Transits of Venus and the American Expeditions of 1874 and 1882

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Transits of Venus and the American Expeditions of
1874 and 1882 Steven J. Dick NASA Chief
Historian GSFC Scientific Coll o quium January
30, 2004
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Courtesy Fred Espenak
Fffff
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Transit Visibility 2004
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Sun-Earth Connection Education Forum
http//sunearth.gsfc.nasa.gov/sunearthday/2004/ind
ex_vthome.htm
Sten Odenwald, Elaine Lewis, Troy Cline
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Transits of Venus

• Importance/Early History
• 2. The American Expeditions
• 3. From Kepler (the Man)
• To Kepler (the Spacecraft)
• 2.

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Introduction To Transits of Venus .
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A Rare Phenomenon
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Importance of Transits of Venus

• Very rare event
• Colorful history of observations
• International cooperation and rivalry space
  race
• Mysterious observational effects black drop
• Bears on greatest problem in history of
  astronomy
• Solar system analogue to extrasolar planet
  detection
• via transits
• At the limits of science, just as extrasolar
  planets

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May 7, 2003 Transit of Mercury From SOHO
Spacecraft
Credits SOHO/ESA NASA
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Circumstances for Transits of Venus

• -13 Venus years nearly 8 Earth years, so every
  8 years
• the Sun, Venus and Earth have nearly the same
  relative position
• Tilt of orbit of Venus to Earth makes event much
  rarer
• (4 times in 243 years at intervals of 105, 8,
  121, 8 years.
• - 8 year pairs due to extended diameter of Sun

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Horrocks Observations, 1639
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B
SUN
A
VENUS
The difference in the chords produced by the
transit as seen from the Earths northern
hemisphere (A) and its southern hemisphere (B) is
a measure of the parallax of Venus, and thus
solar parallax. The chords are different
lengths, which can be determined by timing the 4
transit contacts from each location (Halleys
method). Delisles method used measures of Venus
on the face of the Sun, but required knowledge
of the geographical location of the observing
stations.
EARTH
A
B
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Parallax Measures
Object Closest Geocentric
Parallax Distance
(arcseconds)
(AU/Miles) _______________________
______________________ Mercury
.672/62M 13 Venus
. 279/26M
31 at inferior
conjunction Mars .376/35M
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at perihelic opposition Eros
.161/15M 54
Problems extended objects, Venus invisible when
closest at inferior conjunction, except in
transit!
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Results of 18th Century ToV
Source parallax AU
(Arcseconds)
(Miles) Encke 8.5776 (!)
95, 250, 000 (1824) Modern 8.794148
92, 955, 859
Difference of 2, 294, 141 miles
NOTE Every .01 arcseconds is approx. 100, 000
miles!
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The American Expeditions .
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ToV Expeditions - 1874
U.S. 8 British 12 Russians 26 France
6 Germany 6 Italy 3 Holland 1
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Two AstronomersCrucial to the American Transit
of Venus Story
Simon Newcomb
William Harkness
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Locations of the three Northern and Five
Southern 1874 American Transit of Venus stations
are Indicated on this map of times of First
contact. The map was prepared By George W. Hill
under the Direction of J.H.C. Coffin,
Superintendent of the American Nautical Almanac
Office.

• Peking, China
• Vladivostok, Russia
• Nagasaki, Japan
• Kerguelen Island
• Campbell Town, Tasmania
• Hobart Town, Tasmania
• Queenstown, New Zealand
• Chatham Islands

s
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The Instruments 5-inch Clark Refractor
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The Instruments - Photoheliograph
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The Instruments Broken Transit Howard Clock
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Instruments on Display at U. S. Centennial in
Philadelphia, 1876
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• Black metal boxes filled with instructions,
  log books for recording observations, and other
  papers were sent with each expedition.
• These boxes will also be loaned to the
  Smithsonian for the 2004 exhibition.

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Practice - Spring, 1874
Davis Peters Newcomb Hall Draper
Grounds of U. S. Naval Observatory Washington, DC
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Artificial Transit
Artificial transit device for practice, from
Newcombs Popular Astronomy (1878). The Black
sheet metal disk is one foot in diameter, and was
observed through a Telescope from a distance of
about 1 km.
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American Transit of Venus Party for New Zealand
The New Zealand Transit of Venus party and
ex-officio members, probably outside the Naval
Observatory. Seated (left to right) C. L.
Phillippi, chief photographer Prof. C. H. F.
Peters, Chief of Party Admiral Davis, President,
U. S. Transit of Venus Commission Prof. Henry
Draper, Superintendent of Photographic
Department. Standing (left to right) I. Russell,
First Assistant Photographer E. B. Pierson,
Second Assistant Photographer L. M. Aymé, Third
Assistant Photographer Lt. (U. S. Army) E. W.
Bass, assistant astronomer. From a set of stereo
views of Washington published by J. F. Jarvis,
Washington D.C.
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Black Drop Effect
Second contact, sketched by F. Allerding at
Sydney, Australia, Dec 9, 1874 using a 3.5-inch
refractor. Images are shown in sequence from
left to right.
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Results of 19th Century ToV

• Source parallax AU
• (Arcseconds)
  (Miles)
• Encke 8.5776 (!) 95,
  250, 000
• (1824)
• Harkness 1882 American ToV photos
• (1889) 8.842 92,
  455, 000
• Harkness system of constants
• 8.809 92, 797,
  000
• Modern 8.794148 92, 955,
  859

Difference (ToV observations) 500, 859
miles Difference (system of constants) 158, 000
miles
NOTE Every .01 arcseconds is approx. 100, 000
miles!
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• The observations volume (564 pages) was
  actually set in type and the page proof copy
  dated 1881 was delivered to the U.S Naval
  Observatory,
• BUT
• the observations were NEVER PUBLISHED.

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• The ADS (Astrophysics Data System) has
  recently placed the full text online.

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From Kepler the Man To Kepler the Spacecraft .
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Kepler Mission to Detect Extrasolar Planets via
Transits
PI Bill Borucki (NASA Ames) 100, 000 stars over
4 years Gas giants and terrestrial
planets Launch 2007
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Summary

• 18th Century Transits of Venus did improve
• the value for the Astronomical Unit
• 19th Century Transits of Venus also
• improved the value of the Astronomical Unit.
• BUT results were superseded by other methods
  
• even more accurate (speed of light, etc.)
• Transit of Venus expeditions are an important
• part of the history of American science,
  attempts
• to determine the solar parallax, and
• astronomical constants
• A lot of fun!

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Parting Thought . . .
We are now on the eve of the second transit of a
pair, after which there will be no other till
the twenty-first century of our era has dawned
upon the earth, and the June flowers are
blooming in 2004. When the last transit season
occurred the intellectual world was awakening
from the slumber of ages, and that wondrous
scientific activity which has led to our present
advanced knowledge was just beginning. What will
be the state of science when the next transit
season arrives God only knows. Not even our
children's children will live to take part in
the astronomy of that day. As for ourselves, we
have to do with the present ...   Willi
am Harkness, 1882i