So I recently received a message on my wall/timeline on facebook from a good friend of mine. She and her son asked me "What would it be like if we had two moons? Let's assume they are identical to this one, just two of them." I found this to be a really interesting question. In fact I think I could spend several blog posts discussing the issue without truly answering the question to my satisfaction. So today I am going to describe several possible effects a second moon would have on the Earth using some simplifications to get a general idea in the next couple blog posts. If I blog again after that (haha - I do post things every once in a while - honest) on this subject I will focus on one aspect and try to remove the simplifications.
First, lets discuss where the second moon (lets call it Bob) would be in the sky. We are assuming that Bob is just like out current moon (Luna) with a similar mass and orbit. However, the same orbit can be taken in several ways. I am going to take "same orbit" to mean that Bob follows the same path around the earth, but occupies a different location in that orbit at any given time. If we assume that Bob and Luna don't effect each others' orbits (which is decidedly not true, but I am going with for today) then the most stable places for Bob to be is at the Lagrangian Points, which is where the gravitation forces of two objects (Earth/Luna or Earth/Sun) keep additional objects stable. There are 5 Lagrangian points, however 2 of them would require different orbits, therefore we would be left with Bob being either (i) 60 degrees ahead, (ii) 60 degrees behind Luna, or (iii) exactly opposite (180 degrees) Luna on the other side of the earth.
Now you might ask why does it matter where Bob is? My answer is that whether Bob and Luna are on the same side of the planet or opposite each other determines two things. 1) Whether Bob and Luna are in the sky at the same time (if Bob is opposite Luna it will set just as Luna rises and vis versa); and 2) How the combined gravity of Bob and Luna interact with the Earth. Both of these points can be very important. If Bob and Luna are on opposite sides of the earth then it would mean that there is always a moon in the sky, so there would be significantly more light in the sky at night. And Issac Asimov has postulated the effect always have lights can have on the cultures of intelligent beings. Not to mention the effect more light would have had on prehistoric hunters and animals.
The gravity of Luna is what gives us tides, and is possibly the cause of plate tectonics (moving continents). The combined gravity of Bob and Luna would greatly effect both of these phenomena. If Bob is opposite Luna this would probably amplify the effects that Luna already has on the tides and magma flows (the gravity of Luna pulls water/liquids towards it causing the tides to bulge towards the moon, but the spherical nature of the earth causes the water on the opposite side of the earth from Luna to bulge out as well, meaning is Bob was on the opposite side they would amplify the effects). However, if Bob and Luna are 60 degrees a part this would lead to more complicated interaction on the tides with the size of the tides and forces on the magma increased/decreased but not as much as if they are opposite.
Now you might think that this is already a lot to change if we add Bob to the Earth's planetary system, but these are only two of the more obvious effects (aside from having more eclipses). I will try to describe more possible effects next time, but here are a few things that would change:
- Earth's axial tilt (effect the seasons and weather)
- Number of asteroids that hit the Earth (more or less)
- Earth's orbit of the Sun
- Number of moons (intuition tells me that more moons become easier to acquire more moons/rings)
- The magnetic field of Earth
- Weather
Some links I found on the subject:
http://www.astronomytoday.com/astronomy/earthmoon.html
http://www.astrobio.net/index.php?option=com_retrospection&task=detail&id=2507
http://science1.nasa.gov/science-news/science-at-nasa/2008/17apr_magnetotail/
First, lets discuss where the second moon (lets call it Bob) would be in the sky. We are assuming that Bob is just like out current moon (Luna) with a similar mass and orbit. However, the same orbit can be taken in several ways. I am going to take "same orbit" to mean that Bob follows the same path around the earth, but occupies a different location in that orbit at any given time. If we assume that Bob and Luna don't effect each others' orbits (which is decidedly not true, but I am going with for today) then the most stable places for Bob to be is at the Lagrangian Points, which is where the gravitation forces of two objects (Earth/Luna or Earth/Sun) keep additional objects stable. There are 5 Lagrangian points, however 2 of them would require different orbits, therefore we would be left with Bob being either (i) 60 degrees ahead, (ii) 60 degrees behind Luna, or (iii) exactly opposite (180 degrees) Luna on the other side of the earth.
Now you might ask why does it matter where Bob is? My answer is that whether Bob and Luna are on the same side of the planet or opposite each other determines two things. 1) Whether Bob and Luna are in the sky at the same time (if Bob is opposite Luna it will set just as Luna rises and vis versa); and 2) How the combined gravity of Bob and Luna interact with the Earth. Both of these points can be very important. If Bob and Luna are on opposite sides of the earth then it would mean that there is always a moon in the sky, so there would be significantly more light in the sky at night. And Issac Asimov has postulated the effect always have lights can have on the cultures of intelligent beings. Not to mention the effect more light would have had on prehistoric hunters and animals.
The gravity of Luna is what gives us tides, and is possibly the cause of plate tectonics (moving continents). The combined gravity of Bob and Luna would greatly effect both of these phenomena. If Bob is opposite Luna this would probably amplify the effects that Luna already has on the tides and magma flows (the gravity of Luna pulls water/liquids towards it causing the tides to bulge towards the moon, but the spherical nature of the earth causes the water on the opposite side of the earth from Luna to bulge out as well, meaning is Bob was on the opposite side they would amplify the effects). However, if Bob and Luna are 60 degrees a part this would lead to more complicated interaction on the tides with the size of the tides and forces on the magma increased/decreased but not as much as if they are opposite.
Now you might think that this is already a lot to change if we add Bob to the Earth's planetary system, but these are only two of the more obvious effects (aside from having more eclipses). I will try to describe more possible effects next time, but here are a few things that would change:
- Earth's axial tilt (effect the seasons and weather)
- Number of asteroids that hit the Earth (more or less)
- Earth's orbit of the Sun
- Number of moons (intuition tells me that more moons become easier to acquire more moons/rings)
- The magnetic field of Earth
- Weather
Some links I found on the subject:
http://www.astronomytoday.com/astronomy/earthmoon.html
http://www.astrobio.net/index.php?option=com_retrospection&task=detail&id=2507
http://science1.nasa.gov/science-news/science-at-nasa/2008/17apr_magnetotail/
