Dept of Physics and Astronomy,
University of Glasgow

James Cook (1728 – 1779)

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
Universe divided into two parts:
Corrupt, changeable Earth
Perfect, immutable heavens

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

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

Johannes Kepler
(1571-1630)
’New Astronomy’ published in 1609

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
Galileo Galilei:
(1564 – 1642)
In conflict with
Aristotelian /
Ptolemaic Universe
Published in 1610
‘Sidereus Nuncius’
(The Starry Messenger)

The Observations of Galileo

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
Sun
Heliocentric model
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

Getting the Measure of the Solar System
Planet
Mercury
Venus
Earth
Mars
Jupiter
Saturn
Distance
0.39
0.72
1.00
1.52
5.20
9.54

Getting the Measure of the Solar System
Planet
Mercury
Venus
Earth
Mars
Jupiter
Saturn
Distance
0.39
0.72
1.00
1.52
5.20
9.54

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

Parallax Shift

Parallax Shift

Parallax Shift
do
th

Parallax Shift
Even the nearest star shows a parallax shift of only 1/2000 th the width of the full Moon

Parallax Shift
Even the nearest star shows a parallax shift of only 1/2000 th 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 29 th May
1607
Instead, he ‘discovered’ sunspots

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

th

Pierre Gassendi (1592 – 1655)
Observed a transit of Mercury on 7 th 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 7 th 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 24 th 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 7 th
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 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
R
E

6365 km (0.2% error)

Edmond Halley
(1656 - 1742)
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 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
(1656 - 1742)
In 1716 Halley presented a paper to the Royal Society, appealing 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

The 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

The 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 Results were disappointing:
 Bad weather
 Poor global coverage
 ‘Black Drop Effect’
 Systematic errors

The 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 Results were disappointing: o
 Bad weather
 Poor global coverage
 ‘Black Drop Effect’
 Systematic errors
Astronomical Unit lay between
77 million and 97 million miles
(20% uncertainty)

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”

The 3 rd 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

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
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
Captain James Cook
Jean Baptiste Chappe d’Auteroche died of typhus on 1 st
August 1769, in Baja California

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
Captain James Cook
Jean Baptiste Chappe d’Auteroche 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!

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

The 3 rd June 1769 Venus Transit
Father Maxmilian Hell
(1720-1792) observed the transit from Lapland

3 rd June 1769
20:34 UT
Internal contact at Vardö…

3 rd June 1769
20:34 UT
Internal contact at Vardö…
…meanwhile in
Tahiti…

3 rd June 1769
20:43 UT
Internal contact in Tahiti…

3 rd June 1769
20:45 UT
Internal contact in Tahiti…
…meanwhile at
Vardö…

4 th June 1769
02:22 UT
Internal contact in Tahiti…

4 th June 1769
02:22 UT
Internal contact in Tahiti…
…meanwhile at
Vardö…

4 th June 1769
02:33 UT
Internal contact in at Vardö…

4 th June 1769
02:33 UT
Internal contact in at Vardö…
…meanwhile in
Tahiti…

The 3 rd June 1769 Venus Transit

The 3 rd 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
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 19 th Century, astronomers’ attention switched to Mars.
o Easier to measure positions at night!
o Mars appeared smaller in size o No ‘time limit’ on observations o 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
Captain James Cook
Gill’s observations narrowed the range for 1 A.U.:-
David Gill (1843 – 1914)
Between 92,981,000 miles and 93,235,600 miles
Measured the parallax of
Mars from Ascension Island in 1877
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