Mars as seen from Earth
Mars will be exactly opposite the sun in the sky in a rare cosmic alignment set to take place Tuesday (April 8). Called an opposition, this happens with Mars from Earth’s perspective every 778 days, or 2 years, 1 month and 18 days. Think of Earth and Mars as two cars racing on circular tracks. Because Earth is closer to the sun, it travels faster, completing a circuit in 365 days. Mars is farther from the sun and takes longer, 687 days. By the time Mars has completed one circuit, Earth has a lot of catching up to do to get to a point between the Red Planet and the sun. This is complicated by the fact that the two racetracks are not exact circles. As Johannes Kepler discovered in the 16th century, the planets follow slightly elliptical paths around the sun, sometimes closer to the Sun (perihelion), sometimes farther away (aphelion). [Best Night Sky Events of April 2014: Stargazing Sky Maps (Gallery)] Some planets, like Venus and Earth, follow paths that are almost perfect circles. Other planets, like Mercury and Mars, follow more elliptical orbits, which are described as being more eccentric, or differing from a circle. On the day of opposition, Mars, Earth and the sun fall on a straight line. Six days later, both planets will have moved a little along their orbits, but, because of the eccentricity of its orbit, Mars will be slightly closer to Earth than it was before. More