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