The Transit of Venus

“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

June 5-6, 2012

What is a Transit?Like a mini eclipse of the Sun

Planet

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?

The Orbits Aren’t Aligned

Earth Orbit

Venus Orbit

If Venus is above Earth’s Orbit…

We see:

If Venus is below Earth’s Orbit…

We see:

But if Venus aligns with Earth’s orbit (Astronomers say “on a

node”)…

We see a transit!

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

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.

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

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!

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

Edmund Halley

• In 1716, Halley showed how to use a transit to measure the Astronomical Unit…

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

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:

Measuring the AU• Northern Hemisphere observer sees Venus

travel low across the Sun:

Measuring the AU• Southern Hemisphere observer sees the

higher path:

Measuring the AU• The distance between the two lines, , and

the distance between the two observing points, d, gives the Earth-Venus distance, D.

}

d

D

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…

Venus Atmosphere

D. Kiselman, et al. (Inst. for Solar Physics), Royal Swedish Academy of Sciences. 2004

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

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.

An Unexpected Problem

• The “black drop” effect made accurate measurements impossible!

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

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

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,

Surviving Images

• 11 images survive from 1882– Note gridlines

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…

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.

Importance for the 21st Century

• Spectroscopy of atmosphere– Search for trace elements

• The search for extra-solar planets

The atmosphere

• Composition determined from changes in spectrum– Use this for

planets around other stars too!

HD 209458

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

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

• 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/

projects/transit-of-venus.html

•“On account of their rarity alone, they must afford an exquisite entertainment to an astronomical taste”

- John Winthrop 1769