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What is a
Transit of Venus? What are the applications of the Transit of Venus? History of past
Transits of Venus: Interview
Transcripts: |
A Transit of Venus occurs
when Venus passes between Earth and the Sun and blocks out a bit of the sunÕs
light. Because the inclination
of VenusÕs orbit is different from the EarthÕs orbit, a transit doesnÕt
usually occur when Venus passes Earth.
A transit will only occur if Venus passes Earth close to one of the
two points where their orbit planes intersect. Transits come in pairs 8 years apart, with each pair
occurring about 120 years after the first pair. What are the Applications
of the Transit of Venus? It was thought that by
observing and recording various aspects of the Transit of Venus, the distance
to the sun could be found (see ŌHalley
and his calculationsĶ). This problem of finding the
distance to the sun was one of the noblest problems in Astronomy, and by
using KeplerÕs 3rd law, this one distance could be used to find
all the distances within the solar system. In the 18th
Century, a major problem was finding an accurate method of determining
longitude. It was thought that
determining the scale of the solar system would help with producing
astronomical tables that sailors could use to find their longitude. Jeremiah Horrocks was born in 1619 in Toxteth,
Liverpool. He taught himself astronomy at Emmanuel College, Cambridge.
Horrocks returned to Toxteth in 1635 and used KeplerÕs Laws of Planetary
Motion to prove that the moon orbited the earth elliptically. Following KeplerÕs
prediction that Venus would transit the Sun in 1631, Horrocks calculated that
transits occurred not singularly but in pairs eight years apart. He also
gathered new data to modify the tables for planetary motion; making them more
accurate. Horrocks is believed to be
the first person ever to observe a Transit of Venus. On November 24th
1639, now living in Hoole (near Preston), Horrocks prepared his equipment for
observation of a Transit of Venus. Using a simple telescope set on a wooden
beam, he could project a solar image onto a piece of paper marked with a six
inch graduated circle. Unfortunately Horrocks missed the beginning of the
Transit because he was otherwise engaged by Ōbusiness of the highest
importance which, for these ornamental pursuits, I could not with propriety
neglectĶ. These words were the foundation for the belief that Horrocks was a
clergyman. From 3:15pm he watched the small black dot creep across the sunÕs
face until sunset about half an hour later. As Horrocks said: Ō Éduring the
short time the sun remained in the horizon; for although Venus continued on
the disk for several hours, she was not visible to me longer than half an
hour, on account of his so quickly setting.Ķ He was able to make three
measurements and hence calculate the Transit path for Venus, VenusÕs angular
size and its orbital velocity. He derived a value for the distance of the sun
from earth, and concluded that the earth was further away than previously
thought. His measurement was approximately half the actual distance to the
sun but a factor of ten better than any other calculation at that time. On January 3rd
1641 Horrocks died suddenly at the age of only 22. In 1619, German astronomer
Johannes Kepler (1571-1630) worked out the relative distances of all the
planets from the sun (KeplerÕs 3rd Law). By using KeplerÕs formula, only one inter-planetary
distance needed to be accurately determined to calculate all the distances in
the solar system. Unfortunately
this proved to be a very difficult task, one that was not achieved for
several hundred years. In the late 17th
Century, English astronomer Edmond Halley (1656-1742) realised that the
Transit of Venus could be used to determine the distance to the sun. The transit takes slightly different
times, depending on your location on Earth. By carefully timing the length of the transit at different
locations on the EarthÕs surface, the distance to the sun could be found
using trigonometry and KeplerÕs 3rd law. An alternative method is to precisely measure the times of
ingress and egress (when Venus first touches and last touches the sun) from
different locations of known longitude and latitude. Both these methods required observers
to be situated at far apart locations. Halley had previously tried
to use the more frequent Transit of Mercury to calculate the distance to the
sun. He went to St. Helena in
the South Pacific to observe this phenomenon in 1677 but came to realise that
due to the relatively small distance between the Sun and Mercury, the
differences in the readings would be too small and hence the uncertainties
too great to obtain an accurate result. Halley read a paper to the
Royal Astronomical Society on the topic of using the transit of Venus to
determine the distance to the sun in 1691 and published a revised version of
this paper in 1716. The Royal Astronomical
Society took up his recommendation that observations of the transits set to
occur in 1761 and 1769 be made in as many parts of the world as possible. More than 120 transit
observations were made throughout the world in 1761 but most were of bad
quality or failed due to inexperienced astronomers. The best observation
points for the 1761 transit were India and the East Indies. A particularly unfortunate
story was that of Frenchman Guillaume Le Gentil. Le Gentil travelled to
India to observe the 1761 Transit of Venus from the French colony of
Pondicherry. Unfortunately he never got there. Due to the English/ French war
Pondicherry was a battleground and so Le Gentil was forced to retreat back to
sea. When the transit was occurring Le Gentil was stuck aboard the rolling
decks of his ship. He decided to wait and attempt to observe the transit in
1769. Unfortunately bad weather prevented this from occurring. When he
finally returned home in 1771 he learnt that he had been declared dead. After
a long battle Le Gentil proved that he was in fact still alive. Captain Cook and the 1769
Transit In 1766, the Royal
Astronomical Society started preparing for the next transit, set to occur in
1769. They believed that
accurate observations of this transit would allow them to determine the
distance to the sun, and as this would be their only chance in the next 120
years, the preparations took high importance. To increase the chance of
success, the Royal Society wanted to observe the Transit from as many widely
spaced locations as possible.
This would ensure that the differences in readings would be
significant enough to obtain an accurate calculation and if bad weather
prevented it from being observed in one area, it could still be observed at
other locations. Calculations
had shown that the south pacific would be a suitable place from which to
observe the transit so it was decided to send an expedition to this little
explored part of the world; a major undertaking at this time. King George III agreed to
fund such a trip and a ship from the Royal Navy could be used. The only snag was that the Navy would
not agree to leave its ship in the command of astronomer Alexander Dalrymple,
the man the Royal Society had hoped would command the expedition. (They had probably no forgotten the
bad experiences when astronomer Edmond Halley was left in charge of one of
its ships. This had resulted in
mutiny and near loss of the ship.)
It was decided to instead send James Cook, a junior naval officer. On August 26th
1768, the Endeavour set sail and headed for Tahiti, an island recently
discovered by another English ship.
The transit wasnÕt due to occur for 9 months, but they didnÕt want to
risk still being at sea like Le Gentil when the crucial moment arrived. Endeavour arrived in Tahiti
just over 2 months before the transit was due to occur. The sailors had taken with them a
supply of nails as a previous expedition had found that the islanders were
willing to do almost anything for nails. In the days before the transit, observers were stationed
at three points on the island, so that there would be more chance of
observing the ingress and egress if the weather was bad. As it turned out, the weather was
perfect, Cook noted in his diary, Ōnot a Clowd was to be seen the whole day
and the Air was perfectly clear, so that we had every advantage we could
desire in Observing the whole of the passage of the Planet Venus across the
sunÕs diskĶ The observers were meant to
determine the exact times of ingress and egress. This however caused some difficulty as Cook noted ŌWe very
distinctly saw an Atmosphere or dusky shade round the body of the Planet which
very much disturbed the times of the contactsÉ we differed from one another
in observing the times of contacts by more than could be expectedĶ. This phenomenon has since
come to be known as the ŌBlack DropĶ effect. A black elongated ligament forms between Venus and the
Sun, making it impossible to With the observations
completed, Cook set off in search of the great southern continent believed to
exist at this time. He instead
discovered New Zealand and Australia and charted a lot of their
coastlines. The Endeavour
arrived back in England in July 1771, almost 3 years after it had left. In colonial New Zealand of
1874, the area of Burnham was situated on a barren and open plain. It was home to only a small community
and characterised by its wide and open sky. Due to the fact that railway and telegraphic facilities
were to hand and readily available, it was selected as an observation point
for the Transits of Venus in both 1874 and 1882. Meticulous planning went
into the first of these expeditions.
Four observatory huts were to be built. One of these was to be occupied by Major Palmer,
expedition leader, who would make observations through his equatorial
telescope. Lieutenant Darwin was
to occupy another, taking photos of the transit with his
photoheliograph. In the month
leading up to the Transit, telegraph communication between the stations and
the wider world were checked extensively, as this was expected to prove vital
to making accurate observations and time measurements. By the 8th of December,
these arrangements had been put into place and Major Palmer and his team sat
poised ready to make their observations. What a pity that something
so carefully planned, long awaited and rare could be utterly ruined by
something as unpredictable and entirely uncontrollable as the weather. Clouds and rain caused observations
at both the ingress and egress to be impossible, and the few measurements and
photographs made during the short period in which Venus was visible were
largely without use. Major
Palmer praised the excellent telegraph arrangements made by those at Burnham,
but summed up the feeling of the day with these words; published as part of
his memorandum in the Christchurch Press on December 10th
1984: ŌThe weather failed usĶ. Widespread failure to make
useful observations of the transit due to bad weather in New Zealand at the
crucial moments did not prevent an additional expedition being made to the
same sight in 1884. Luckily this
time the weather held out, with Colonel Tupman and Lieutenant Coke,
expedition leaders, agreeing that atmospheric conditions were Ōgenerally most
favourableĶ. On the 7th
December 1884, the two men and their wives made satisfactory observations of
the transit throughout the day.
They found the experience to be Ōmost gratifyingĶ and stayed on at
Burnham to carry out further astronomical observations, before returning to
England 10 days later. To this day, observation
pillars from this historical expedition remain at Burnham and a plaque
commemorates the success of the 1882 observations. The point from which the observations were made was also
the point used to calculate NZ Standard time from Greenwich Mean Time. The distance to the sun has
now been accurately determined using other methods and the Transit of Venus
that will occur on the 8th of June this year has little scientific
importance, but has massive historical and cultural significance. Transcript
of Interview with Dr. William Tobin of the Physics and Astronomy Department
at Canterbury University Why does a Transit of
Venus take place? The Transit of Venus takes
place because Venus in its orbit manages to go directly between us and the
sun and blocks out a little bit of the sunÕs light as it creeps across the
disk of the sun. Another issue is why are they so rare because you might
think that every time we pass Venus, which happens about once every nine
months, Venus would be in the way. The fact that we don't always get a
transit is because the inclination of Venus's orbit is different from the
earth's orbit and there are essentially only two places where Venus's orbit
crosses ours and we have to both be simultaneously in these places. ThatÕs
why itÕs very rare and also why you only get transits in June and December
because where the earth is at that point is June and December. What can you tell us
about Jeremiah Horrocksâs observations of the Transit of Venus? Very little other than heÕs
reputed to have made them on a Sunday and because he was a clergyman he had
to rush between his pulpit and his telescope to make the observations. Why was it considered
important to find the distance of the earth from the sun? It was a crucial issue in
understanding how the planets were moving in the sky. In the history of
astronomy from about the seventeenth century through to the end of the
nineteenth century, the major issue in astronomy was understanding how the
planets moved. And a key question in that was how far away was the sun from
the earth. It also had practical applications because the motions of the
planets and the stars were being used. For example to determine
longitude. There was a method
for determining longitude on land which worked very well, which was to
observe when the eclipses of JupiterÕs moons occurred. That you could do with a telescope
from a nice stable base like terrafirma but that didnÕt work from the rolling
deck of a ship. Some other method was needed and this was a great quest until
the middle of the eighteenth century. The great quest was determining a means
of measuring longitude and one way that people thought that they were going
to do it at sea was to look at the relative position of the moon and/or the
sun against each other or the background of stars. That was a measurement that could be made from a heaving
deck but to be able to make your tables of predictions of where the moon
should be you needed to understand all the motions in the solar system and
the distance to the sun was a key element in that. Why use Venus and not
some other planet? The direct way to determine
the distance to the sun was essentially to triangulate it, just like when you
look at a finger with one eye and the other eye you see a shift in the
position of your finger against the distance. If you know how far apart your eyes are and you know how
much the shift is, you can work out how far away the finger is. The idea was
to observe with two telescopes at different points on the earthÕs surface and
triangulate (the simplest idea) the edge of the sun, but the trouble with
that is that the sun is far away and you wonÕt get very good accuracy. Now already Kepler had realized in
the sixteenth century or so that there was a law about how the planets were
distributed in the solar system- KeplerÕs Third law itÕs called - and he had
managed to make a scale model of the solar system. He knew what the relevant
spacing between the sun, mercury, Venus, earth and mars was, he just didnÕt
know what scale to put on the map. He had a scale map but he didn't know the
scale. So if you could measure any distance on that scale map then you had
the scale of the whole solar system and particularly the distance to the
sun. So the name of the game was
to find a measurement that you could make accurately. What it was believed that you would
be able to measure most accurately was the object that was closest and so
Venus when it came between us and the sun was the object of choice. Following that there was mars, though
not in the direction of the sun but the opposite direction for conjunction
was about twice as far away as Venus at the best, and that was the second
method of choice In fact the very first accurate measurement of the distance
to the sun, one that was in the right ballpark, not factors of dozens
incorrect, was a measurement of the opposition of mars in 1762 where
astronomers in Paris and astronomers in South America measured the parallax,
the difference in direction to mars and by knowing the difference in the
distance between Paris and South America were able to work out how big the
solar system was, at least with a reasonable sort of accuracy. So everybody
was looking for the Transits of Venus as being the best method, the closest
object and Edmond Halley, of comet fame, in 1677 who had actually seen a
transit of mercury across the disk of the sun. Transits of mercury occur every few years. I saw one
several years ago from a telescope in this building. He realized that you could do the
same thing with Venus. That was in 1677 and he knew that the next transit of
Venus was in 1761- it was going to be quite some time to wait So what happened and how
successful were the 1761 and 1769 transits? They were not hugely
successful. A number of expeditions were sent out - we know here in New
Zealand about Captain CookÕs expedition to Tahiti. There was also a measurement
at the other end of the sight line, the other eye, which was particularly
crucial and was one made in Norway.
People in the nineteenth century were very suspicious of that
measurement, in fact some astronomers even thought that that measurement was
a fabrication, but the person that made that measurement was cleared of that
charge subsequently. Why there
were many expeditions was then you have many measurements of roughly the same
quantity then you can compare them and make sure theyÕre all coherent and
consistent and the problem with the measurements from the eighteenth century
was the ultimately on one end of the sight line, one end of the space between
the two eyes. There was only one eye so although you had several eyes on one
end there was only one measurement on the other end and that made people very
uncertain as to whether the measurements were really reliable. Did any benefits come
from the values they calculated in the eighteenth century? I think that you always
have benefits if you understand what you're doing better and what the
problems are. I donÕt know if there were obvious practical benefits but they
did keep working on improvements to navigation and Captain Cook on his third
voyage, not the voyage that he went to measure the transit of Venus had one
of HarrisonÕs chronometers with him and ultimately it was Harrison, a clock
maker, who discovered the way of telling the time at Greenwich when you were
elsewhere on the earthÕs surface because thatÕs essentially what the problem
of the longitude is. You need to
tell your local time which you can always tell by looking at the stars or sun
and you need to know what the time at Greenwich as your reference time. The
difference is how far away you are. We are twelve hours ahead of Greenwich
time because we are roughly 180 degrees longitude to the east of Greenwich.
The moons of Jupiter eclipsing or the relate position of the moon against the
background of the stars was the clock that people were wanting to use to tell
the time in Greenwich, so they could compare their local time to Greenwich
time and find out where they were on the surface of the earth Why repeat the whole
ordeal in the nineteenth century? Because people werenÕt
happy with the measurements from the eighteenth century and they thought that
theyÕd be able to do better in the nineteenth century. In that they were
unfortunately disappointed because the hoped for improvements in accuracy
didnÕt eventuate. It turned out
that different astronomers in measuring the transit very much more disagreed
about the moment of contact; when the planet of Venus has just got its limb
in contact with the limb of the sun; which was the crucial thing that people
needed to measure within a second of accuracy. In fact they seemed to disagree from each other with about
half a minute or a minute so it proved that they weren't able to make the
measurements as well. A publishing from the Paris observatory shows an
engraving of what the problem is. Essentially when you tried to determine the
exact moment when Venus was over the disk of the sun, you got a little black
drop effect and the images seemed to stick together. It wasn't clear the
exact moment when you wanted to measure where Venus and the sun were and that
made the measurements very difficult. There was hope that photography would
be a great utility in being more objective than the human eye but that didnÕt
work out. The measurement of the transit of Venus didnÕt lead to an improved
measurement of the distance to the sun. Ultimately what people then went on
to realize was that what they would do better was to measure smaller objects
even though they were further away and they began to measure asteroids which
are further out than mars even, but very small so they are very tiny
pinpoints of light which you could much more easily triangulate on. Did you still need two
observers at different points on the earthÕs surface? Yes or the other option was
instead of having two observers at different places in the world you had one
observer and you just let the world turn or move in its orbit, but it became
apparent that there were other ways you could deal with that problem. Given that we appear to
have solved this big problem, the distance of the earth from the sun, what is
the significance of the Transit of Venus this year? Its significance I think is
itÕs cultural. Here in New
Zealand, the transit of Venus was an important part in the initial discovery
and exploration of these islands by Cook (its European discovery). ItÕs a rare
event. One of the things is that
some people think that they observed the transits of Venus in the eighteenth
and nineteenth century because they were rare but no, there was an absolute
scientific reason for observing them and if the transits had occurred once
every fortnight, they would have observed them once every fortnight. But itÕs
a rare event - there wonÕt be another pair until the twenty-second century so
if you want to see a transit of Venus you've got to do it either on the 8th
June this year or in 2012. Transcripts
of Interview with Rev. Ian Crumpton, an astronomy enthusiast, from
Westmelton, Christchurch, New Zealand What is the transit of
Venus? A transit of Venus is where
Venus gets between us and the sun so the sun is a great big ball and we are
looking at it and across the face of the sun goes the little tiny disk Venus-
a little black dot and you see it coming on to the sun and it takes several
hours tot ravel right across and move off at the other side. How often do they occur? They occur 122 years apart in pairs, which are eight
years apart. So you get one then eight years later another then after 122
years another pair also eight years apart. What is the importance
of this yearÕs transit of Venus? The importance for science
this year is zero, the professional astronomers wonÕt be looking at it at
all. TheyÕre able to determine the distances to planets and planetary objects
to within a few metres by using things like radar and radio signals from
spacecraft. When these were observed on the last two occasions they were
vital. Astronomers had no idea of the scale of the solar system at that time.
KeplerÕs Laws had shown them how the planets are spaced and where theyÕre
spaced but the scale of it was unknown. It was Edmond Halley who worked out a
mathematical means of using these transits to determine the distance of the
earth to the sun and that distance is called the astronomical unit or the
A.U. It proved quite difficult to do but that was what it was all about- that
was why there was so much fuss on those two previous occasions. This yearÕs
is a very exciting event for people.
We're seeing something that no one else who's living now will see
again. This is the first pair that the general public, ordinary amateurs like
me and ordinary people like you, can watch, see and enjoy. IÕm looking
forward to it. Venus television has the unique quirk of a coming live to you only 4 times in 243 years Đ that is every time a Transit of Venus occurs. Venus Television was established in 1639, when an amateur cameraman caught J HorrocksÕ candid flight out of St. Michaels Church in England. An organization was formed with the sole mission statement being: ŌTo inform the wider world about Transits of Venus and their applications.Ķ Since then, Venus Television had had the honour of being present at many historically significant events, such as Edmond HalleyÕs presentation to the Royal Society of England, and CookÕs voyage from Tahiti to New Zealand. This year Venus Television has produced an extra special edition including the highlights of all previous shows (with the exception of 1761, when footage was destroyed due to unfavourable wartime conditions). So join us as we count down to this phenomenal
historical astronomical sensation, otherwise weÕll see you in 2004. We would like to
acknowledge and thank very much the following people who have lent us equipment
and helped us in various ways: Shane Campbell Mr. Wiltshire Mrs. Ray Mr. Groves Mr. Warren Mrs. Keer Mr. Leigh Olivia Hedges Dr William Tobin Rev. Ian Crumpton Mathew Westbrooke Catie Nobes Katie Lock Mrs. Johnson Burnside High School Christchurch Public Library Canterbury Museum Pirates Island Mini Golf
Course www.sil.si.edu/exhibitions/chasing-venus/measuring.htm http://southseas.nla.gov.au/biogs/P000085b.htm http://skolor.nacka.se/samskolan/eaae/summerschools/TOV2.html www.venus-transit.de/1639/horrox.htm www.uclan.zc.uk/facs/science/physastr/misc/horrock.htm http://sunearth.gsfc.nasa.gov/eclipse/transit/venus0412.html www.aas.org/publications/baas/v27n4/aas187/S035002.html http://transitofvenus.auckland.ac.nz/astronomy/noblest_problem.html
Endeavour
CD-Rom The
Christchurch Press, Dec 10 1874 The
Christchurch Press, Dec 7 1882 The
Christchurch Press, Dec 8 1882 The
Christchurch Press, Feb 15 1996 The
Christchurch Press, Feb 22 1996 Transcripts of the Royal Astronomical Society Cook,
James, The journals of Captain James Cook on his voyages of discovery, Cambridge,
1955-1974 This website and its material
was written by Lucy Dalton, Hannah Smeele and Caitlyn Westbrooke from
Burnside High School, Christchurch, New Zealand |
Transit of Venus History and Applications