oppositions of Mars
The Earth-Mars distance fluctuates between about 56 million and 400 million kilometers (35 million and 250 million miles), as the two planets swing around their respective orbits. The best times for telescopic viewing are when Mars is in opposition – that is, close to the Earth and in the opposite part of the sky from the Sun. These happen every 780 days. But the most favorable oppositions of all, when the two planets approach as close as possible, occur only at intervals of between 15 and 17 years, and always in July, August or September. On August 27, 2003, the opposition distance of Mars from Earth was the closest it has been for about 60,000 years. Calculations suggest that the last time they were this close together was when Neanderthals roamed the planet, on September 12, 57,617 BC. Other significant encounters happened during the month of August in 1924, 1845 and 1766 but there won't be another close approach for 284 years. Earth has a more-or-less circular orbit while Mars takes a more elliptical path around the star. The two planets travel at different speeds around the Sun, the Earth eventually catching and overtaking Mars. This means Mars passes closer to Earth than usual at various times in history, with the 2003 opposition being particularly memorable.