Astronomy Picture of the Day . ... 31.12.1999 . ... Credit: Apollo 8 Crew, NASA Explanation: When the second millennium began , people generally knew that the Earth was round , but few saw much of it beyond their local village. ... The Earth started as the center of everything , but became a planet placed in the Solar System , which became placed in a Galaxy , which became placed in the Local Group of Galaxies , which became placed in an expanse so vast we call it just the Universe . ... 1999 2000 . ...