Hubble Science Briefing

The Transit of Venus:

A Cosmologist Prepares

3 May 2012Dr. Susana Deustua

p. 1p. 11

Transits of Venus

• What are they?• How often do they

occur?• Why are they

interesting?• Where can I see one?• How can I observe a

transit this year?

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Credit: NASA/LMSAL

When Venus is between Earth and Sun (Inferior Conjunction)

ANDWhen Venus crosses the Earth’s orbital plane

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Inferior Conjunction

Figures are from http://astrobob.areavoices.com/2010/10/28/allow-me-to-introduce-you-to-venus-dark-side/

Venus-Earth Model

The image above shows the model of the orbit of Venus, relative to the Earth, made by Benjamin Cole in London to explain the transit of Venus to the Fellows of the Royal Society before 1761 (courtesy Science Museum, London). 

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Venus Transits from Earth

Node: direction of Venus motion in its orbit when it crosses the plane of the Earth’s orbit.

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• Why all the excitement?– Because we want to know– How Big is/are

• The Universe• The Galaxy• The Solar System• Stars• Planets

– Which means we want to know distances between objects

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0 1 10 102 103 104 105 106 107 108 109 1010

Distance in parsec

Stars in the Milky Way Galaxy

Galaxies, Quasars, Galaxy Clusters

Distant Galaxies

Star Clusters in the Milky Way Galaxy

Nearby Galaxies

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How do you measure the size of things?

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How do you measure the size of things?

With a ruler, of course!

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What do you do when you can’t use a ruler at all?

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Parallax

Parallax is conceptually very simple - all one needs to do is measure anglesand know the exact value of the Earth-Sun distance

1 AU = X milesBut how to measure it?

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A2 + B2 =C2

sin a = B/Ccos a = A/Ctan a = B/A

sin b = A/Ccos b = B/Ctan b = A/B

• Predicted and observed the 1639 transit of Venus

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The Transit of 1639- Jeremiah HorroxThe Transit of Venus over the Sun:

orAN ASTRONOMICAL TREATISE

onTHE CELEBRATED CONJUNCTION

ofVENUS AND THE SUN

ON the 24th of NOVEMBER, 1639.By JEREMIAH HORROX.

“In the second place, the distance between the centres of Venus and the Sun I found, by three observations, to be as follows :The Hour. Distance of the Centres.At 3.15 by the clock. 14’ 24”“ 3.35 “ 13’ 30”“ 3.45 “ 13’ 00” ” 3.50 the apparent sunset.The true setting being 3.45, and the apparent about 5 minutes later, the difference being caused by refraction. The clock therefore was sufficiently correct.”

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Horrox used a piece of paper with degrees drawn on it

Predicted and observed the 1639 transit of VenusHe based his predictions on Kepler’s lawsPublished his results in Venus in Sole Visaand determined the diameter of Venus to be ~1/30th of the Sun’s and the relative distance of Venus:Sun and Earth:Sun to be 72333:100000

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• In 1676 a young Edmund Halley sets sail for the island of St. Helena, with a 5-foot sextant, a 24-foot long telescope, and a clock

• In November 1677 he observed and accurately recorded a transit of Mercury

• Consequently he realized that if a transit were observed at different latitudes, the solar parallax could be calculated from the apparent paths of the planet across the Sun.

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tan(SP) = Radius of Earth Earth-Sun Distance 1 AU = Radius of Earth tan (SP)

The Solar Parallax

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120 observers at 60 stations, among which were French and English astronomers who voyaged thousands of miles to set up observation posts.

Results solar parallax 8.28 – 10.60 arcsecondsearth-sun distance 77 x106 – 97 x106 miles

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Some veterans of the 1761 expeditions and new actors: 1.Guillaume Le Gentil –left Manila and headed to Pondichery, India (where he was clouded out!)2.Alexandre Pingre – Santo Domingo3.Jean Baptise Chappe d’Autreroche – San Jose del Cabo in Baja California (died, typhus epidemic)4.Father Maximilian Hell - Vardö, Lapland5.Stepan Rumovsky – Kola Peninsula6.Joseph Dymond & William Wales- Hudson Bay, Canada7.James Cook & Charles Green to the South Seas (Tahiti)

If anything, more enthusiasm for the next transit. Results600+ papers written solar parallax:8.43 – 8.80 arcsecondsearth-sun distance 90 x106 – 94 x106 miles

Better – but still not accurate enough at 5%.

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Captain James Cook’s & Charles Green’s observations of the transit of Venus in 1769

Note the ‘Black Drop’.

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Nevertheless, in 1891 Simon Newcomb, American Astronomer, re-analyzed ALL the Venus transit data using meta-analysisHis Results:solar parallax: 8.79 arcsecondsearth-sun distance: 92.99 x106 milesNB: 1771 analysis by Thomas Hornby, English scientists, based on observations from 5 locations solar parallax:8.65 arcsecondsearth-sun distance93.72 x106 miles for an earth radius of 3985 miles

1874 & 1882 Transits of Venusdidn’t generate much enthusiasm. A century of poring over ‘old’ numbers, and realization that the latest techniques were not satisfactory – would not lead to the sought for improvements in accuracy

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Astronomers use RADAR echoes (just like state troopers with their radar guns) to measure distance to Venus from Earth over a period of time

20th Century

•radio signal travels at light speed•measure time for signal roundtrip •calculate

distance = velocity x time

Today’s accepted valuessolar parallax 8.794148 arcsecondEarth-Sun distance 92,958,329 miles

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Pete Lawrence’s montage of the 2004 transit, including an annular eclipse of the planet.

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In January, 2012 HST practiced for the June 5/6 2012 Transit of Venus•by acquiring data of two craters on the moon: Hipparchus-C and Dolland-E•to test the orbit at a similar moon phase as during the 2012 transit of Venus, select an appropriate location to target on the Moon and fine-tune the exposure times for the different instruments and filters used.

35Ref: Visit Status Report

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Venus reflected off the moon

“In a relatively near future, numerous transiting extrasolar planets will be discovered {gaseous giant planets, Earth-size planets and temperate Uranus in the form of "Ocean-planets"}. Space telescopes operating in the UV-optical-IR will allow the study of their atmospheres. We have to show if and how these observations will give access to the detection of atmospheric species, particularly when telluric (Earth-like) planets will be observed, to demonstrate that life may be possible on one of them. For that purpose, we propose to use the unique event of the century, the Venus transit in 2012 {next Venus transits are in 2117 and 2125!}, to demonstrate the feasibility of these observations and show precisely what a Venus-like planet will look-like. To observe the Venus transit with similar conditions as extrasolar planets {no spatial resolution}, we propose to observe the solar light reflected on the Moon during the Venus transit on June 5-6 2012, lasting about 7h 40mn, i.e. about 4 HST orbits. A total of 5 HST orbits will allow us to obtain high S/N transit spectra and reference spectra to reveal the detectable atmospheric species with current space instrumentations. Similarly, in a companion proposal, we propose to observe the Earth transit on the Moon through the reflected light during a total Moon eclipse to directly compare the observed atmospheric signatures of Earth-like and Venus-like extrasolar planets.”

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http://archive.stsci.edu/proposal_search.php?id=12537&mission=hst

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5/6 June 2012 Transit of Venus

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Sun Funnel

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• http://cdn.transitofvenus.org/docs/Build_a_Sun_Funnel.pdf• http://www.transitofvenus.org/june2012/eye-safety

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A pinhole cameraYou will need 1.a white screen:

1. blank sheet of white paper, a sheet on a wall, a white wall

2.a pinhole: 1. index card, sheet of

cardboard 1. cut out a 2cm-sized

hole (round or square or triangle or any shape really)

2. tape aluminum foil over hole in cardboard

3. Poke hole in foil with pin

OR2.cover window except for a very small area with black material/paper

3. Project image of sun through pinhole onto screen. –The larger the distance between pinhole and screen the larger the image

http://www.exploratorium.edu/eclipse/how.html

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And, of course,NEVER

stare directly at the SUN.

Be Safe

Check with your local astronomy clubs, community colleges, science centers, museums and similar organizations for transit events in your community.

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See Viewing the Transit & Eye Safety at june2012/eye-safety/280-viewing-the-transit-eye-safety for definitive advice on viewing the sun safely; by B. Ralph Chou, MSc, OD.  (linked from http://www.transitofvenus.org/june2012/eye-safety )

"It is never safe to look at the sun without proper eye protection.  No filter should be used with an optical device (e.g. binoculars, telescope, camera) unless that filter has been specifically designed for that purpose and is mounted at the front end (i.e., end towards the Sun). Unsafe filters include all color film, black-and-white film that contains no silver, photographic negatives with images on them (x-rays and snapshots), smoked glass, sunglasses (single or multiple pairs), photographic neutral density filters and polarizing filters, computer disk media. Most of these transmit high levels of invisible infrared radiation which can cause a thermal retinal burn.  The fact that the Sun appears dim, or that you feel no discomfort when looking at the Sun through the filter, is no guarantee that your eyes are safe.  A person with eye damage from improper viewing may not notice the damage until hours later."

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• http://astro.ukho.gov.uk/nao/transit/V_2012/index.html

• http://transitofvenus.com/• http://www.usno.navy.mil/USNO/astronomical-

applications/data-services/transit-us• http://eclipse-maps.com/Eclipse-Maps/

Transits.html• http://transitofvenus.org/• http://www.exploratorium.edu/venus/• http://eclipse.gsfc.nasa.gov/OH/transit12.html

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Questions?