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 thetheCosmic Cosmic YardstickYardstick

James Cook (1728 – 1779)

Retrograde motion of Mars

Early Greek AstronomyEarly Greek AstronomyThe Greeks inherited ideas from Babylonia and Egypt, but approached astronomy in a scientific way

Plato (428 – 347 BC): Plato (428 – 347 BC): reality a distorted reality a distorted shadow of a shadow of a Perfect Perfect FormForm..

Circle = most perfect form in nature

All celestial motions are combinations of circular motions

Aristotle (384 – 322 BC):Aristotle (384 – 322 BC):

Universe divided into two parts:

Corrupt, changeable Earth

Perfect, immutable heavens

Early Greek AstronomyEarly Greek AstronomyThe Greeks inherited ideas from Babylonia and Egypt, but approached astronomy in a scientific way

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

NicolausCopernicus

(1473 – 1543)

“In the true centre of everything resides the Sun”

De Revolutionibus Orbis (1543)

The Copernican RevolutionThe Copernican Revolution

The Copernican RevolutionThe Copernican Revolution

Simpler explanation why Venus and Mercury appear close to the Sun

Tycho Brahe(1546-1601)

Uraniborg observatoryHven, between Denmark and Sweden

Uraniborg observatoryHven, between Denmark and Sweden

Tycho Brahe(1546-1601)

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, The Spectacle Vendor by Johannes Stradanus, 15821582

Hans Lippershey’s 1608 patent of a device for

"seeing faraway things as though nearby."

Porta’s sketch of a telescope, August 1609

Galileo Galilei:(1564 – 1642)

The Observations of GalileoThe Observations of Galileo

Autumn/Winter 1609, observed: Craters of the Moon Moons of Jupiter Phases of Venus

Published in 1610 ‘Sidereus Nuncius’ (The Starry Messenger)

In conflict with Aristotelian / Ptolemaic Universe

The Observations of GalileoThe Observations of Galileo

Galileo Galilei:(1564 – 1642)

The Moon is an imperfect world with mountains and valleys, just like the Earth

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 GalileoEarlier observed phases of Venus

Geocentric model Sun

The Observations of GalileoThe Observations of GalileoEarlier observed phases of Venus

Geocentric model Heliocentric model Sun

Sun

The Observations of GalileoThe Observations of GalileoEarlier observed phases of Venus

The Observations of GalileoThe 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 SystemIn the Heliocentric model it was easy to

determine the relative distances of the planets, using the geometry and trigonometry of the Greeks…

Earth

Sun

Venus

We can use Pythagoras’ theorem!!

Getting the Measure of the Solar System

Planet Distance

Mercury 0.39Venus 0.72Earth 1.00Mars 1.52Jupiter 5.20Saturn 9.54

Getting the Measure of the Solar System

Planet Distance

Mercury 0.39Venus 0.72Earth 1.00Mars 1.52Jupiter 5.20Saturn 9.54

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

Eratosthenes: (c 276 – 195 BC)Eratosthenes: (c 276 – 195 BC)

360501

Eratosthenes: (c 276 – 195 BC)Eratosthenes: (c 276 – 195 BC)

360501

Syene – Alexandria = 5000 stadia

Circumference of the Earth = 250000 stadia

Aristarchus (310 – 230 Aristarchus (310 – 230 BC):BC):Earth – Moon distance Earth – Moon distance from eclipse geometryfrom eclipse geometry

Aristarchus (310 – 230 Aristarchus (310 – 230 BC):BC):Earth – Sun distance Earth – Sun distance from phases of the Moonfrom phases of the Moon

Aristarchus (310 – 230 Aristarchus (310 – 230 BC):BC):Earth – Sun distance Earth – Sun distance from phases of the Moonfrom phases of the Moon

Sound method, but angle between Sun and Moon hard to measure precisely.

Aristarchus (310 – 230 Aristarchus (310 – 230 BC):BC):Earth – Sun distance Earth – Sun distance from phases of the Moonfrom 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

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

Parallax Shift

Nearby stars do show an annual parallax shift, but it is tiny! First detected only in the mid 19th Century.

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

Parallax Shift

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 itKepler 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)

Edmond Halley (1656 - 1742)

Halley travelled to St Helena in 1677, to map the Southern SkiesHe 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 SkiesHe 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) measuredkm6365ER (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

“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)

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 1769He 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 yearso ‘Public outreach’ description by James Fergusono Franco-British cooperation, despite being at war!o 120 astronomers observed from about 60 locations

o Observations meticulously planned, for many yearso ‘Public outreach’ description by James Fergusono Franco-British cooperation, despite being at war!o 120 astronomers observed from about 60 locations

The 6th June 1761 Venus Transit

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

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 yearso ‘Public outreach’ description by James Fergusono Franco-British cooperation, despite being at war!o 120 astronomers observed from about 60 locations

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

John Harrison

‘H4’1763

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

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 Green died of malaria.

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 Green died of malaria.

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

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 Green 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!

Captain James Cook

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

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

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

Venus Abandoned

In the 19th Century, astronomers’ attention switched to Mars.o Easier to measure positions at night!o Mars appeared smaller in sizeo No ‘time limit’ on observationso No black drop or atmospheric effects

View from 0 N, 0 Wo o

View from 0 N, 180 W (opposite side of the Earth)oo

Captain James Cook

Venus Abandoned

David Gill (1843 – 1914)

Measured the parallax of Mars from Ascension

Island in 1877

Captain James Cook

Venus Abandoned

David Gill (1843 – 1914)

Measured the parallax of Mars from Ascension

Island in 1877

Captain James Cook

Venus Abandoned

David Gill (1843 – 1914)

Measured the parallax of Mars from Ascension

Island in 1877

Gill’s observations narrowed the range for 1 A.U.:-

Between 92,981,000 miles and 93,235,600 milesSuperceded the photographic observations of Venus transits in 1874 and 1882

Captain James Cook

Venus Reclaimed

Observations of the Near-Earth asteroid Eros allowed even greater precision

Captain James Cook

Venus Reclaimed

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)

Captain James Cook

Venus Reclaimed

Irwin ShapiroBounced RADAR echoes from Venus in 1968

Captain James Cook

Venus Reclaimed

Irwin ShapiroBounced RADAR echoes from Venus in 1968 In 1976 IAU adopted:-

1 A.U. = 92,958,329 miles

= 149,597,870 km

Captain James Cook

Venus Reclaimed

Irwin ShapiroBounced 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