Mariner 5 . ... The Mariner 5 spacecraft was the fifth in a series of spacecraft used for planetary exploration in the flyby mode. Mariner 5 was a refurbished backup spacecraft for the Mariner 4 mission and was converted from a Mars mission to a Venus mission. ... The closest flyby distance was 3,990 kilometers (2,480 miles) and, with more sensitive instruments than its predecessor, Mariner 5 was able to shed new light on the hot, cloud-covered planet and on conditions in interplanetary space. ...

... The greater the distance, the smaller the parallax. ... That triangle has the same proportions as the much larger triangle ABC, and therefore, if the distance B'C to the thumb is 10 times the distance A'B' between the eyes, the distance AC to the far landmark is also 10 times the distance AB. ... The most distant objects our eyes can see are the stars, and they are very far indeed: light which moves at 300,000 kilometers (186,000 miles) per second, would take years, often many years, to reach them. ...