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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)
Galileo Galilei:
(1564 – 1642)
Galileo Galilei:
(1564 – 1642)
Earth-centred model
Sun-centred model
Sun
Sun
How big is
this angle?
Sun
Earth
Venus
Sun
Earth
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)
Syene – Alexandria
= 5000 stadia
Circumference of the
Earth = 250000 stadia
1
50
 360
Aristarchus (310 – 230 BC):
Earth – Moon distance
from lunar eclipse
Parallax Shift
Parallax Shift
Even the nearest star shows a
parallax shift of only 1/2000th
the width of the full Moon
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)
Halley travelled to St Helena in
1677, to map the Southern Skies
He observed a transit of
Mercury on November 7th
Edmond Halley
(1656 - 1742)
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
RE  6365 km
(0.2% error)
The 6th 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 Results were disappointing:




Bad weather
Poor global coverage
‘Black Drop Effect’
Systematic errors
o Astronomical Unit lay between
77 million and 97 million miles
(20% uncertainty)
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):
1 A.U. = 93,726,900 miles
(between 90
Captain James
Cook
and 94 million miles)
Cassini de Thury
“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!”
Mapping the Solar System
Captain James Cook
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