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Prof Martin Hendry School of Physics and Astronomy, University of Glasgow Captain Cook and the Cosmic Yardstick

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James Cook (1728 1779)

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Ptolemy: 90 168 AD

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Nicolaus Copernicus (1473 1543)

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Johannes Kepler (1571-1630)

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Galileo Galilei: (1564 1642)

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Galileo Galilei: (1564 1642)

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Galileo Galilei: (1564 1642)

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Sun Earth-centred modelSun-centred model

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How big is this angle?

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Earth Sun Venus

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Earth Sun Venus We can use Pythagoras theorem!

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Getting the Measure of the Solar System PlanetDistance Mercury0.39 Venus0.72 Earth1.00 Mars1.52 Jupiter5.20 Saturn9.54

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Getting the Measure of the Solar System PlanetDistance Mercury0.39 Venus0.72 Earth1.00 Mars1.52 Jupiter5.20 Saturn9.54 How far is an astronomical unit?

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http://www.scottishsolarsystem.org.uk

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...

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Eratosthenes: (c 276 195 BC) Syene Alexandria = 5000 stadia Circumference of the Earth = 250000 stadia

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Aristarchus (310 230 BC): Earth Moon distance from lunar eclipse

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Parallax Shift

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Even the nearest star shows a parallax shift of only 1/2000 th the width of the full Moon Parallax Shift

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Johannes Kepler predicted a transit of Mercury on 29 th May 1607 Instead, he discovered sunspots

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May 7 th 2003: Transit of Mercury

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Pierre Gassendi (1592 1655) Observed a transit of Mercury on 7 th November 1631

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November 24 th 1639 Jeremiah Horrocks (c1619 1641) The Founder of English Astronomy (Eyre Crowe, Walker Art Gallery) William Crabtree (1610 - 1644) Crabtree watching the transit of Venus (Ford Madox Brown, Manchester Town Hall)

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Edmond Halley (1656 - 1742) Halley travelled to St Helena in 1677, to map the Southern Skies He observed a transit of Mercury on November 7 th Transit observations could measure the astronomical unit!

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Methods relied on an accurate estimate for the radius of the Earth In 1669 Jean Picard (1620 1682) measured (0.2% error)

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The 6 th June 1761 Venus Transit o Results were disappointing: o Astronomical Unit lay between 77 million and 97 million miles (20% uncertainty) Bad weather Poor global coverage Black Drop Effect Systematic errors o Observations meticulously planned, for many years o Public outreach description by James Ferguson o Franco-British cooperation, despite being at war! o 120 astronomers observed from about 60 locations

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Captain James Cook The 3 rd June 1769 Venus Transit Endeavour arrived in Tahiti on 13 th April 1769 constructed a fort, and an observatory, at Point Venus Transit observed by Cook, Green and Solander

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Captain James Cook The 3 rd June 1769 Venus Transit After years of analysis, the results of the 1769 observations were published. e.g. Thomas Hornsby (1771): Cassini de Thury 1 A.U. = 93,726,900 miles (between 90 and 94 million miles) Happy is our Century, to which has been reserved the glory of being witness to an event which will render it memorable in the annals of the Sciences!

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Captain James Cook Mapping the Solar System Irwin Shapiro Bounced RADAR echoes from Venus in 1968 Shapiro Effect time delay also a test of General Relativity In 1976 IAU adopted:- 1 A.U. = 92,958,329 miles = 149,597,870 km

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The next transit: June 5 th /6 th 2012 www.transitofvenus.org

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Waikoloa, Hawaii

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www.transitofvenus.org

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http://www.scottishsolarsystem.org.uk