### On the figure eight orbit of the three-body problem

#### Abstract

A new solution to the three-body problem interacting through gravitational forces with equal masses and zero angular momentum, has been recently discovered. This is a periodic symmetric orbit where the particles follow a figure eight trajectory in the plane. They alternate between six isosceles-aligned positions and six isosceles triangle positions in a periodic orbit composed by twelve equivalent segments. The condition of zero angular momentum is considered assuming that the three masses can be equal or different, yielding in both cases the same final expression for the kinetic energy. We found that the property of this orbit of having isosceles configurations, is a general feature to be found in any orbit of the equal-mass case, associated with an increase of $\pi/6$ in one angle of our set of coordinates. The figure-eight solution is determined by expanding two of our coordinates in a Fourier series of that angle, by using the Jacobi-Maupertuis principle as opposed to the standard Lagrangian action. The time and the angle conjugated to the angular momentum are also expressed in terms of that same angle.

#### Keywords

Three-body problem; zero angular momentum; equal-mass case; figure-eight orbit; Jacobi's action

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Revista Mexicana de Física

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Bimonthly publication of Sociedad Mexicana de Física, A.C.

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