Dr Martin Hendry Dept of Physics and Astronomy, University of Glasgow Captain Cook and the Cosmic Yardstick

James Cook (1728 – 1779)

Retrograde motion of Mars

Early Greek Astronomy The Greeks inherited ideas from Babylonia and Egypt, but approached astronomy in a scientific way Plato Aristotle

Ptolemy: 90 – 168 AD Ptolemy proposed an Earth-centred Universe

Nicolaus Copernicus (1473 – 1543) In the true centre of everything resides the Sun The Copernican Revolution

Galileo Galilei: (1564 – 1642) The Observations of Galileo

Autumn/Winter 1609, observed: Craters of the Moon Moons of Jupiter Phases of Venus Published in 1610 Sidereus Nuncius (The Starry Messenger) In conflict with an Earth-Centred Universe The Observations of Galileo Galileo Galilei: (1564 – 1642)

The Moon is an imperfect world with mountains and valleys, just like the Earth The Observations of Galileo

Moons of Jupiter: supported idea of Earth moving through space, contradicted Aristotelian view of all motions around Earth The Observations of Galileo

Earlier observed phases of Venus

Geocentric model Sun The Observations of Galileo Earlier observed phases of Venus

Geocentric modelHeliocentric model Sun The Observations of Galileo Earlier observed phases of Venus

The Observations of Galileo Phases of Venus impossible to explain in an Earth- centred model Clear evidence that the Earth went round the Sun, and not the other way round

Getting the Measure of the Solar System In the Sun-centred model it was easy to determine the relative distances of the planets, using the geometry and trigonometry of the Greeks…

Earth Sun Venus We can use Pythagoras theorem!!

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

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?…

Eratosthenes: (c 276 – 195 BC)

Syene – Alexandria = 5000 stadia Circumference of the Earth = stadia

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

Aristarchus (310 – 230 BC): Earth – Sun distance from phases of the Moon

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

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

May 7 th 2003: Transit of Mercury

Pierre Gassendi (1592 – 1655) Observed a transit of Mercury on 7 th November 1631 Predicted by Kepler in 1629, although he didnt live to see it

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

Edmond Halley ( ) 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!

Edmond Halley ( ) 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!

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

Edmond Halley ( ) In 1716 Edmond Halley appealed to astronomers to observe the Venus transits of 1761 and 1769

I recommend it, therefore, again and again, to those curious astronomers who (when I am dead) will have an opportunity of observing these things, that they would remember this my admonition, and diligently apply themselves with all their might to the making of this observation; and I earnestly wish them all imaginable success; in the first place that they may not – by the unseasonable obscurity of a cloudy sky – be deprived of this most desirable sight; and then, that having ascertained with more exactness the magnitudes of the planetary orbits, it may redound to their eternal fame and glory. Edmond Halley ( )

Edmond Halley ( ) In 1716 Edmond Halley appealed to astronomers to observe the Venus transits of 1761 and 1769 He predicted the astronomical unit could be measured to an accuracy of 1 part in 500

6 th June 1761 Venus Transit 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

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 6 th June 1761 Venus Transit o Results were disappointing: Bad weather Poor global coverage Black Drop Effect Systematic errors

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

Neville Maskelyne ( ) I am afraid we must wait till the next transit, in 1769…before astronomers will be able to do justice to Dr Halleys noble proposal

Captain James Cook The 3 rd June 1769 Venus Transit Captain James Cook set sail for Tahiti in August 1768, onboard the Endeavour with astronomer Charles Green

John Harrison H4 1763

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

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

Captain James Cook The 3 rd June 1769 Venus Transit: Tahiti

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 The Endeavour explored for two more years, before returning to Britain. During the voyage Charles Green died of malaria.

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 The Endeavour explored for two more years, before returning to Britain. During the voyage Charles Green died of malaria. Jean Baptiste Chappe dAuteroche died of typhus on 1 st August 1769, in Baja California

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 The Endeavour explored for two more years, before returning to Britain. During the voyage Charles Green died of malaria. Jean Baptiste Chappe dAuteroche died of typhus on 1 st August 1769, in Baja California Guillaume-Joseph-Hyacinthe-Jean-Baptiste Le Gentil wins the award for the unluckiest astronomer!

Captain James Cook The 3 rd June 1769 Venus Transit: Vardö, in Lapland

The 3 rd June 1769 Venus Transit Father Maxmilian Hell ( ) observed the transit from Lapland

The 3 rd June 1769 Venus Transit

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!

Kepler mission, Proposed launch 2007 Will detect hundreds of ESP transits