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Captain Cook and the Cosmic Yardstick. Prof Martin Hendry School of Physics and Astronomy, University of Glasgow. James Cook (1728 – 1779). Ptolemy: 90 – 168 AD. Nicolaus Copernicus (1473 – 1543). Johannes Kepler (1571-1630). Galileo Galilei : (1564 – 1642). - PowerPoint PPT Presentation
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Prof Martin HendrySchool of Physics and Astronomy, University of Glasgow
Captain Cook and the
Cosmic Yardstick
James Cook (1728 – 1779)
Ptolemy: 90 – 168 AD
NicolausCopernicus
(1473 – 1543)
Johannes Kepler(1571-1630)
Galileo Galilei:(1564 – 1642)
Galileo Galilei:(1564 – 1642)
Galileo Galilei:(1564 – 1642)
Sun
Sun
Earth-centred model Sun-centred model
How big is this angle?
Earth
Sun
Venus
Earth
Sun
Venus
We can use Pythagoras’ theorem!
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?…
http://www.scottishsolarsystem.org.uk
...
Eratosthenes: (c 276 – 195 BC)
360501
Syene – Alexandria = 5000 stadia
Circumference of the Earth = 250000 stadia
Aristarchus (310 – 230 BC):
Earth – Moon distance from lunar eclipse
Parallax Shift
Even the nearest star shows a parallax shift of only 1/2000th the width of the full Moon
Parallax Shift
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
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!
Methods relied on an accurate estimate for the radius of the Earth
In 1669 Jean Picard (1620 – 1682) measured
km6365ER (0.2% error)
The 6th 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
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
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!”
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
The next transit: June 5th/6th 2012
www.transitofvenus.org
Waikoloa, Hawaii
www.transitofvenus.org
http://www.scottishsolarsystem.org.uk
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