Dr Martin HendryDr Martin HendryDept of Physics and Astronomy, Dept of Physics and Astronomy, University of GlasgowUniversity of Glasgow

Captain Cook Captain Cook and and thethe

Cosmic Cosmic YardstickYardstick

James Cook (1728 – 1779)

Retrograde motion of Mars

Ptolemy: 90 – 168 AD

Ptolemy proposed an Earth-centred Universe

NicolausCopernicus

(1473 – 1543)

“In the true centre of everything resides the Sun”

The Copernican RevolutionThe Copernican Revolution

Galileo Galilei:(1564 – 1642)

The Observations of GalileoThe Observations of Galileo

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

The Observations of GalileoThe Observations of Galileo

The Observations of GalileoThe Observations of Galileo

Earlier observed phases of Venus

Geocentric model

Sun

The Observations of GalileoThe Observations of Galileo

Earlier observed phases of Venus

Geocentric model Heliocentric model

Sun

Sun

The Observations of GalileoThe Observations of Galileo

Earlier observed phases of Venus

The Observations of GalileoThe 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

Planet Distance

Mercury 0.39

Venus 0.72

Earth 1.00

Mars 1.52

Jupiter 5.20

Saturn 9.54

Getting the Measure of the Solar System

Planet Distance

Mercury 0.39

Venus 0.72

Earth 1.00

Mars 1.52

Jupiter 5.20

Saturn 9.54

How far is an astronomical unit?…How far is an astronomical unit?…

Johannes Kepler predicted a transit of Mercury on 29th May 1607

Instead, he ‘discovered’ sunspots

Johannes Kepler predicted a transit of Mercury on 29th May 1607

Instead, he ‘discovered’ sunspots

May 7th 2003: Transit of Mercury

Pierre Gassendi (1592 – 1655)

Observed a transit of Mercury on 7th November 1631

Predicted by Kepler in 1629, although he didn’t live to see it

November 24th 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)

Edmond Halley (1656 - 1742)

Halley travelled to St Helena in 1677, to map the Southern Skies

He observed a transit of Mercury on November 7th

Transit observations could measure the astronomical unit!

Edmond Halley (1656 - 1742)

Halley travelled to St Helena in 1677, to map the Southern Skies

He observed a transit of Mercury on November 7th

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

km6365ER (0.2% error)

Edmond Halley (1656 - 1742)

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

Edmond Halley (1656 - 1742)

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

Neville Maskelyne

(1732 - 1811)

“I am afraid we must wait till the next transit, in 1769…before astronomers will be able to do justice to Dr Halley’s noble proposal”

Captain James Cook

The 3rd June 1769 Venus Transit

Captain James Cookset sail for Tahiti in August 1768, onboard the Endeavour with astronomer Charles Green

Captain James Cook

The 3rd June 1769 Venus Transit

Endeavour arrived in Tahiti on 13th April 1769 – constructed a fort, and an observatory, at Point Venus

Captain James Cook

The 3rd June 1769 Venus Transit

Endeavour arrived in Tahiti on 13th April 1769 – constructed a fort, and an observatory, at Point Venus

Transit observed by Cook, Green and Solander

Captain James Cook

The 3rd June 1769 Venus Transit: Tahiti

The 3rd June 1769 Venus Transit

Father Maxmilian Hell(1720-1792) observed the transit from Lapland

Captain James Cook

The 3rd 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!”