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# If a planet has multiple moons

## 15 posts in this topic

Interesting question but if a planet as multiple moons, how do they measure a day? By which moon?

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I could be mistaken, but I do not think that the moons matter as far as calculating the length of a day; it has to do with a 360* rotation of the planet itself...

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I could be mistaken, but I do not think that the moons matter as far as calculating the length of a day; it has to do with a 360* rotation of the planet itself...

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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

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

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

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As has been stated the of the length of a day of a planet is the time it takes the planet to spin once on its axis and is irrespective of the number of moons it has (Mercury and Venus both have days but neither of them have a moon).

To make life a little more confusing, astronomers have two ways to measure the length of the planet's day. These are the "sideral day" and the "mean solar day".

The siderial day is the time it takes the planet to rotate once about its axis and can be measure (in simple terms) by the time it takes the same, fixed, star to pass over head twice.

The mean solar day is the average time it takes the sun (or the hypothetical planet's home star) to go from one noon (over head) to the next.

If the Earth (or any other planet) was simply rotating on its axis these two measurements would be the same but the fact that the Earth is also orbiting the sun complicates this.

For the Earth the mean solar day is 24 hurs long but the siderial day is slightly shorter at 23 hours, 56 minutes and 4 seconds.

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If you're not sure what Waspie is talking about, this diagram helped me a long while back when I was studying this

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If you're not sure what Waspie is talking about, this diagram helped me a long while back when I was studying this

That takes me back a few years to my O-level astronomy. Ouch, it was 1983.

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That takes me back a few years to my O-level astronomy. Ouch, it was 1983.

1983?!?!

(...completely tongue in cheek...never would I insult a moderator...to forgive is devine... )

Anyway, great info guys.

I think the bottom line for our OP is that a day is the time it takes for the planet to spin around once.

The moon or moons aren't a consideration...

And, with guys like Gav and Waspie, you're getting a great and detailed Astronomy lesson in the process of that simple explanation.

Nice work!

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Some great science here, I followed MID to this section of UM. Thanks MID!

Just to add - remember that we can also see our moon during the early AM - After noon to late PM. It does not play into our measure of day - only cool to see a full moon during a blue sky. Kind of cool.

Ok - this was random, I admit it!

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.......

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

1 Metis 7h 4m 29s

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

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.

......

Blimey Gavsto, lot of effort in that posting, nice one!!

Regarding the storm on Jupiter, I wonder what sort of wind speeds it carries - however strong, I bet it would mess up one's hairdo lol.

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The moon has to do with a monthly cycle. The day is measured by the earths rotation around the sun and axial tilt.

Mars, for instance has two moons. The solar day (or sol) on Mars is only slightly longer than an Earth day: 24 hours, 39 minutes, and 35.244 seconds.

Mars' axial tilt is 25.19 degrees, which is similar to the axial tilt of the Earth. As a result, Mars has seasons like the Earth, though Mars' are about twice as long given its longer year. Link

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Are you asking this because of the thread on the length of Saturn's day?

If you are, they only mentioned Saturn's moon in that thread to say that it slows down the planets magnetic field, which means they can't calculate the length of it's day by looking at that field. Nothing to do with using the moon itself to calculate it.

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The moon has to do with a monthly cycle. The day is measured by the earths rotation around the sun and axial tilt.

The last sentence is incorrect. The day is the rotation of the earth on it's axis. The rotation (or orbit) around the sun is the year. The length of both of these is totally independent of axial tilt, which as you correctly state, is responsible for the seasons.

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Interesting question but if a planet as multiple moons, how do they measure a day? By which moon?

Interesting question, indeed! Neptune has how many moons? Neptune has 13 moons. 3 of those 13 orbit opposite the spin of the planet. You know, I think they fixed that when they started with the solar days, but to measure the days according to the moons, I think they divide the total by the retrograde which in this case would be 4 1/13 and they decided that if there is 4 moons on the sun's side of the planet, that makes the day.

Uranus has 27 moons. 8 of the 27 orbit the opposite way. 27 / 8 = 3 1/9. So, when there is 3 moons on the sun's side of the planet, that starts the day.

That sounds so good! I should be the next Einstein!

Oh, and the planets without moons, they just pick any day.

Edited by greggK

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Interesting question, indeed! Neptune has how many moons? Neptune has 13 moons. 3 of those 13 orbit opposite the spin of the planet. You know, I think they fixed that when they started with the solar days, but to measure the days according to the moons, I think they divide the total by the retrograde which in this case would be 4 1/13 and they decided that if there is 4 moons on the sun's side of the planet, that makes the day.

Uranus has 27 moons. 8 of the 27 orbit the opposite way. 27 / 8 = 3 1/9. So, when there is 3 moons on the sun's side of the planet, that starts the day.

That sounds so good! I should be the next Einstein!

Oh, and the planets without moons, they just pick any day.

This makes sense to you I suppose? How small does an object have to be to be considered a moon in your crazy mixed up system? The reason I ask is that all four giant planets have rings which are made up of miilions upon millions of orbiting objects ranging from the size of a building to tiny specks of dust. Now how are you going to calculate the days?

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