Martian Orbital Parameters

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Posted 1/2/2013

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)

 

TABLE 1 – THE MARTIAN MONTHLY CALENDAR

MARS

MONTH

END SOL

START SOL

DAYS

Degrees per month

ΔLs Degrees

per Day

1

61

0

61

30

0.491803

2

127

61

66

30

0.454545

3

193

127

66

30

0.454545

4

258

193

65

30

0.461538

5

318

258

60

30

0.5

6

372

318

54

30

0.555556

7

422

372

50

30

0.6

8

468

422

46

30

0.652174

9

515

468

47

30

0.638298

10

562

515

47

30

0.638298

11

613

562

51

30

0.588235

12

669

613

56

30

0.535714

 

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.

 

TABLE 2 - ORBITAL PARAMETERS

SOURCE:

 

 

MARS

EARTH

RATIO

Semimajor axis (106 km)

227.92

149.60

1.524

Sidereal orbit period (days)

686.980

365.256

1.881

Perihelion (106 km)

206.62

147.09

1.405

Aphelion (106 km)

249.23

152.10

1.639

Mean orbital velocity (km/s)

24.13

29.78

0.810

Max. orbital velocity (km/s)

26.50

30.29

0.875

Min. orbital velocity (km/s)

21.97

29.29

0.750

Orbital Inclination (deg)

1.850

0.000

--

Orbit eccentricity

0.0935

0.0167

5.599

Sidereal rotation period (hrs)

24.6229

23.9345

1.029

Length of day (hrs)

24.6597

24.0000

1.027

Obliquity to orbit/axis tilt (deg)

25.19

23.44

1.075

  

 

Figure 1 - I found it necessary to correct the REMS Team and JPL about which month it was on Mars back in August 2012. They made the correction, but the level of mistakes made has been high enough to raise questions about what is really going on with the REMS Team. Are they being prevented from revealing the truth about Martian air pressure, or were they simply incapable of distinguishing between hPa and Pa pressure units?

Figure 2 - Comparison of Martian and Terrestrial Orbits.