Black drop effect

This led to the failure of the attempts during the 18th century transits of Venus to establish a truly precise value for the astronomical unit.

However, it is now thought by many to be an optical effect caused by the combination of the extreme darkening of the Sun's disk near its apparent edge and the intrinsic imperfection of the viewing apparatus.

The Shuckburgh telescope of the Royal Observatory, Greenwich in London was used for the 1832 transit of Mercury.

[6] It was equipped with a micrometer by Dollond and was used for a report of the events as seen through the small refractor.

[6] By observing the transit in combination with timing it and taking measures, a diameter for the planet was taken.