Transit12
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Slideshow for the Transit of Venus, June 5, 2012.
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Transit12
- 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?
- 4. The Orbits Aren’t Aligned Earth Orbit Venus Orbit
- 5. If Venus is above Earth’s Orbit…
- 6. We see:
- 7. If Venus is below Earth’s Orbit…
- 8. We see:
- 9. But if Venus aligns with Earth’s orbit (Astronomers say “on a node”)…
- 10. We see a transit!
- 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:
- 20. Measuring the AU • Southern Hemisphere observer sees the higher path:
- 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…
- 23. Venus Atmosphere D. Kiselman, et al. (Inst. for Solar Physics), Royal Swedish Academy of Sciences. 2004
- 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.
- 26. An Unexpected Problem • The “black drop” effect made accurate measurements impossible!
- 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,
- 30. Surviving Images • 11 images survive from 1882 – Note gridlines
- 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
Notes de l'éditeur
- 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.
- TRACE will observe the transit