Transit of Venus

1
Transit of Venus

• Pattern of transit and why
• How transit can be used to determine astronomical
  unit
• Historical importance

2
Pattern

• Occurs in 8 year pairs
• Pairs separated by gaps of 121.5 years and 105.5
  years
• Cycles every 243 years

Why?

• Venus orbit is tilted by about 3.3 degrees in
  relation to Earth

3
Determining AU

• Venus appears to pass in front of Sun in two
  parallel lines from two positions, must know
  distance between lines
• Transit takes 7 hours, Halley predicts must know
  time to within 2 seconds to be accurate

4
Historical Importance
Early Method to Determine AU

• Halley(1656-1742) realizes Venus close enough to
  use parallax
• Halley developed detailed plan for 1761,1769
  transits
• Halley predicated accuracy within 1/40th Suns
  Arc sec
• Multiple expeditions worldwide to measure time of
  transit
• Black Drop effect prevents accuracy