Communities

Writing
Writing
Codidact Meta
Codidact Meta
The Great Outdoors
The Great Outdoors
Photography & Video
Photography & Video
Scientific Speculation
Scientific Speculation
Cooking
Cooking
Electrical Engineering
Electrical Engineering
Judaism
Judaism
Languages & Linguistics
Languages & Linguistics
Software Development
Software Development
Mathematics
Mathematics
Christianity
Christianity
Code Golf
Code Golf
Music
Music
Physics
Physics
Linux Systems
Linux Systems
Power Users
Power Users
Tabletop RPGs
Tabletop RPGs
Community Proposals
Community Proposals
tag:snake search within a tag
answers:0 unanswered questions
user:xxxx search by author id
score:0.5 posts with 0.5+ score
"snake oil" exact phrase
votes:4 posts with 4+ votes
created:<1w created < 1 week ago
post_type:xxxx type of post
Search help
Notifications
Mark all as read See all your notifications »
Q&A

Can a planet's moon rise at the same time every night?

+0
−0

Is it possible for a planet's moon to always rise at the same time of the night? And in this way be used as a timekeeping device? What if there are two moons?

History
Why does this post require moderator attention?
You might want to add some details to your flag.
Why should this post be closed?

This post was sourced from https://worldbuilding.stackexchange.com/q/8533. It is licensed under CC BY-SA 3.0.

0 comment threads

1 answer

+0
−0

In order for this to be possible, the moon would have to be in the same place relative to the planet and the star throughout the day - in other words, if you drew a line from the center of the star through the center of the planet, then drew a line from the center of the planet through the center of the moon, you would have two lines that intersected at an angle that is forever constant.

For those who got past that first paragraph sentence, there are two places that would work: the Lagrangian points $L_1$ and $L_2$. The five Lagrangian points are laid out like this:

Lagrangian points

The "holes" and overall shape of space here is an analogy for gravity wells, so ignore that, but treat the green lines connecting the star, planet and points as rigid - in other words, as the planet revolves around the star, the green lines rotate with it, as do the Lagrangian points.

$L_1$ and $L_2$ will satisfy your scenario. However, $L_1$ should be discarded because putting the moon there would make it only appear in the daytime! So we'll take $L_2$.

The problem is that the only stable Lagrangian points are $L_4$ and $L_5$ notice how the others are near the metaphorical depressions of space. This indicates that if an object at them shifts, it will move away. For $L_4$ and $L_5$, the object will merely return.

To stay at $L_1$ or $L_2$, stationkeeping is required. So you'd have to attach thrusters to your moon to keep it in the same place! Alternatively, put it in a Lissajous orbit. You'll still need some thrusters, but it should be stable. Here's what the orbit will look like:

Lissajous orbit

This kind of orbit would be very tough - if not impossible - to put a very massive object in.

Two moons would make this job nearly impossible, because if they were near each other, they'd most likely perturb each other's orbits and ruin any stability. Your odds of getting this to work for one moon are slim to none; your odds for two moons are slightly worse.

Or you can use a little handwavium to solve all your technical difficulties!

History
Why does this post require moderator attention?
You might want to add some details to your flag.

0 comment threads

Sign up to answer this question »