vulcan

The closest planet to The Sun is Mercury. However, there WAS, at one time, an even closer planet to The Sun. This was Vulcan, which is no longer easily visible. Its orbital period was  19 days and 17 hours, which is 19.70833 Earth days. Below is a scan from the book Oddities, by Rupert T. Gould, which provides an account of how this period was measured.