The Earth and Moon Planetary System . ... Credit: Northwestern University , JPL , NASA Explanation: How similar in size are the Earth and the Moon ? A dramatic visual answer to this question is found by combining photographs taken by the Mariner 10 spacecraft that headed out toward Venus and Mercury in 1973. The Moon can be seen to have a diameter over one quarter that of Earth, relatively large compared to its planetary companion . ... NASA Web Site Statements, Warnings, and Disclaimers . ...