Comet Encke was "discovered" in 1786, 1795,1805, and 1818 by different astronomers. These comets were found to be the same only after orbital calculations in 1819 by the German astronomer Johann Encke, who then predicted its return in 1822.

A new look at how light bends as it travels through the universe could point to an alternative theory of gravity

Retrograde motion is an effect of changing perspective. Inferior planets Mercury and Venus show retrograde motion on either side of inferior conjunction. They "overtake" Earth as they pass between Earth and the Sun.