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Captain Cook and the
Cosmic Yardstick
Dr Martin Hendry
Dept of Physics and Astronomy,
University of Glasgow
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 (428 – 347 BC):
reality a distorted shadow of
a Perfect Form.
Circle = most perfect form in
nature
All celestial motions
are combinations of
circular motions
Early Greek Astronomy
The Greeks inherited ideas from Babylonia and Egypt,
but approached astronomy in a scientific way
Aristotle (384 – 322 BC):
Universe divided into two parts:
Corrupt, changeable Earth
Perfect, immutable heavens
Ptolemy: 90 – 168 AD
Ptolemy proposed a
model which could
explain planetary
motions – including
retrograde loops
John of Holywood (c. 1200)
Author of ‘The Sphere’, standard
textbook on spherical trigonometry
The Copernican Revolution
Nicolaus
Copernicus
(1473 – 1543)
“In the true centre of
everything resides the Sun”
De Revolutionibus Orbis (1543)
The Copernican Revolution
Simpler explanation why Venus and
Mercury appear close to the Sun
Tycho Brahe
(1546-1601)
Uraniborg observatory
Hven, between Denmark and Sweden
Tycho Brahe
(1546-1601)
Uraniborg observatory
Hven, between Denmark and Sweden
Tycho Brahe
(1546-1601)
Johannes Kepler
(1571-1630)
‘Mysterium Cosmographicum’
published in 1596
’New Astronomy’
published in 1609
Johannes Kepler
(1571-1630)
The Spectacle Vendor by Johannes Stradanus, 1582
Hans Lippershey’s 1608 patent of a device for
"seeing faraway things as though nearby."
Porta’s sketch of a telescope, August 1609
The Observations of Galileo
Galileo Galilei:
(1564 – 1642)
The Observations of Galileo
Autumn/Winter 1609, observed:
 Craters of the Moon
 Moons of Jupiter
 Phases of Venus
In conflict with
Aristotelian /
Ptolemaic Universe
Galileo Galilei:
(1564 – 1642)
Published in 1610
‘Sidereus Nuncius’
(The Starry Messenger)
The Observations of Galileo
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
The Observations of Galileo
Earlier observed phases of Venus
Geocentric model
Sun
The Observations of Galileo
Earlier observed phases of Venus
Geocentric model
Heliocentric model
Sun
Sun
The Observations of Galileo
Phases of Venus
impossible to explain
in geocentric model
Clear evidence
that the Earth
went round the
Sun, and not the
other way round
“Cynthiae figuras aemulatur mater amorum”
Getting the Measure of the Solar System
In the Heliocentric model it was easy to determine
the relative distances of the planets, using the
geometry and trigonometry of the Greeks…
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?…
Eratosthenes: (c 276 – 195 BC)
1
50
 360
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
eclipse geometry
Aristarchus (310 – 230 BC):
Earth – Sun distance from
phases of the Moon
Aristarchus (310 – 230 BC):
Earth – Sun distance from
phases of the Moon
Sound method, but angle between Sun and
Moon hard to measure precisely.
Aristarchus (310 – 230 BC):
Earth – Sun distance from
phases of the Moon
Sound method, but angle between Sun and
Moon hard to measure precisely.
Heliocentric model (Sun much larger than the Earth).
Not widely accepted, because no parallax shift
Parallax Shift
A
B
A and B line up the tree with different mountains,
because they see it along different lines of sight
Parallax Shift
Parallax Shift
Parallax Shift
Nearby stars do show an annual
parallax shift, but it is tiny!
First detected only in the mid
19th Century.
Parallax Shift
Even the nearest star shows a
parallax shift of only 1/2000th
the width of the full Moon
Parallax Shift
Even the nearest star shows a
parallax shift of only 1/2000th
the width of the full Moon
But parallax would be the key to measuring the A.U…
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
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
Kepler also predicted a transit of Venus in December
1631, but it occurred after Sunset in Europe
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!
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!
Method relied on an accurate estimate
for the radius of the Earth
In 1669 Jean Picard (1620 – 1682) measured
RE  6365 km
(0.2% error)
In 1716 Halley presented a paper
to the Royal Society, appealing to
astronomers to observe the
Venus transits of 1761 and 1769
Edmond Halley
(1656 - 1742)
“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
Edmond Halley
(1656 - 1742)
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.”
In 1716 Halley presented a paper
to the Royal Society, appealing to
astronomers to observe the
Venus transits of 1761 and 1769
Edmond Halley
(1656 - 1742)
He predicted the astronomical
unit could be measured to an
accuracy of 1 part in 500
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
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
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)
“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”
Neville Maskelyne
(1732 - 1811)
The 3rd June 1769 Venus Transit
Captain James Cook
set sail for Tahiti in August
1768, onboard the Endeavour
with astronomer Charles Green
Captain James Cook
John Harrison
‘H4’
1763
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
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
The Endeavour explored for two
more years, before returning to
Britain. During the voyage
Charles
Captain James
CookGreen died of malaria.
The 3rd June 1769 Venus Transit
Endeavour arrived in Tahiti on
13th 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
Captain James
CookGreen died of malaria.
Jean Baptiste Chappe d’Auteroche died of typhus on 1st
August 1769, in Baja California
The 3rd June 1769 Venus Transit
Endeavour arrived in Tahiti on
13th 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
Captain James
CookGreen died of malaria.
Jean Baptiste Chappe d’Auteroche died of typhus on 1st
August 1769, in Baja California
Guillaume-Joseph-Hyacinthe-Jean-Baptiste Le Gentil wins
the award for the unluckiest astronomer!
The 3rd June 1769 Venus Transit: Vardö, in Lapland
Captain James Cook
The 3rd June 1769 Venus Transit
Father Maxmilian Hell
(1720-1792) observed
the transit from Lapland
3rd June 1769
20:34 UT
Internal contact
at Vardö…
3rd June 1769
20:34 UT
Internal contact
at Vardö…
…meanwhile in
Tahiti…
3rd June 1769
20:43 UT
Internal contact
in Tahiti…
3rd June 1769
20:45 UT
Internal contact
in Tahiti…
…meanwhile at
Vardö…
4th June 1769
02:22 UT
Internal contact
in Tahiti…
4th June 1769
02:22 UT
Internal contact
in Tahiti…
…meanwhile at
Vardö…
4th June 1769
02:33 UT
Internal contact
in at Vardö…
4th June 1769
02:33 UT
Internal contact
in at Vardö…
…meanwhile in
Tahiti…
The 3rd June 1769 Venus Transit
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 and 94 million miles)
Captain James
Cook
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!”
Venus Abandoned
In the 19th Century, astronomers’ attention switched to Mars.
o
o
o
o
Easier to measure positions at night!
Mars appeared smaller in size
No ‘time limit’ on observations
No black drop or atmospheric effects
Captain James Cook
o
o
View from 0 N, 0 W
o
o
View from 0 N, 180 W (opposite side of the Earth)
Venus Abandoned
Captain James Cook
David Gill (1843 – 1914)
Measured the parallax of
Mars from Ascension Island
in 1877
Venus Abandoned
Captain James Cook
David Gill (1843 – 1914)
Measured the parallax of
Mars from Ascension Island
in 1877
Venus Abandoned
Gill’s observations narrowed the range
for 1 A.U.:-
Captain James Cook
David Gill (1843 – 1914)
Measured the parallax of
Mars from Ascension Island
in 1877
Between 92,981,000 miles
and 93,235,600 miles
Superceded the photographic
observations of Venus transits
in 1874 and 1882
Venus Reclaimed
Captain James Cook
Observations of the Near-Earth
asteroid Eros allowed even
greater precision
Venus Reclaimed
Captain James Cook
Observations of the Near-Earth
asteroid Eros allowed even
greater precision
Harold Spencer Jones (1900
– 1960)
1 A.U. = 93,005,000 miles
(less than 0.1% uncertainty)
Venus Reclaimed
Captain James Cook
Irwin Shapiro
Bounced RADAR echoes from
Venus in 1968
Venus Reclaimed
Captain James Cook
Irwin Shapiro
Bounced RADAR echoes from
Venus in 1968
In 1976 IAU adopted:-
1 A.U. = 92,958,329 miles
= 149,597,870 km
Venus Reclaimed
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
Faro – Helsinki, 3479 km
Waikoloa, Hawaii