Venus
The Romans thought Venus was hot. Too right! She is the hottest thing in the Solar System. A bit smaller than the Earth, with an atmosphere full of CO2, she boasts a ground temperature of 475°C, enough to melt lead. This is what happens when the greenhouse effect goes crazy.
Astronomers use the Astronomical Unit (AU, the radius of the Earth orbit) for distances. In the 1600’s, it was already known that Venus’ orbit radius is about 0.7 AU. The trouble was that they only had a vague idea of the actual size of the AU. Then in 1716 Edmund Haley proposed a method to calculate it using the transit of Venus.
When Venus is aligned with the Sun, she looks like a black spot that crosses the Sun surface. You can make a simulation at home with an orange and a knitting needle like on the photo. Look at the needle head path on the orange as you gently rotate it. You can also tilt it back and forth to simulate what is seen from the North or the South of the Earth.
To calculate the AU, we need two observers. The North observer sees the North path, the South observer sees the South path. We measure the angle between the two paths. Calculating the AU is then a matter of simple geometry. About 150 million km.
If you wonder what James Cook was doing in Tahiti in 1769, he was having a go at the South job.
References
Kepler: https://en.wikipedia.org/wiki/Johannes_Kepler
Venus facts: https://solarsystem.nasa.gov/planets/venus/overview/
Transit of Venus calculations: https://www.exploratorium.edu/venus/question4.html
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