The distance to the Sun was first determined with a good accuracy from worldwide observations of the transit of Venus. A transit of Venus is a phenomenon where Venus passes in front of the Sun. By measuring the time Venus spends crossing in front of the Sun from two or more locations on the surface of the Earth, it is possible to calculate the distance to the Sun.
Flat-Earthers insist that it is impossible to determine the distance to the Sun in the globe model because sun rays are practically parallel. They are wrong. Using simple geometry, the distance to the Sun can be determined from the observations of the transit of Venus.
In 1663, James Gregory came up with the idea of using Venus or Mercury transits to determine the distance to the Sun. In 1691, Edmond Halley published all the detailed calculation to determine the distance to the Sun using observations of the transit of Venus.
Halley died in 1742, but from his publication, scientists from all over the world were able to prepare and worked together to observe the transits of Venus of 1761 and 1769. Many of the European powers were at war during the time, and many countries made the exemption to allow opposing ships with the scientific mission to pass unharmed.
Their result was 93,726,900 miles. This is only a difference of about 0.8% from the radar-based value used today.
Gory Details
References
- 1769 Transit of Venus observed from Tahiti – Wikipedia
- A New Method of Determining the Parallax of the Sun, or Hist Distance from the Earth – Dr. Edmund Halley
- The Mathematics of the Transit of Venus – Dr. Sten Odenwald – NASA
- Measuring the size of the Solar System – ESA
- James Cook and the Transit of Venus – NASA
- Transit of Venus – Wikipedia
- Calculating the Astronomical Unit during a Transit of Venus using Satellite Data – NASA