What effects would an extra few moons have on my planet?

Question

While I was worldbuilding I thought about how cool it would be if my planet had maybe two, maybe three moons. Does this change the world itself? For example:

• How are the tides and ocean affected? How does this affect the ocean floor/animals/vegetation?

• What kind of cultural ideas or stories (medieval-ish time period) might come from several moons?

• How would the moons look? Would they line up evenly or would some be bigger than others? Would solar eclipses still happen? Would you be able to see them during the day? Would all of the moons go through different cycles?

• Does adding extra moons multiply the effects? Like how different would a two-moon planet be from a three-moon planet?

For context, the planet is pretty much exactly like Earth.

Answer 1

You bring up a lot of factors, but i'll do my best to cover them all. Earth would be able to adapt to having 2 moons with little major difference to the fauna, but nocturnalism would be a less effective adaptation because the nights would be brighter. Look at the last paragraph for more on that.

The Tides would change, if both moons were next to each other, the tide would be higher with their combined force, so living on the coastline would be nearly impossible. If the moons orbits are off of each other, there could be more than 2 tides a day, but the tides would be less severe as the ocean would be pulled in 2 directions instead of 3. Either way, multiple moons makes sailing and coastal dwelling nearly impossible, so travel overseas would have to be in the air or not at all.

Often in mythology, 2 celestial bodies like this could either be expressed as immortalized lovers, or as enemies doomed to chase each other across the sky for all eternity.

The moons appearance can vary greatly, one moon could be smaller than the other, or orbit faster, and that would change how it would look. It is unlikely they would line up, because 2 moons that close together might cause their gravities to pull them together and that wouldn't be good. Solar eclipses would still happen, but the light of the other moon would mitigate most of the shadow effect, so it would only be what we know as a solar eclipse if the second moon was on the other side of the planet, so it would be an even more rare occurrence. The Lunar cycles are caused by the moon's position in relation to the sun, and would be consistent even with more moons because they wouldn't effect each other enough to matter. The moons would not be on the same cycle, as that would mean they were close together and that would cause problems. Lunar calendars would have to be based off a system of partial-month in order to accommodate the different moons.

The more moons you add, the higher chance there is of a potentially world-ending collision, so don't add too many. The more moons on one side of the planet, the higher and more unpredictable the tides, and the less land area your people have to live on. More moons also affects Earth's tilt on its axis and rotation with the different pulls of conflicting gravities. The one moon Earth has is already slowing Earth's rotation, so it is logical to assume seasons would be different and days might be longer or shorter depending on the various moons placements. The tides would also likely change with the seasons.

Any moons you add would have to be significantly smaller than our current moon, or you will need to shrink the size of the current moon to accommodate, or they will likely eject or collide.

That was a lot of questions in one. Next time, try to split it up or narrow down your inquiry. (Though not gonna lie I had a lot of fun answering this) Here are my sources if you want them. 2 moon Earth. 3 moon Earth.

Answer 2

Earth is a pretty ordinary planet, but the Moon(lets refer to it as Luna) is very much NOT. It is much larger, and further out from its primary, than moons tend to be. Earth-Luna is very nearly a binary planet, if Luna was 15% bigger it would have been.

If you have multiple Luna-sized moons, at anything like the sort of orbital distance that Luna is or less, then their orbits will not be stable over astronomical timespans. If they are at greater distance, they will very soon wander off around the sun, rather than sticking to Earth.

Multiple moons simply have to be smaller relative to their primary, to form stable-ish systems.

Effects of having multiple, smaller, moons around Earth in various well-separated orbits:

• Tides will be predominantly only due to the Sun. Each moon will induce a bit of a tide, but the much smaller sizes will reduce the effect greatly, and the presense of multiple moons would blur the effect. Possible exception is if a super-conjunction is possible.
• It is absolutely by unlikely coincidence that Luna's apparent diameter is so very close to the Sun's apparent diameter as seen from Earth surface. Eclipses in a multi-moon system will be much more common, but will very likely be partial or transit (moon smaller than sun disk)
• Visually, each moon would look different, and have a different orbital period. But they all would follow the exact same orbital rules and phases depending on their location relative to the sun.
• most significant moons will orbit in roughly the same plane as the planet does, and in the same rotational direction. Exceptions are possible, becoming more likely for smaller, further out moons.
• It is possible for a moon to have an orbital period, thus "month" of less than one day of its planet! Mars's Phobos is a prime example, it orbits around its planet 3 times each day. This causes it to appear to move in the wrong direction in the sky, despite still being in the same rotational plane as the other moon, Deimos.
• It is possible for a moon to die by coming to close to its planet. If it gets closer than the "Roche limit", it will break apart and form a ring. If (when) Phobos gets about 30% closer to Mars, it will break up in this way. And because it orbits faster than the surface rotation, its own tides are slowing it down, pulling the moon closer.