okay, so I was thinking: what if there was a moon (c) of an other planet and it circled around planet b but also around itself (see image) How would its day and night cycle look like?
That's a cool idea
assuming its rotation is a bit faster than its revolution it would have a couple short nights and then a long night
>>8018435
The fact that the moon c is orbiting planet b would not change its day/night cycle. Its day night cycle would only be determined by its rotational period (i.e. how quickly it spins on its axis and the tilt of this axis).
However, depending on how the moon's orbital plane around b is oriented, occasionally the moon would fall in b's shadow and it would experience an eclipse.
In the long term (millions or billions of years), the moon's rotational period would eventually slow due to tidal forces and it would become tidally locked to b. I.e. the same side would always face b (much like our moon). So, in other words b's day/night cycle would gradually grow longer and longer until eventually it would equal its orbital period (i.e. one month).
>>8018461
>only be determined by its rotational period
Well, that is the case for our Moon since the Moon's orbit is concave as seen from Polaris.
If on the other hand this satellite's orbit was fast at relatively large axes and the (green) planets orbit was slow, then that assumption does not quite hold and the parallax to the sun would be noticeable. In that case the day/night duration would depend a bit on the moon's position in its orbit around the planet.
Admittedly a bit contrived. For a SF story it might be cool.
>>8018486
>creating a fictional world
Reading Nightfall is probably obligatory reading then.
Sure, lots of moons would be cool. And scientifically speaking, wouldn't more moons mean life would be more romantic?
In any case, what could beat rings?? https://youtu.be/UT2sQ7KIQ-E
More coolness: have a small white dwarf instead of Jupiter and one brown dwarf in each of the L4 and L5 of this white dwarf, and nights would be very, very variable.
>>8018486
Sounds reasonable to me. Although I don't think that the timing of the eclipses would necessarily be exactly regular. If the moon's orbital plane lines up exactly with the planet's orbital plane around the sun, then the eclipses would indeed occur on a regular schedule (about once a month). However, it's more likely that the moon's orbital plane would be tilted a bit. In that case, eclipses would not be as common and may only happen a few times a year. Check out the Wikipedia article on "lunar eclipse" and "eclipse cycle".
One cool thing about these "eclipse nights" is that, depending on the size and atmosphere of planet b, instead of c being cast into total darkness it might instead experience a prolonged red "sunset" effect. If you look at pictures of lunar eclipses of our own moon, the moon will turn red at the start of the eclipse (due to sunlight scattering through Earth's atmosphere), then it will get darker towards the peak of the eclipse, then it will fade out to red again as the eclipse ends. Depending on the composition of b's atmosphere (perhaps it is a gas giant), you might even get other colours during the eclipse. Something to keep in mind.
One final point: you may be wondering what are plausible amounts of time for c's "month" (i.e. its orbital period around b). Basically, the orbital period of a moon depends on how far away it is from its planet (the farther away it orbits, the longer the month), and also on the mass of the planet (the more massive the planet, the shorter the moon's orbital period for a given distance). For instance, Earth's moon is about 370,000 km away from earth and has an orbital period of 28 days, whereas Jupiter's main moons orbit at distances of 420,000 km to 1.8 million km, with orbital periods of 1.7 days to 17 days.
So if you want to have a really short period between "eclipse nights", then you could make b really massive and have c orbit fairly closely.
>>8018519
>If on the other hand this satellite's orbit was fast at relatively large axes and the (green) planets orbit was slow, then that assumption does not quite hold and the parallax to the sun would be noticeable. In that case the day/night duration would depend a bit on the moon's position in its orbit around the planet.
(Not OP). I think I understand what you're saying. The problem with that is that a moon's orbital radius can't be very large with respect to its parent's orbital radius around the sun. If the moon is too far away from its parent planet, then its lunar orbit is no longer stable (i.e. the planet is no longer the body of greatest gravitational influence; the sun is). I think that for any reasonable lunar orbital radius, this parallax effect would be pretty small.
>>8018443
>>8018486
Why the fuck would there be short nights and long nights? Do you mean an eclipse that lasts several days? Largely depends on size differences and distance of planet and moon, and even then it probably wouldn't ever be that extreme. Our lunar eclipses are the solar eclipse for the moon. It only lasts several minutes. It wouldn't even happen for every revolution unless the sun, planet, and moon were all on a perfect plane. Not even Earth and moon do this.
It would just be a normal day/night cycle dependant on rotation.
>>8018435
This has made me wonder; why don't moons have moons? Why doesn't anything orbit even Ganymede?
>>8018566
Well, using Jupiter's moon Io as an example:
It has an orbital period of 1.7 days. So, if its orbit were lined up exactly with Jupiter's own orbital plane around the sun (which it isn't), then it would experience an eclipse about once every 1.7 days. But given its distance from Jupiter, it would likely only be in Jupiter's shadow for maybe an hour or less? I'm pulling numbers out of my ass here. But the point is that the eclipse would be much shorter than its orbital period of 1.7 days.
>>8018574
It's hard to get stability over astronomical timescales with so many nearby bodies influencing their gravity. The best we have is the rings of Rhea.
dont know if im correct but i think i get it:
the sun us pretty big and the planet is relatively small, because of this the 'shadow' of planet b is not that big, this means that there is only an eclipse if the small moon falls behind that shadow so if you want 'regular' eclips I suggests that you make planet b big and c close to b and have planet c circle b so the sun and planets b & c are on the same horisontal plane if that makes sense...
>>8018628
Yep, makes sense to me. However, one further thing to consider is the Roche Limit (look it up on Wikipedia).
Basically, two bodies such as a planet and a moon can only orbit each other so closely, before tidal forces tear one or both of them apart.
This sets a lower limit on the orbital distance of a moon around a planet. It depends on the relative sizes of the two bodies.
>>8018566
Not necessarily, that depends on the orbital planes. You assume all are aligned but if the case is more similar to our Moon not being co-planar with the ecliptic (Earth's orbital plane around the sun) then eclipses can be much rarer.
Though moons are unlikely to have moons you can always have troyans in Lagrange points L4 and L5.
>>8018570
I made a drawing, pic (way off scale). Dark green arrow is speed of planet A, light green is speed of moon B around A and light blue filled arrow is the net speed summing the vectors. As you see the motion would be a bit peculiar.
I drew in the white dwarf (brown) along with two brown dwarves (yellow, lost control of colors there...). Given relative velocities our friends on A and B would have no night for a bit more than half of their year and normal night for about half a year. With the right geometries you could have a lot of weird effects.
>>8018744
Yeah, I get what you're saying about the parallax effect. But for a more realistic orbital situation (take the Earth-Moon-Sun system as in my diagram), the parallax effect would be negligible. The ratio of the Moon's orbital radius (370,000 km) to the Earth's orbital radius (150,000,00 km) is 1:405.
>>8018435
pretty fucked up
