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# 9: The Two Body Problem in Two Dimensions

In this chapter we show how Kepler’s laws can be derived from Newton’s laws of motion and gravitation, and conservation of angular momentum, and we derive formulas for the energy and angular momentum in an orbit. We show also how to calculate the position of a planet in its orbit as a function of time. It would be foolish to embark upon this chapter without familiarity with much of the material covered in Chapter 2. The discussion here is limited to two dimensions. The corresponding problem in three dimensions, and how to calculate an ephemeris of a planet or comet in the sky, will be treated in Chapter 10.

• 9.1: Kepler's Laws
Kepler’s law of planetary motion are as follows: 1. Every planet moves around the Sun in an orbit that is an ellipse with the Sun at a focus. 2. The radius vector from Sun to planet sweeps out equal areas in equal time. 3. The squares of the periods of the planets are proportional to the cubes of their semi major axes.
• 9.2: Kepler's Second Law from Conservation of Angular Momentum
Kepler's second law. that argued a line joining a planet and the Sun sweeps out equal areas during equal intervals of time, can be derived from conservation of angular momentum.
• 9.3: Some Functions of the Masses
• 9.4: Kepler's First and Third Laws from Newton's Law of Gravitation
• 9.5: Position in an Elliptic Orbit
• 9.6: Position in a Parabolic Orbit
When a “long-period” comet comes in from the Oort belt, it typically comes in on a highly eccentric orbit, of which we can observe only a very short arc. Consequently, it is often impossible to determine the period or semi major axis with any degree of reliability or to distinguish the orbit from a parabola. There is therefore frequent occasion to have to understand the dynamics of a parabolic orbit.
• 9.7: Position in a Hyperbolic Orbit
If an interstellar comet were to encounter the solar system from interstellar space, it would pursue a hyperbolic orbit around the Sun. To date, no such comet with an original hyperbolic orbit has been found, although there is no particular reason why we might not find one some night. However, a comet with a near-parabolic orbit from the Oort belt may approach Jupiter on its way in to the inner solar system, and its orbit may be perturbed into a hyperbolic orbit.
• 9.8: Orbital Elements and Velocity Vector
In two dimensions, an orbit can be completely specified by four orbital elements. Three of them give the size, shape and orientation of the orbit. They are, respectively, a , e and ω . The fourth element is needed to give information about where the planet is in its orbit at a particular time. Usually this is T , the time of perihelion passage.
• 9.9: Osculating Elements
In practice, an orbit is subject to perturbations, and the planet does not move indefinitely in the orbit that is calculated from the position and velocity vectors at a particular time. The orbit that is calculated from the position and velocity vectors at a particular instant of time is called the osculating orbit, and the corresponding orbital elements are the osculating elements.
• 9.10: Mean Distance in an Elliptic Orbit
It is sometimes said that “ a ” in an elliptic orbit is the “mean distance” of a planet from the Sun. In fact a is the semi major axis of the orbit. Whether and it what sense it might also be the “mean distance” is worth a moment of thought.

Tuhmbnail: Two bodies with similar mass orbiting a common barycenter external to both bodies, with elliptic orbits—typical of binary stars. (Public Domain, Zhatt).