Astronomy timeline

Eratosthenes estimates the radius of the Earth
The first steps were taken by Eratosthenes who combined reports of distances, in paces, from Aswan to Alexandria and then measured the difference in angle of the Sun above the horizon at noon (i.e. greatest angle) at the two places.

The circumference of the Earth is then [360 degs/angle difference] times the number of paces, and the radius is the circumference/[2 pi]. Solar parallax is then defined as [radius of the Earth/distance from Sun to Earth]

Guesses of the distance to the Sun
Up to the mid seventeenth century there was agreement that the sun was remote from the earth. However, the various guesses made were all far too low (for example, the Greeks were off by a factor of 20) and based on woolly arguments such as what distance would fit in harmoniously with the sizes of the planets! Real measurements were first made from observations of the parallax of Mars, which can be made whenever it passes near to a bright (but distant and practically fixed) star. But they were not very consistent, other methods were needed.

Maskelyne weighs the Earth
Each of the really fundamental force laws of nature is very general, and needs to be linked to our own particular universe by measuring the value of a universal constant embedded in the law. In the case of Newton it is G, the gravitational constant (as distinct from g which is merely the acceleration at the surface of the Earth and so of local interest only).

Once you know G here, it is assumed that you know it everywhere in the Universe. Maskelyne measured it in Scotland, in fact, in 1776. A mountain pulls a pendulum sideways and the Earth pulls it downwards.

Measuring the (very small) deflection allows the pull of the mountain alone to be deduced, and that separates out the value of G from the value of the product GM which occurs in Newton 's analysis of gravity. The method may sound crude but it was a long time before a markedly more accurate way was found.

The thickening plot of aberration
Fizeau made some of the first terrestrial measurements of the speed of light (Romer's method was astronomical), and went on to start to clarify the mechanism of stellar aberration. Airy made a bizarre but brilliant extension of the work in looking at aberration with a water filled telescope. This observation, by hindsight, brought him to the brink of discovering special relativity, but the theoretical framework to support it was not in place - that took another 50 years. Stellar aberration can only be properly analysed and understood in terms of special relativity.

Transits of Venus
The two transits in the 19th century (1874 and 1881) benefited from substantial international backing, for a large number of expeditions. With so many results to pool, random errors were indeed reduced. But there was clearly a danger of systematic errors, and those could only be tackled by looking at other, independent, methods.

Eros geometrical measure
Eros, an asteroid, is part of a cluster between Mars and Jupiter, and can be used to successfully make geometrical measurements of the scale of the Solar System - and because it's closer to Earth than Mars, it's easier to measure.

Einstein's theories of relativity
Einstein's theories of relativity are dated 1905 (special) and 1913 (general - i.e. gravity) Astrophysics would have come to a halt at the beginning of the 20th century without relativity to bring an understanding of light, nuclear reactions, and the large scale structure of the Universe. It would have come to a halt again in the 1950's without particle physics to feed speculation about detailed processes in the very early, very hot, compact Universe.

Movement and structure of galaxies
Herschel first saw galaxies around 1785 but, surprisingly, it was not until about 1930 that a clear idea of their size and structure emerged, following the recognition of individual star types within galaxies. This allowed estimates of the luminosity, and hence distance, of different types of galaxies, to be made on the basis of their visible structure. Galaxies are seen in groups, and around the 1950's the dynamics of these clusters began to be understood, pushing distance estimates even further. Hubble and others had already recognised the overall streaming motion called the recession of galaxies in 1936. Fitting all these dynamical properties together has required a huge amount of observational and theoretical work, and is still in progress.

The discovery of Neutron Stars
At this point, the timeline reaches a gateway which leads into a vast new area of modern astronomy - the link between astrophysics and the physics of elementary particles. The understanding of stellar structure triggered by Eddington rapidly came together with the discovery of the neutron by Chadwick in 1932, in Oppenheimer and Snyder's prediction of neutron stars, discovered by Jocelyn Bell Burnell and Anthony Hewish in 1967, in the form of pulsars. But that is another story, and we will not pass through the gateway.

Big Bang cosmology
The recession of the galaxies clearly implies that the Universe was more compact in the past, and the present abundances of light elements suggest strongly that it was also much hotter. How far back can we push these speculations? At this level, our ordinary ideas of distance are no longer of any use, and the story of links to the AU is at an end. Our ordinary ideas of astronomy are no longer a guide, and the story is entirely in terms of the quantum physics of elementary particles.

In his 1950 BBC radio series, The Nature of the Universe, Fred Hoyle mockingly called this idea the "big bang" considering it preposterous. Yet the theory - and the derisive term - have become mainstream, not only in astronomy but in society as well.

Hipparcos spacecraft measures a million stars
It is difficult to do better than Bessel from the Earth's surface because of turbulence in the Earth's atmosphere. From space, triangulation can reach out much further. A spacecraft called Hipparcos did an excellent job despite being accidentally launched into the wrong orbit. It catalogued a million distances to stars in our Galaxy. A planned successor, GAIA, should be able to do a billion.

Doppler wobbles reveal other planetary systems
A planetary system rotates about its centre of gravity (or more exactly centre of momentum) which is displaced from the centre of the parent star (e.g. the Sun). Doppler measurements of the mean line-of-sight speed of the star with respect to the Earth will, if the star has a planetary system and if they are accurate enough, therefore show a periodic oscillation as the star rotates off-centre. Dr G. Marcy and his co-workers were pioneers in this method of detecting extra-solar planets.

Official definition of the AU
The definition of the AU as a quantity to measure is "the (mean) radius of the Earth divided by the angle (in radians) that the radius subtends from the centre of the Sun". If those distances could be surveyed directly (i.e. with, in principle, a tape measure and theodolite) that would be the end of the story, everything would be given in metres. Clearly they cannot be measured directly, and the indirect methods involve the value of the gravitational constant G, itself rather poorly known. The International Astronomical Union (IAU) sidesteps this so that all the astronomical distances which depend on the AU can be put on a common footing. So we have two units, the metre and the AU A very similar dodge is used for very large distances, results for many of which depend on the (uncertain) value of the Hubble constant. So the Hubble constant is treated as a unit in itself. (It is actually a unit of inverse time i.e. frequency, but that is another story!).

What is the Astronomical Unit?