Martian Orbital Parameters

Understanding where Mars is in its orbit. The solar longitude Ls is the Mars-Sun angle, measured from the Northern Hemisphere spring equinox where Ls=0. Ls=90 thus corresponds to summer solstice, just as Ls=180 marks the autumn equinox and Ls=270 the winter solstice (all relative to the northern hemisphere). In trying to understand where Mars is in its orbit, it’s important to know that while all Martian months involve a change of 30 degrees, there is a large variation in number of days in each of the 12 months there, and also a large variation in angular speed around the sun (see Figure 1 and Table 1 below). 

While the slightly greater tilt of the Martian axis was not much of a factor around the equinoxes (Month 1, Sol 1; and Month 7, Sol 373), the tilt does make a significant difference at the solstices (Month 3, Sol 193; and Month 10, Sol 515). Note that aphelion (when Mars is furthest from the sun) and perihelion (when Mars is closest to the sun) does not correspond to the start of any Martian month. The next perihelion for Mars will be on December 12, 2014 which on the Martian calendar is Month 9, Sol 485, Ls 250.7. It will then be 1.38 AU from the sun. The next aphelion for Mars will be on January 2, 2014 which on the Martian calendar is Month 3, Sol 151, Ls 70.6. It will then be 1.67 AU from the sun.

       For the sake of quick reference, Table 2 contains a few useful facts for comparing Earth and Mars orbital parameters and orbital elements.