Mars at Opposition: 2012-2025

As Earth has a smaller orbit than that of Mars, it passes between Mars and the Sun about every 26 months. At such times Earth lies essentially between the Sun and Mars. Said another way, it is when Mars and the Sun—viewed from Earth—are in opposite directions. When this happens, we say that Mars is in opposition.

Because the orbits of both planets are not circular, Mars may be closer to Earth than 0.39 AU at some oppo- sitions (as in 2018), but further than 0.64 AU from it at others (as in 2025)! The closest approaches between Mars and Earth occur only about every 15 to 17 years, when Mars is somewhat near its own perihelion and Earth is somewhat near its own aphelion. This has important implications for missions to the Red Planet, for which overall travel times and distances are paramount concerns.

In addition to the oppositions of Mars, the above diagram reveals the equinoxes of both Earth and Mars. In this sense, these are the locations of each planet in its orbit when the Sun appears to move north and south over its equator. For Earth, of course, we call these respectively the March and September equinoxes. Un- fortunately, Mars does not repeatedly reach its own equinoxes on the same Earth dates, so a number of methods have been devised to implement a practical Timekeeping on Mars.

As the orbits of the planets are not precisely circular, the time for any planet to move in its orbit from one equinox to the next is not the same as the time to then complete its orbit back to the first equinox. For Earth this difference is only a little over a week, so it has minimal effect on our seasons. The same cannot be said for the Martian seasons. Succeeding travels between the equinoxes of Mars vary by about two and a half Earth-months, and this has important implications for the Martian seasons.

The Martian seasons are therefore considerably more variable in length than seasons on Earth. Moreover, as Mars's perihelion is considerably closer to the Sun than its aphelion, and as—when Mars is nearer perihelion than aphelion—its Southern Hemisphere "leans toward" the Sun, this hemisphere therefore receives appreci- ably more intense solar radiation than its counterpart during their respective summers. As a consequence summer in Mars's Southern Hemisphere is notably warmer than summer in its Northern Hemisphere! This may become a significant factor in deciding where the first colonies on Mars are established. In addition, from the orientation of the lines that can be drawn between each planet's equinoxes, it should be obvious that Earth and Mars often are not experiencing the same seasons when Mars is in opposition!

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