ASTRONOMY ( HERCHEL)

Measuring the Universe

Martin Griffiths, Senior Lecturer in Earth and Space Sciences, Glamorgan University,on 5 October 2001

This talk was an historical and cultural examination of our unfolding understanding of the cosmos and our attempts to determine the distance, firstly, of the Sun and the planets, then of the nearby stars and finally of the size of our galaxy and the distances of the others to the uttermost reach of our present-day telescopes.

Mr Griffiths dealt with early measurements of the size of the Earth, the Copernican Revolution and the first real step in modelling the solar system made by Johannes Kepler's analysis of Tycho Brahe's accurate observations. We were entertained by character sketches of these two astronomical giants. Although Kepler revealed the relative distances of the planets from the Sun, the final piece in the jigsaw was the actual distance of one of them from the Sun.

We learned the extraordinary story of the attempts and failures of so many astronomers and navigators from rival European countries to determine what was known as the `solar parallax' from observations of the `transit of Venus' in the 17th and 18th centuries. As Venus orbits the Sun, infrequently it can be seen in silhouette against the solar disc. Observations of such transits of the Sun from points on the Earth of measured separation enabled the distance of Venus to be calculated and hence a scale to be derived with which, coupled with Kepler's third law, the distances of all the planets was determined. It was necessary to travel to quite remote parts of the Earth to observe this phenomenon and we could only admire the lengths to which these pioneers were prepared to go and commiserate with the hardships and misfortunes
which befell them.

Once the diameter of the Earth's orbit was known, it became possible to measure the distance to a star by measuring the difference in its direction when observed from the Earth on either side of its orbit (i.e. at 6-monthly intervals). Observers including William Herschel failed in the attempt to do this because the stars were so distant that their `stellar parallax' was smaller than they were able to observe with the instruments of the day. Frederick Bessel succeeded in 1838 in measuring the parallax and hence determining the distance of the star 61 Cygni.

Mr Griffiths then recounted the history of measurement of more distant stars in our galaxy and in others by the discovery that some pulsating stars, called `Cepheid variables', have a period of variability which is related to their intrinsic brightness. By comparing their apparent brightness with the brightness derived from their period, their distances could be calculated. Such stars were observed in other galaxies which enabled the vast distances of these `island universes' to be measured. The distances of galaxies too remote for their `Cepheids' to be observable are measured using other `standard candles' such as `Type II supernovae' - very bright exploding stars which always have the same intrinsic brightness.

Finally, Edmund Hubble related the reddening of light, caused by the stretching of light-waves as galaxies speed away from each other (and us) to their distance (measured using Cepheid variables). This cosmological `red-shift' enables us to measure the distance of galaxies which are a substantial way to the edge of the observable Universe.

This quick excursion through the unimaginable gulfs of Space with a variety of `rulers' was conducted with humour and with vignettes of the human fortitude and failings of the remarkable pioneers in this great challenge to mankind that of measuring the Universe.

Richard Phillips