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Comment by snakeboy | Hacker News Reader

> In our numerical model, we do not integrate the orbit of the Moon and instead use a single particle with the combined mass of the Earth and the Moon.

Is that a standard assumption for this kind of model? For such a chaotic simulation, would it not be very important to approximate the effects of the Earth and moon very precisely? Especially since these bodies would have such dominating effects early on?

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Only the initial launch has the car near the earth and moon, but the calculations are already factoring in a range of starting points - I’d assume the moon’s effects are contained in that range.

After it moves away from the earth the distance ought to be negligible, although even if they do reduce the earth moon to a single point I don’t know if they model a wobble.

Yes, for things "far" away from the Earth. They said they started their simulation on Feb 10, the launch was on Feb 6, to the roadster was far enough away at that time to ignore the Earth and Moon being separate. I didn't see where they mentioned it in their paper, but they likely started with either published location/velocity data, or used observations to compute its position and velocity for the start on the 10th.

The mass of the moon is only about 1.2% of that of earth.

Also, “early on” earth is much, much closer than the moon. The gravitational pull of the sun is >100 times as large as that of the moon there, and both are minute compared to that of earth (you don’t feel much lighter or jump higher when the sun or the moon are overhead, do you?)

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