ASTRONOMY/HERSCHEL
Meetings chaired by the convenor Richard Phillips unless otherwise stated

The Renaissance of Astronomy & Scaling the Solar System

Christopher Baddiley

Quinetiq, Malvern

2 April 2004

This extensive talk gave a quick overview of the history of astronomy from the 6^th century BC, when Parmenides discovered that the earth was spherical, to 1781, when Herschel discovered Uranus, and then went on the explain Newton's Laws of Motion and the determination of the size of the solar system – a complete 10-week course of astronomy lectures summarised in an hour and a half! The speaker achieved it by skipping over the mathematical sections, concentrating on the principles, and summarising historical discoveries in one sentence each.

He opened by showing a video picture by a webcam of the changes in the appearance of the Moon over a month. This demonstrated the phases and also an apparent change in size, which is due to the varying distance of the Moon from the Earth. It also showed a 'wobble' due to the elliptical orbit of the Moon around the Earth, in which it varies in speed, and its rotation at a constant speed on its own axis. The difference results in the same face of the Moon always pointing towards the Earth – nearly – which, over a month, shows as the 'wobble'. Also sometimes the Moon is above and sometimes below the Earth's orbit plane, which introduces another variation in the view.

He listed the Greek astronomers and their achievements, from Parmenides who believed that the Earth was a sphere in a spherical universe in the 6^th century BC, and Thales (624-547 BC), who successfully predicted an eclipse and thus stopped a war, to Ptolemy (120-180 AD) who perfected the geocentric system with epicyclic movement of planets, which was adopted by the Roman Catholic church and enforced until the 17^th century. Pythagoras (572-500 BC) believed the earth was spherical; Eudiscus (408-355 BC) and Aristotle (348-322 BC) developed the concept of concentric spheres and Aristarchus (310-230 BC) proposed a heliocentric system with the Earth revolving round the Sun once a year. Eratosthenes (276-196 BC) who measured the Earth's circumference and Hipparchus (140BC), who made a star catalogue, determined the precession of the equinoxes and measured the distance to the Moon, were amongst others listed. Unfortunately, time prevented him reviewing Chinese, Egyptian and Babylonian astronomers.

This led on to the medieval astronomers. Copernicus proposed in 1530, as an aid to calculation, a heliocentric solar system. Pope Gregory devised his new calendar. Leonard Digges, an Englishman, invented the telescope in 1570, preceding Hans Lipershey, the Dutchman usually credited with that invention in 1608. Bruno was burnt in 1600 for saying that the Earth moves around the Sun

Kepler, initially assistant to Tycho Brahe, (1546-1601) formulated his three Laws of Planetary Motion in 1609 and 1619:

• The orbit of each Planet is an ellipse with the Sun at one focus and the other focus empty
• The radius vector sweeps out equal areas in equal times
• The siderial period (of rotation around the Sun) squared is proportional to the mean distance (from the Sun) cubed.

Other key dates, people and events are:

Leonard Digges (d. 1571) invented the telescope.

1639 Horrocks & Crabtree predicted and saw the Transit of Venus on 24 November.

1651. Riccoli produced a lunar map

1655 Huyghens discovered Titan, the largest moon of Saturn and markings on Mars

1665 Newton formulated his Laws of Motion:

For every action there is any equal and opposite reaction.

Flamstead. first Astronomer Royal, founded the Royal Observatory at Greenwich.

1676 Romer determined the speed of light correctly within 10% using the position of the moon Io of Jupiter to determine the time for light to cross the Earth's orbit round the sun (18 minutes). Halley produced a catalogue of the Southern stars. Newton published Principia Mathematica proving the solar system was heliocentric

1705. Halley's Comet appeared and he recognised it as having appeared before and predicted its next appearance

1725. Flamstead's tables published postumously

52. Lacalle's star catalogue

1762. Bradley's star catalogue and the measurement of aberration in starlight

1767. Maskelyne's nautical almanac

1776. Harrison's clock for navigation

1781. Herschel's discovery of Uranus.

The Scaling of the solar system was the final topic of this talk. The speaker showed methods of

• measuring the distance to the Moon by parallax (Aristrachus's method) and from the radius of the Earth (Hipparchus's method)
• measuring the radius of the Earth by Eratosthenes
• the precession of the equinoxes (Hipparchus)
• the Earth's distance from the Sun using the transit of Venus in 1767
• the distance of planets from the Earth
• the distance of the near-by stars by parallax
• the mass of the Earth and the gravitational constant
• the mass of the Sun from the period of the Earth
• the masses of double star systems.

Most of these methods use simple mathematics involving mass, velocity, acceleration, gravity time, and angles. The formulae were available but not discussed.

He also described the discovery of Neptune by calculations based on peturbations of the orbits of other planets and described and explained the Transit of Venus. Peturbations also affect the orbits of comets, which may be captured by the solar system, and asteroids.

Donald Lovell