The Historical Search for the Astronomical Unit's Value and the Subsequent Study of Venus's Transits

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The Historical Search for the Astronomical Unit's
Valueand the Subsequent Study of Venus's
Transits As Explained by the book Venus in
Transit by Eli MaorKayleigh Ayn Bohemier
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What is a Transit, Anyway?

• A planet transits when it passes in front of the
  sun. This happens fairly rarely because planets
  are usually inclined to Earth's orbit, and both
  planets need to be in the right place at the
  right time.

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The Mechanics of Transits

• http//en.wikipedia.org/wiki/ImageTransit_diagram
  _angles.png

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Transit Periodicity

• http//www.nao.rl.ac.uk/nao/transit/periodicity/in
  dex.html

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The Astronomical Unit

• http//upload.wikimedia.org/wikipedia/en/7/77/Edmu
  nd_Halley.gif

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Halley's Proposal to the Royal Society

• 1716

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• There are many things exceedingly paradoxical,
  and that seems quite incredible to the
  illiterate, which yet, by means of mathematical
  principles, may be easily solved. Scarce any
  problem will appear more difficult than that of
  determining the distance from the sun to the
  earth, very near the truth but even this, when
  we are made acquainted with some exact
  observations, taken at places fixed upon and
  chosen beforehand, will, without much labour, be
  effected. And this is what I am desirous to lay
  before this illustrious Society (which I foretell
  will continue for ages), that I may explain
  beforehand to young astronomers, who may perhaps
  live to observe these things, a method by which
  the immense distance of the sun may be truly
  obtained to within a five-hundredth part of what
  it really is.

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THE PLAN
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Use Sun as Billboard to Measure Venus's Parallax
from Different Earth-based Locations

• http//www.futura-sciences.com/uploads/tx_oxcsfutu
  ra/comprendre/d/images/venusen2.jpg

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Schedule of Transits

• 1518 May 26
• 1526 May 23
• 1631 Dec 7
• 1639 Dec 4
• 1761 Jun 6
• 1769 Jun 3
• 1874 Dec 9
• 1882 Dec 6

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August 1760 Delisle's Method

• 1. Observe Moment of Ingress
• 2. Observe Moment of Egress

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Discovery of Venus's Atmosphere

• Mikhail Vasilievitch Lomonosov

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The Distance from Earth to the Sun

• Using the Venus-Transit method, the parallax
  measurements are between 8.97'' and 8.83''.
• Solar distance range
• 92 570 000 to 93 000 000 miles
• Uncertainty
• 0.5 million miles

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• Scientist Hey, let's use Venus's parallax to
  help us determine the value of the Astronomical
  Unit!
• The Crowd That sounds like a brilliant idea!
• Fast-forward to 1874
• Scientist Um . . . look, guys, this just isn't
  working. Maybe we had better use other methods of
  finding the Astronomical Unit.
• The Crowd Oh, well. Transits of Venus still look
  pretty cool.

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Other Methods

• Observe the parallax of Mars and asteroids.
• Solar parallax 8.806''
• Mean Solar Distance 92 830 000 miles

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Transit of Venus Today

• The next transit is on 2012 Jun 6.
• Anyone wanna go?