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Author: Alan Cooper

Astronomy timeline

Updated Tuesday, 1st June 2004
How do the Transits of Venus fit into the development of modern astronomy?

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The Transit of Venus might seem at first glance to be little more than an interesting diversion - however, it's played a vital part in the development of the history of astronomy, and has been right at the heart of the development of science, as our timeline shows. Indeed, as you can see here, study of transits of other stars is integral to the development of major theories in astrophysics and cosmology, as well as the search for the AU.

c250 BC
Solar System Geometry

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.
graphical representation of the equation described
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]

130 BC -
AD 1400

Stellar Geometry

Early star catalogues
Astronomy as a science cannot get started without star catalogues, and the first ones were produced a very long time ago and were remarkably accurate and reliable. (There were no lenses, and so no telescopes, but they used extremely long sighting lines). Two particularly dedicated early astronomers are Hipparchus (Greece , c.130 B.C.) - hence the HIPPARCOS satellite - and Ulugh Begh (Samarkand c. 1400, murdered because his colleagues were afraid of his knowledge).
Before 1600
Solar System Geometry

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.

Late 1500s
Solar System Geometry
Tycho Brahe (1546- 1601), son of a Danish nobleman, was so struck as a child by an eclipse which occurred at the predicted time that he pledged his life to astronomy, and his family's wealth supported his work. He contributed to many areas of astronomy but there are two for which he is especially remembered. In 1572 he saw, and recognised the significance of, a bright new star, now known as Tycho's supernova. He also plotted the tracks of the planets with unprecedented care and passed the results on to one of his students, Johannes Kepler. The tracks are very complicated as seen from the moving platform of the Earth, but thanks to the accuracy of the data, Kepler was able to extract underlying simple rules of planetary behaviour, which Newton was able to explain through his even more fundamental laws of motion and gravity.
Kepler discovers the rules of planetary motion
Johannes Kepler assembled many years of planetary measurements, especially from Brahe, and found purely empirically (ie not deduced from an underlying theory) that they obey 3 simple rules, of which the one we need here is that planetary distances are proportional to the 2/3 power of their orbital period (their "year"). Or in symbols: T2 is proportional to R3.
graphical representation of the equation described
The underlying laws are those of Newton, written in absolute terms but involving one of the six universal constants (see The Gravitational Constant). So measuring the planetary "years" - easy for the closer ones if not for the very distant ones - gives all the RATIOS of all distances. That is why it was so essential to measure one distance in absolute terms. We can then, quite literally, relate everything right back to the length of Eratosthenes' stride.
Newton's theories of dynamics and gravity
Newton greatly clarified the ideas of earlier scientists such as Galileo, and set them in mathematical form so that they had predictive power. He then created a theory of gravity and tested it on the Moon. In the process, Kepler's rules for the planetary orbits appeared as straightforward consequences of the laws. So his work is the mathematical structure behind all orbital analysis until the mid 20th century, when Einstein's general relativity cleared up some anomalies left by Newtonian analysis, and in its turn made predictions for extreme conditions such as neutron stars, for which Newton 's Laws fail to be even approximate.
Solar System Geometry
Romer measures the speed of light
It's not hard to show the speed of light is very high (e.g. you and a friend with a horse; both have a lantern and a hat to put in front of it to make a signal - you can imagine the rest). But how fast? Infinitely fast? Ole Romer in 1676 used the satellites of Jupiter in much the same way as the lantern, with Jupiter as the hat. He discovered the timings of the satellite eclipses or transits changed as Jupiter came closer to us (or moved away), in our different orbits. He got 225,000 km/sec - 30% too low, but showing clearly that it was not infinite.
Solar System Geometry
Halley suggests using Venus' Transits to measure the Astronomical Unit
If we can triangulate any solar system body it gives the scale to the whole Solar System thanks to Kepler's Laws. So we want to choose the body closest to us which is orbiting the Sun, to give the best chance of getting a measurement. The candidates are Mars, any asteroid which comes within the orbit of Mars, and Venus. The crucial point is that it needs something as a backdrop against which to measure the chosen body. A frequent opportunity is given by watching Mars occult, or nearly occult, any distant star. Venus is inside Earth's orbit and so offers a special but rare opportunity when it passes between us and the Sun. The Sun itself provides the backdrop. We can then measure the distance of Venus and deduce the distance of the Sun. When it became possible to measure the distance of passing asteroids directly by timing radar pulses, measuring the AU was a solved problem. It was Halley who suggested Venus would be perfect for this; Delisle suggested a similar, but distinct, method.
Solar System Geometry
Bradley discovers stellar aberration
Since light does not travel infinitely fast, the direction in which we perceive a given star is changed slightly by the speed of the Earth in its orbit - the effect will be in the opposite sense in six months from now when the Earth will be on the opposite side of the orbit. This was an amazingly clever idea for 1728 when it was measured by Bradley. All stars show this to the same extent, it is not related to the distance of the star.
graphical representation of the equation described
The aberration angle gives the speed - it was the first measurement of the speed of the Earth. Since we know the duration of a year this gives us the circumference and hence the radius of the Earth's orbit. In effect this upstaged transits as a method of measuring the AU, but these were new ideas and they had to be tested.

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.
graphical representation of the equation described
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.

Stellar Geometry
Bessel discovers the parallax of a distant star
The next step was to discover the distance of nearby stars in the night sky. The diameter of the Earth is far too short to hope to triangulate the distance to the stars. But the diameter of the Earth's orbit - by this stage known well enough to be used - provides a much longer baseline.
graphical representation of the equation described
Fortunately there is an abundance of very distant stars to provide an almost fixed backdrop. Bessel was the first person to achieve a successful measurement, of 61 Cygni (0.3 arc sec). Before that no-one really had any idea how far away the stars are. Measuring stellar distances is called astrometry.

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.

19th century
Solar System Geometry

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.

Solar System Geometry

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.

1905 - 1913

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.

Eddington makes the sun shine
There was no good reason known for the stars to shine, and to shine for so long, before Eddington (1926) calculated the balance between gravity holding stars together and nuclear energy tending to blow them apart. Huge amounts of subsequent computation have led to sufficient understanding of stars to know how luminous different types of stars must be. By recognising types of stars in other galaxies, galactic distances can be found.
This in turn determines the age and size of the whole observable universe. Successive links have taken us from a walk to Alexandria to an imaginary journey to the limits of the universe!
The temperature of the early universe can be found by the helium produced in thermonuclear reactions, so we know directly and for sure that it was very hot.
The idea of a Big Bang is not so direct, it involves a lot more assumptions.

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.

Stellar Geometry

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.

Stellar Geometry

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.

Solar System Geometry

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?

Stellar Geometry
SuperWasp is an array of specialised digital cameras with the ability to capture data about an enormous numbers of stars. Using this information it is possible to detect planets around other stars.
Stellar Geometry

Darwin space telescopes search for extrasolar planets
From the surface of the Earth we can get evidence of extrasolar planets, but it is indirect, we can never see them. A group of infra red telescopes in space, acting as an interferometer, could achieve the resolution needed to see planets round another star - and measure their AU for them! The ESA are currently planning to do this with the Darwin space telescopes. Perhaps somebody elsewhere has done it for us! (Besides good resolution, the array needs to "null" out the glare of the parent star.)


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