The move away from an Earth-centered view of the Universe

Web Guides

Many of the web links below have been traced with the help of these web sites:

• Imagine the Universe at the High-Energy Astrophysics Learning Center
• The NASA web site.

The above sites, and many more resources, are listed in 'Astronomy and space in UK colleges and schools, a resources guide for teaching' by Kerry Parker and Margaret Penson Oct 1999.

There are many web sites with useful and interesting information on astronomy and cosmology. Some have background information on the major figures whose work developed and improved our understanding of 'where we area'. You should note that some of these sites - including several of those suggested in this web guide - go well beyond the requirements of the Science for Public Understanding course. You should be selective in using these sites and not worry too much if you find some of the information rather difficult.

Early theories of the Universe: the Ptolemaic and Copernican models

To appreciate the early theories of the Universe it is useful to look at the site at the School of Mathematical and Computational Sciences at the History of Cosmology which has a brief history of cosmology, with links to short biographies of all the main figures involved from Ptolemy to Einstein. The same site at St. Andrews also provides a short summary of Greek astronomy, with biographies of the main figures involved, including Ptolemy. These pages are quite difficult, though, and go into much more detail than you need, though you may find them interesting. The greatest achievement of ancient Greek astronomy was Ptolemys model of the solar system. His geocentric model involved complicated circles and epicycles, to explain the retrograde motion of the planets.

Copernicus proposed a heliocentric model of the solar system. The Copernican model was much simpler than the complicated model of Ptolemy, though Copernicus had to add a lot of extra detail to try to make it fit better with observation. More data was required to allow the Copernican model to be accepted and even Brahe, the greatest astronomical observer of his day, did not accept the heliocentric model of Copernicus. He developed instruments which could measure the positions of stars very accurately. This enabled him to estimate the distance to a comet and show that it was further away than the moon, and not within the Earth's atmosphere as believed at the time, but even that did not convince him. It was Johannes Kepler who succeeded in establishing a heliocentric model of the solar system. After several years of painstaking calculations (using Tycho's data) on the orbit of Mars, he concluded that the planets move in ellipses, not circles. This insight enabled him to develop a very accurate heliocentric model. You can see Keplers Laws described here in both mathematical terms and through animations.

Galileo's telescopic observations
Much of Galileo's life and work was devoted to persuading others to accept the heliocentric model of the solar system. The site includes pictures of some of the instruments he used, details of experiments conducted and even letters exchanged between him and his eldest daughter. Galileo acquired one of the newly-invented telescopes and put it to good use in making astronomical observations. His observations of the moon convinced him that heavenly bodies were not 'perfect' spheres, challenging the accepted view of Aristotle and Ptolemy. Using his telescope, Galileo observed the moons of Jupiter, showing that bodies could orbit round centres other than the Earth.

Obstacles to the acceptance of a heliocentric model
One important objection to the Copernican model was that it could not easily explain the absence of stellar parallax and even when there was clear cut evidence against the Ptolymaic view, as with Galileo's observations of the moons of Jupiter, that evidence led to conflict with the religious authorities.

Grounds for confidence in the precision of our understanding of the solar system
It needed Isaac Newton to realise that the idea of universal gravitation could explain why the planets (and the moon) moved in their orbits. Then, for the first time, an explanation was provided for the observed motions of the bodies in the solar system.

Further evidence came as the existence of the planet Neptune was predicted mathematically to account for anomalies in the movement of Uranus. A young English mathematician John Couch Adams, used data on Uranus to predict the position of this'unknown' planet. Adams' however, had difficulties in persuading more senior astronomers to take his work seriously. So it was another independent prediction by the French astronomer LeVerrier that led Johann Galle in Berlin to look for, and find, Neptune. The anomalies in the motion of Uranus, instead of challenging the accepted model of the solar system, led to its greatest triumph it had led to the discovery of a new planet!

In the 20th century the two Voyager missions provided evidence of just how accurate and precise our knowledge of the solar system now is. The journeys of Voyager 1 and 2 took them close to all the outer planets. The Voyager missions sent back detailed information and photographs of the outer planets and many of their moons.

Evidence for an expanding Universe
A first step to providing evidence of an expanding Universe lay in being able to measure the distances to stars, and then to galaxies. Once the heliocentric model had become accepted, astronomers started to look for changes in the position of the stars due to parallax. Ths provides a way of measuring the distances to the stars. Follow the link to 'parallaxa', and then to 'how astronomers use parallax to measure distances to the stars'. A site at the University of Washington explains how the method works; follow the links to 'stellar parallax'. Photographic methods can help to show up the small movement of some stars against the fixed background.
The first person to observe and measure stellar parallax was Friedrich Bessell. He showed that the stars were immense distances away the closest is over 4 light years from Earth succeeding in measuring a parallax angle of 0.31 second to determine the distance to 61 Cygni in 1838.

However, measurement of parallax only works for the closest stars. To measure distances to more distant stars, a range of methods is needed. Between August 1989 and August 1993 the Hipparcos satellite measured the positions, parallaxes and proper motions of thousands of stars to an accuracy never attained before. This is more advanced than you need for the course.

A further important clue came from work on the colour of stars, which enabled them to be grouped by spectral class. The painstaking work of Annie Jump Cannon and her women colleagues at Harvard played a crucial role. By observing stars of known distance (or in clusters which are all at almost the same distance), astronomers then noticed a link between colour and intrinsic brightness. This then enabled them to estimate the intrinsic brightness of other stars from their colour and then, by comparing this with their observed brightness, to estimate how far away they are.

Another important piece of evidence came from the work of Henrietta Leavitt on variable stars. Other short biographies of Leavitt can be found on the site of the University of California at Los Angeles. Leavitt studied a particular group of stars called Cepheids and found that their period (from maximum brightness to minimum and back to maximum) was linked to their brightness. Hence they can be used to measure distances. This enabled other astronomers to measure distances to stars much further away, including those in other galaxies.

In 1912 Vesto M Slipher, working at the Lowell Observatory in Arizona, noted that most of the spiral galaxies he was observing had their spectral lines red shifted showing that they are moving away from us at high speed. The greater the shift, the faster they are moving away. This is an example of the Doppler effect.

Edwin Hubble measured the distance to several galaxies (by observing Cepheid variable stars in them) and also measured the speed at which they were moving away (from their red shift). This led him to Hubble's Law - the further away a galaxy is, the faster it is moving away. (Another, rather more advanced, discussion of this can be found on Ned Wright's site at UCLA.)

Estimates of the age and scale of the Universe
By 1920, a major issue in astronomy was whether our Milky Way galaxy was the whole of the universe, or one of many 'island universes'. This was the subject of a celebrated debate between Harlow Shapley and Heber Curtis. One thing it showed is that public debates are not the way in which scientific knowledge advances! We now believe that there are many galaxies of which the Milky Way is just one. The solar system is not at the centre of the galaxy, but about two-thirds of the way out on one spiral arm. In 1924, Edwin Hubble observed Cepheid variable stars in the Andromeda galaxy, and showed that it is over a million light years away another galaxy far outside the Milky Way.

How the Universes age and rate of expansion is determined,(select universe then select 'our universe' on left hand bar. Then scroll down and select 'The Age of the Universe') together with a range of questions and answers on this area, are provided by two sections of a NASA site. Whether the Universe continues to expand depends on the amount of matter in it and this prompted the search for dark matter.

The Big Bang theory and the evidence for it
The Universe is now thought to have begun with a Big Bang some 15 billion years or so ago. The evidence for this is the presence of the cosmic microwave background radiation (select universe - then Big Bang Tests on left hand bar - then scroll down and select 'The cosmic microwave background (CMB) radiation) discovered by Robert Wilson and Arno Penzias in 1964. The Cosmic Background Explorer (COBE) spacecraft has mapped this background radiation and details are available on its site. An alternative to the Big Bang, the Steady State, was proposed in 1948 by Hermann Bondi, Thomas Gold and Fred Hoyle. It is no longer accepted by most cosmologists as it has no explanation for the background radiation.
John Kierein explains why he feels the Big Bang theory is wrong. It is an interesting alternative perspective. What do you think? Why?

Space and time and Einstein's theory of relativity
In 1915 Albert Einstein published his General Theory of Relativity with the idea that the mass of an object alters the properties of space and time around it, this being confirmed by the eclipse of 1919. With this theory Einstein calculated the orbits of the planets and, with one minor exception, got almost the same answers as found with Newtonian mechanics. Indeed, the new theory predicted an anomaly with Mercury's orbit that had remained unexplained for more than fifty years. For much greater detail on both General and Special Relativity, far more than needed for the course, you could refer to Ned Wright's Relativity Tutorial.

NOTE:

If you are interested in having more information on the planets and seeing some spectacular views of them and their moons, together with details of famous astronomers, then look at the Views of the Solar system. Or for a multimedia tour of the nine planets and their moons go to The Nine Planets Multimedia Tour site. For the latest on spaceflight you would do well to go to the NASA and ESA (European Space Agency) sites. This is the web site of the Russian Space Research Institute which is in charge of the long-range planning and research of space programmes. Particularly useful sections on planetary science and space physics.

Links to a useful range of astronomy history resource.
For more detailed information and frequently asked questions (FAQs) and answers on the whole topic of Cosmology you might like to look at Ned Wrights Cosmology Tutorial

Finally the newly set up National Schools Observatory site promises to be a key resource of interest.

This site moves you through from a view of the milky way, to the earth, right through to the DNA in a leaf cell.

This site is good for astronomy.

Other resources
'Exploring the sun CD ROM'
Interactive multi-media exploration of the sun from the Solar and Heliospheric Observatory.