Indeed, they measure it by the planet. All though it is quite possible for the moons to have different day lengths, infact a synodic period on our moon is 29.5 days

Assumign your question was actually meant to say how do they calculate a MONTH, then the word month derives from the word moon so each of Jupiters's 63 companion satellites would have its own individual month length,

And as you might expect those orbiting closest to the planet have the shortest months and those furthest away have the longest months (they both have further to go and go more slowly). The biggest four, known since Galileo trained his telescope on them in January 1610, are all quite close in, at positions 5-8 inclusive,

In order ourwards from the planet, measured in Earth days, hours and minutes

(Note that for Metis and Adrastea the day is longer than the month, )

1 Metis 7h 4m 29s

2 Adrastea 7h 9m 30s

3 Amalthea 11h 57m 22.67s

4 Thebe 16h 11m 17s

5 Io 1.77 days

6 Europa 3.55 days

7 Ganymede 7.15 days

8 Callisto 16.69 days

9 Themisto 129.87 days

10 Leda 241.75 days

11 Himalia 250.37 days

12 Lysithea 259.89 days

13 Elara 261.14 days

14 S/2000 J 11 287.93 days

15 Carpo 458.62 days

16 S/2003 J 12 482.69 days

17 Euporie 538.78 days

18 S/2003 J 3 561.52 days

19 S/2003 J 18 569.73 days

20 Thelxinoe 597.61 days

21 Euanthe 598.09 days

22 Helike 601.40 days

23 Orthosie 602.62 days

24 Iocaste 609.43 days

25 S/2003 J 16 610.36 days

26 Praxidike 613.90 days

27 Harpalyke 624.54 days

28 Mneme 627.48 days

29 Hermippe 629.81 days

30 Thyone 639.80 days

31 Ananke 642.02 days

32 S/2003 J 17 672.75 days

33 Aitne 679.64 days

34 Kale 685.32 days

35 Taygete 686.67 days

36 S/2003 J 19 699.12 days

37 Chaldene 699.33 days

38 S/2003 J 15 699.68 days

39 S/2003 J 10 700.13 days

40 S/2003 J 23 700.54 days

41 Erinome 711.96 days

42 Aoede 714.66 days

43 Kallichore 717.81 days

44 Kalyke 721.02 days

45 Carme 721.82 days

46 Callirrhoe 722.62 days

47 Eurydome 723.36 days

48 Pasithee 726.93 days

49 Cyllene 731.10 days

50 Eukelade 735.20 days

51 S/2003 J 4 739.29 days

52 Pasiphaë 741.09 days

53 Hegemone 745.50 days

54 Arche 746.19 days

55 Isonoe 750.13 days

56 S/2003 J 9 752.84 days

57 S/2003 J 5 758.34 days

58 Sinope 762.33 days

59 Sponde 771.60 days

60 Autonoe 772.17 days

61 S/2003 J 14 776.02 days

62 Megaclite 792.44 days

63 S/2003 J 2 1077.02 days

The other rotation period that is of interest is that of the Great Red Spot, Jupiter's best known feature, a persistent anticyclonic storm located 22° south of the equator that is larger than Earth. It is known to have been in existence since at least 1831 and possibly since 1665.

Mathematical models suggest that the storm is stable and may be a permanent feature of the planet. The storm is large enough to be visible through Earth-based telescopes.

The oval object rotates counterclockwise, with a period of about 6 days. The Great Red Spot's dimensions are 24–40,000 km × 12–14,000 km. It is large enough to contain two or three planets of Earth's diameter

**Edited by Gavsto, 27 March 2007 - 02:10 PM.**

"We live in a society exquisitely dependent on science and technology, in which hardly anyone knows anything about science and technology." - Carl Sagan