1. The Transit of Venus
June 5-6, 2012
“The sight, which is by far the
noblest astronomy can afford, is
denied to mortals for a whole
century, by the strict laws of motion”
- Edmond Halley 1691
2. What is a Transit?
Like a mini eclipse of the Sun
Planet
3. Why Do They Happen?
• An inner planet
(Mercury or Venus)
passes directly
between the Sun and
Earth
• So why don’t they
happen every time?
11. Series of Venus Transits
• Combination of tilt and orbital speeds mean
transits occur in a series:
– 8 years apart, 105.5 years, 8 years, 121.5 years,
8 years, 105.5…
• Previous pair was in December1874 and 1882
• Last one was June 2004
• Next pair will be in December 2117 and 2125
12. Transits before Kepler
• Babylonians, Mayans and Chinese regularly
observed the Sun and Venus and probably saw a
transit, but there are no clear records.
– The Egyptian tablet appear to be a transit record, but is
too damaged to be sure.
13. Predictions of
Transits
• Kepler’s Rudolphine tables
published in 1627
– Predicted first Venus transit
1631
• Visible over North America,
but not Europe!
• Missed predicting 1639 transit
14. Jeremiah Horrocks
• Predicted time and location over Britain
– Observed only the middle of the transit in Lancashire.
– His friend, William Crabtree was mostly clouded out in
Manchester (picture is from Manchester town hall)
– Venus was much smaller than expected!
15. Safe Solar Viewing in 1639
• Project an image through the ‘scope
– By 19th century, telescopes were getting too
big
• Large lenses (and mirrors) can overheat, warp, crack
or melt!
• Need a filter or an “aperture stop” to prevent too
much light through the lens
16. Edmund Halley
• In 1716, Halley showed how to use a transit
to measure the Astronomical Unit…
17. The Astronomical Unit (AU)
• Kepler’s laws give relative distances in
Solar System in terms of the Earth-Sun
distance: the AU.
– E.g. Kepler showed Venus is 0.72 AU from
Sun, BUT, not how big an AU is!
• The “yardstick” for astronomy
– Foundation for all distances in the universe
18. Measuring the AU
• The method was derived by Halley from
ideas presented by James Gregory
– Solar parallax measured by timing transit from
multiple places on Earth:
19. Measuring the AU
• Northern Hemisphere observer sees Venus
travel low across the Sun:
21. Measuring the AU
• The distance between the two lines, θ, and
the distance between the two observing
points, d, gives the Earth-Venus distance,
D.
D
d θ
θ
}θ
22. 1761
• 176 reported observations
• 7 year war and bad
weather caused many
problems
– No AU measurement
• Mikhail Lomonosov
discovered the atmosphere
as a faint halo around
Venus…
24. 1769
• First organized
American observations
– State House observatory
in Philadelphia where
Declaration of
Independence was read
(image is replica at the
Henry Ford in Dearborn,
MI)
– Franklin ensured
Philadelphia observations
were published in Europe
25. 1769
• International Expeditions Launched
– Many had multiple goals: Cook’s Voyage to Point Venus
Tahiti and find “Terra Australis Ignota” or the Unknown
Southern Land.
27. Not Just a Problem for Venus
• This image of the
transit of Mercury
2003 shows same
effect
• There were high
hopes that
photography could
solve some problems
in 19th century
28. 1874 & 1882
• New Technology
– Photography
• Measurement could be made when Venus crossed Sun’s
meridian
• solve the black drop problem?
– The Heliostat
• could be driven
slowly and evenly
to track the Sun
29. UofM connections
• James Craig Watson,
director of the Detroit
Observatory, led US
expedition to Peking for the
Dec 1874 transit.
• Aseph Hall Sr. led
expedition to Vladavostock
– Trained at the Detroit
Observatory under Brünnow
– Father of 4th director Asaph
Hall Jr,
31. Accuracy of AU
• Astronomers measured the Earth-Sun
distance to within 0.2%
– That’s an accuracy of about 300,000 km or
200,000 miles
– Great for the time, but…
32. 20th Century Accuracy of AU
• Radar and laser measurements have an
uncertainty of less than a meter!
• Transits are no longer necessary (or even
useful) for this distance measurement.
33. Importance for the 21st Century
• Spectroscopy of atmosphere
– Search for trace elements
• The search for extra-solar planets
34. The atmosphere
• Composition
determined from HD 209458
changes in
spectrum
– Use this for
planets around
other stars too!
35. Detecting transits of Extra-Solar
Planets
• As a planet
passes in front of
a star, it blocks a
tiny bit of light
• Kepler has found
more than 240
planets orbiting
other stars
36. Viewing the Transit
• NEVER LOOK DIRECTLY AT THE SUN
– Indirect methods
• Pinhole cameras, projection screens
– Filters
• Eclipse glasses, telescope filters
– A filter is adequate only if you cannot see to horizon
through it
37. •“On account of their rarity alone, they must afford
an exquisite entertainment to an astronomical taste”
- John Winthrop 1769
• http://sunearthday.nasa.gov/transitofvenus/
• http://www.exploratorium.edu/venus/
• http://www.transitofvenus.org/
• http://venustransit.nasa.gov/transitofvenus/
• http://www.astronomerswithoutborders.org/index.php/proje
cts/transit-of-venus.html
Editor's Notes
Quote from Philosopical Transactions, possibly out of knowledge he wouldn ’ t see one Halley 1657 - 1742 born 18 years after 1639 transit, died 19 years before next transit Arguably responsible for making the18th and 19th century transits of Venus some of the most important events in astronomical history
Venus has inside orbit - moves faster than Earth Synodic period is 19 months Kepler ’ s 2nd law: speed of orbit changes with distance to the Sun: fewer days between Sept 21 & March 21 than between March 21 and Sept 21. Earth ’ s longer orbital period makes if harder to match up exactly.
Inclination is actually 3.4 degrees Sun is 0.5 degees diameter Must be close to line of nodes for transit. 8 years between repeated alignemnts The next transit occurs 8 years latter, but then they don ’ t line up again for more than a century.
Inclination is actually 3.4 degrees Sun is 0.5 degees diameter Must be close to line of nodes for transit. 8 years between repeated alignemnts The next transit occurs 8 years latter, but then they don ’ t line up again for more than a century.
Inclination is actually 3.4 degrees Sun is 0.5 degees diameter Must be close to line of nodes for transit. 8 years between repeated alignemnts The next transit occurs 8 years latter, but then they don ’ t line up again for more than a century.
For details see Jean Meeus “ Transits ” It ’ s been 121.5 years since last transit 8 years ‘ till next 105.5 years till next pair
The Babylonian tablet may refer to a transit of Venus. However the text is damaged so the translation is questionable Aztecs were Sun worshipers and may have observed a transit if occurred at sunset or sunrise. The Chinese observed sunspots and did enough observing they may have also seen a transit and not known what they were seeing.
The Rudolphine tables contained predictions of planet positions based on Kepler ’ s laws of planetary motion. The laws were highly accurate, but the calculations weren ’ t always… Predicted 134 years to next transit
Horrocks (also horrox) and Crabtree met at Cambridge Horrocks missed beginning of trasit, probably because he was conducting services at his church, and missed the end due to sunset. Crabtree saw only a few moments of the transit due to clouds. Showed Venus was MUCH smaller than the Sun, and much smaller than expected Mural at Manchester town Hall by Ford Maddox Brown
Kepler ’ s laws give all distances in the solar system in terms of the AU. AU is the basis for all distance observations
Demo thumb parallax w/ 2 distances Demo two paths Note observers doing pics by hand: hard to get accurate path, measure time instead since path length is proportional to the time. Need to know distance between observers to get distance to planet - longitude poorly known
Demo thumb parallax w/ 2 distances Demo two paths Note observers doing pics by hand: hard to get accurate path, measure time instead since path length is proportional to the time. Need to know distance between observers to get distance to planet - longitude poorly known
Demo thumb parallax w/ 2 distances Demo two paths Note observers doing pics by hand: hard to get accurate path, measure time instead since path length is proportional to the time. Need to know distance between observers to get distance to planet - longitude poorly known
Demo thumb parallax w/ 2 distances Demo two paths Note observers doing pics by hand: hard to get accurate path, measure time instead since path length is proportional to the time. Need to know distance between observers to get distance to planet - longitude poorly known
Lomonosov assumed atmosphere similar to Earth - jungle planet 1761 expeditions plagued by 7 year war and bad weather 1769 expeditions to do more than just transit watching Cook ’ s voyage Several obersatories set up in American Colonies (image: replica of State House observatory in Philadelphia where Declaration of Independence was read at the Henry Ford in Dearborn Mi) Franklin ensured Philadelphia observations were published in Europe John Winthrop aided Harvard in setting up observatories
Lomonosov assumed atmosphere similar to Earth - jungle planet 1761 expeditions plagued by 7 year war and bad weather 1769 expeditions to do more than just transit watching Cook ’ s voyage Several obersatories set up in American Colonies (image: replica of State House observatory in Philadelphia where Declaration of Independence was read at the Henry Ford in Dearborn Mi) Franklin ensured Philadelphia observations were published in Europe John Winthrop aided Harvard in setting up observatories
Lomonosov assumed atmosphere similar to Earth - jungle planet 1761 expeditions plagued by 7 year war and bad weather 1769 expeditions to do more than just transit watching Cook ’ s voyage Several obersatories set up in American Colonies (image: replica of State House observatory in Philadelphia where Declaration of Independence was read at the Henry Ford in Dearborn Mi) Franklin ensured Philadelphia observations were published in Europe John Winthrop aided Harvard in setting up observatories
Optical illusion caused by combination of Venus ’ and the Sun ’ s atmospheres and diffraction in the eye or the telescope aperture.
Images: Transit of Mercury Simon Newcomb Heliostat
Images: Simon Newcomb Heliostat
11 images survive Can see degradation of the plates in these images.