How can I get a total solar eclipse to be a yearly event?
In our world, a solar eclipse is visible from the planet a few times a year, in certain locations. However, a total solar eclipse that can be seen across the planet generally happens every three or four centuries. I am building a world similar to Earth. What I need is for a total eclipse to happen once every year, at the six month mark. The problem here is two fold. The moon rotates and revolves around the planet at a certain speed and distance. To accomplish my needs, I figure that I would need to speed up one of these processes, or introduce some other new change to our orbit. The problem is that the moon has certain gravitational effects on our planet that may change due to any alterations that I would make.
What would be the ramifications of altering our relationship with the moon in this way? How would I be able to create the effects that I desire without significantly damaging the planet as it is now?
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1 answer
The simplest way to achieve this is to have the Moon be on a perfectly circular orbit that lasts twelve months and lies in Earth's orbital plane; this would mean that at each new moon, we would get a solar eclipse. Let's say we keep Earth and the Moon at the same masses. We'd then need the Moon to orbit about six times as far away as it currently does, by Kepler's third law. However, then the Moon would be too small to produce a total eclipse - and the Moon would likely lie outside Earth's Hill sphere! There are two solutions:
- Make the Moon larger and place it in this new orbit
- Keep the Moon's size the same, put it in this new orbit, and reduce the size of the Sun to compensate (correspondingly, perhaps making the Sun hotter to compensate for the reduction in flux you'd get otherwise)
- Lower the Earth's mass and keep the Moon in its current orbit
I suspect that the final solution is preferable, as the first two would result in substantially lower tidal forces (which scale as an inverse cube law) unless we drastically increased the mass of the Moon to an unrealistic value; additionally, the Moon would likely lie outside Earth's Hill sphere, quickly making its orbit unstable. Therefore, if we want to Moon to remain in its current orbit and still have an orbital period of twelve months. However, Kepler's third law tells us that this would require Earth to have mass of roughly 0.5% of its current mass.
I suspect the best solution is a compromise between the three: Make the Earth slightly lighter while moving the Moon further away, increasing its size and making the Sun smaller and hotter. Its possible that this would result in a binary planet-like arrangement.
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