages over reflectors as research instruments.

In case (iii), the key advance is the development of silver-on-glass (later aluminium-on-glass) mirrors. These remove the disadvantages of speculum: they are easier to make and maintain, not so temperature sensitive, and have higher reflectivity.

The technological advances are included in the discussion above: you need good mounts (from 1680s onward, improving steadily as the Industrial Revolution improves the quality of precision engineering) and good optics (requires the achromatic objective, as invented by Dollond in the 1750s, and the improved glass technology of Fraunhofer, which made larger achromats possible). The successful measurement used a heliometer, which was a 19^th century invention, though it isn't clear that this was really needed.

Choosing the right star required astronomical advances: Galileo wanted to use double stars, which he thought were chance alignments. Michell demonstrated that this was very improbable (too many close pairs to arise by chance) and Herschel later proved by observation that binary stars do orbit each other. This also showed that stars are not all equally bright, which means that using the brightest stars as targets won't necessarily work - hence the shift to more promising stars of high proper motion.

Demonstrating reliability required advances in experimental technique: Bessel's approach involved careful calibration of the instrument, and his paper is much more modern in its careful treatment of experimental error than most work of the time.