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 |
|
130 BC - |
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 |
| Late 1500s Solar System Geometry |
Brahe
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. |
| c.1610 Astrophysics |
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. 
Used with permission
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. |
| 1660 Astrophysics |
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. |
| 1676 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. |
| 1680 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. |
| 1728 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. 
Used with permission
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. |
| 1776 Astrophysics |
Maskelyne weighs the Earth 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. |
| 1810 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. 
Used with permission
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. |
| 1871 Astrophysics |
The thickening plot of aberration |
| 19th century Solar System Geometry |
Transits of Venus |
| 1901 Solar System Geometry |
Eros geometrical measure |
| 1905 - 1913 Cosmology |
Einstein's theories of relativity |
| 1926 Astrophysics |
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. |
| 1930s Cosmology |
Movement and structure of galaxies |
| 1932 Astrophysics |
The discovery of Neutron Stars |
| 1950s Cosmology |
Big Bang cosmology 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. |
| 1989 Stellar Geometry |
Hipparcos spacecraft measures a million stars |
| 1990 Stellar Geometry |
Doppler wobbles reveal other planetary systems |
| 1996 Solar System Geometry |
Official definition of the AU |
| 2004 Stellar Geometry |
SuperWASP
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. |
| 2015 Stellar Geometry |
Darwin space telescopes search for extrasolar planets |
![Comet Borrelly [Image:NASA, JPL]](/openlearn/files/ole/imported/9120/PIA03500_NASA_JPL_prog.jpg)
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After Einstein I doubt Astronomy
I don't agree with astronomy after the Einstein experiments. I am also not a believer, in neutrons either. Or the other particles that sprung into existence after the neutron.
I believe that a similar thing has taken place before. When great mathematicians calculated the paths of planets, necessary to make earth, the center of the universe.
Although the paths were mathematically indisputable. They relied on the earth being in the center of the universe. The math was mind boggling, and taken to extremes of accuracy, the common fellow could not duplicate.
The common fellow could not find interest in understanding the math and formulas, now comical formulas that made no sense to him, and rightly so.
Whole generations memorized strange spiraling paths of planets in our solar system. For nothing. People dedicated their lives to this spiraling of the planets. At the time you could die for even mentioning that most people were total fools, poor mathematicians, and even worse scientists, for believing the earth was the center of the universe.
There was no proof for the common man. Just strange hieroglyphics that made the wrong people with wrong ideas, the "higher class" of citizen. The hieroglyphics made the wrong people the teachers, teaching nothing. All because they belonged to the same cult.
Of course the calculations were totally useless and unrealistic, however they persisted for many years across many countries boundaries. You would think men of such exacting complex math would have realized the truth. Evidently not. So today, I submit that it has happened again to most of the citizens of earth.
I believe that we have done something similar with our calculations and basics of matter and gravity.
However it will probably require hundreds of years, and imprisoned scientists, to bring it to light. Even though it was totally proven by an amazing lot of scientists. Scientists that were shunned by governments.
I believe when good scientists requested that poor scientists prove a force of attraction exists within this universe. And not a soul could, real science and education were doomed. As attraction was taught as the basis, of understanding gravity, and matter, to many a young mind.
Just my opinion on it.
Sincerely,
William McCormick
Doubts about astronomy
I am guessing that you are referring Eddington's experiment to measure the deflection of starlight during an eclipse. While the results of that were marginal, it has been repeated many times since and in particular, as we have discussed before, the Hipparcos Mission verified to to high accuracy over more than 170 degrees angle from the Sun. The modern results are indisputable.
Given we have neutron factories churning them out all over the world, you might as well tell a fireman you don't believe in water.
Are you talking about the Ptolemaic System? There was no "spiraling" in that, the whole point was that they thought objects moved in perfect circles! That's the only system I know that put the Earth at the center of the "universe" or what we would now call the inner Solar System. To be honest, I don't think you have the faintest idea what you are talking about.
It would be many centuries before Tycho Brahe's years of careful observations would provide enough detail for anyone to work out the planetary paths well enough to show they were based on ellipses, not circles. The men of Ptolemy's time had no possible way of working that out.
Now what are you talking about William. Your hero, Ben Franklin, knew electrostatics both attracted and repelled, that is how he created the instrument with which he tested polarity by hanging a pith ball between two bells. If they were oppositely charged, the ball would touch one picking up the same charge and then be repelled by that one and attracted to the other causing it to swing between them. If the bells had the same charge, the pith ball would be repelled by both so the instrument didn't ring. Oh and don't waste our time repeating the old Le Sage gravity nonsense, I am talking about electrostatic force. His idea for gravity would vapourise the planet instantly as was pointed out to him at the time and again as I have proved to you several times before.
George
My hero Benjamin Franklin had
My hero Benjamin Franklin had to speak using simple words so the highest English scientists could understand his amazing vision. Ha-ha.
Actually his use of the word attraction was merely a description of what was still at that time, pre American Revolution, unknown ground on a subatomic level.
However the current atomic model is comically flawed with its notion that there is an attraction force present within the atom somehow holding particles of electricity in orbit, around a proton. Scientifically, there is no such force.
Matter is made of a nucleus, basically a ball or sphere, of particles of electricity. Held in that shape because a sphere offers the least resistance to bombardment from all sides.
Very high velocity, free, individual, particles of electricity, invisibly, and obviously undetected by modern science, bombard matter, giving matter its structure and density. During a bomb blast the rather inert and low potential energy matter, making up the bomb, creates a structure that slows down, blocks, the undetected high velocity, particles of electricity. To give us X-rays, light, heat and whatever else goes with that particular type of bomb.
The different radiations, are usually caused by either volume of the event, or the actual physical size of the particles at the surface of a radio active substance. Very large atoms can cause, a polarization within sensitive equipment. Because of the actual rays, and the distance between the rays, leaving a radio active substance, a substance that is made up of very large atoms. Modern scientists often mistake this for, different particles, or different radiation.
The all repulsive universe is working well as we speak. The fact that no scientist can ever prove an attraction force exists, is all the proof you need to look else where. From there you can work out the error in your math, in calculating the earths destruction from very high velocity particles of electricity.
If you look at the original X-ray machines. Those machines were proven to spew particles of electricity at very high velocity. The voltages were in the millions of volts. As the voltage went higher, instead of the particles being stopped, or absorbed, in the heavy blocks of steel. many inches thick. They started to penetrate the plate effortlessly.
If you really want to get to the bottom of it you will have to consider that there is just one subatomic particle in our universe. Very simple basic universe actually. That allows the most complex creations ever dreamed of.
Sincerely,
William McCormick