Introduction to Astrophysics Lecture 2: Landmarks in the history of astronomy NB: The material in this lecture is covered in almost any introductory astronomy textbook, so it is not really necessary to take notes. The slides will also appear on the WWW.

Prehistoric astronomy There is endless evidence of prehistoric attempts to measure various astronomical phenomena. The main motivation would be to use the sun and the moon to judge times, for example for planting of harvests. Many civilisations were very sophisticated, eg in China in 2159BC two astronomers were executed for failing to predict an eclipse!

The Ancient Greeks By far the most famous early astronomers are the ancient Greeks. Between about 500BC and 100BC, they built a picture of the Universe which dominated for over 1000 years.

Thales ( BC) Realised that celestial objects were at different distances, that the Earth was spherical, and that the light of the moon was reflected sunlight. Pythagoras ( BC) Produced the first geocentric model of the Universe, with everything making perfectly circular orbits around the Earth. Invented the idea of epicycles, later `perfected’ by Ptolemy. Plato ( BC)

Aristarchus ( BC)  He created the first heliocentric cosmology; that is, he was the first to propose that the Earth, and the other planets, went around the Sun. 1,750 years later Copernicus will claim the credit.  Aristarchus is also famous for devising a way of measuring the size of the moon in terms of the size of the Earth.  Hipparchus later used this to conclude that the moon had about one quarter the radius of the Earth (modern value 0.27) and that its distance was about 60 Earth radii away (modern value 60.4). However, his calculations did not give absolute values; only those relative to the unknown radius of the Earth.  Aristarchus also measured the relative distances of the Sun and Moon. He underestimated that badly, but even so realised that the Sun was bigger than the Earth.

Eratosthenes ( BC) Measured the circumference of the Earth with amazing accuracy. He did so with a particularly powerful piece of observational technology, namely a long stick. Hipparchus ( BC) Powerful insights into many aspects of astronomy. Invented the magnitude scale, worked out the size of the moon, developed star and eclipse catalogues. Perfected the geocentric model with epicycles. It will go unchallenged for 1300 years. Ptolemy (127 - ??? AD)

An example of an epicycle.

Nicolaus Copernicus ( ) Credited with the heliocentric model of the Solar System. He divided the planets into the `inferior’ ones closer to the Sun than the Earth, and the `superior’ ones outside the Earth’s orbit. Only planets out to Saturn were known at that time. The orbits were all taken as circular.

Finally we shall place the Sun himself at the centre of the Universe. All this is suggested by the systematic procession of events and the harmony of the whole Universe, if only we face the facts, as they say, `with both eyes open'. Nicolaus Copernicus

Tycho Brahe ( ) Almost as famous for his silver nose (he lost the original in a duel) as for his observations. With the support of the King of Denmark, he developed instruments of unprecedented quality, capable of positional accuracies of one arcminute. He is less remembered for his cosmological model, an attempted compromise in which the Sun goes round the Earth but the planets round the Sun.

Johannes Kepler ( ) Painstaking studies of Tycho’s observations led him to the now- accepted conclusion that planets moved not on circles but on ellipses. He went on to formulate his three laws of planetary motion, which we will be studying in a later lecture.

Didn’t invent the telescope (and might well not have dropped cannonballs off the leaning tower of Pisa either). Galileo Galilei ( ) But he is considered the inventor of the modern scientific method, with its emphasis on experimental verification of theoretical models.

Galileo was the first to properly exploit the telescope for astronomical purposes. Considering that his original telescope had a magnification power of only three, he made amazing new discoveries, including the four large moons of Jupiter, sunspots, and the rings of Saturn. His belief in Copernicus’s heliocentric Universe nearly led him to a nasty end. GALILEO NOW

Isaac Newton ( ) Widely acknowledged as one of the greatest scientific geniuses of all time. Co-inventor of calculus and founder of the modern theories of dynamics and optics, he also developed a theory of gravity. His theory explains Kepler’s Laws, and shows that the gravity we feel on Earth is the same as the gravity that governs planetary orbits.

Despite the development of the telescope, it would be many years before the remaining planets of the solar system were discovered. Uranus was discovered in 1781 by William Herschel.

Neptune was first observed in 1846, after its position was predicted by Adams and Leverrier by analysing perturbations to the motion of Uranus... … while Pluto was not discovered until 1930, and its moon Charon only in 1978.

Next on `Introduction to Astrophysics’ There will be no class tomorrow morning. The next lecture will be on Monday at 11:00.