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r a is the radius at apoapsis (also "apofocus", "aphelion", "apogee"), i.e., the farthest distance of the orbit to the center of mass of the system, which is a focus of the ellipse. r p is the radius at periapsis (or "perifocus" etc.), the closest distance.
In gravitationally bound systems, the orbital speed of an astronomical body or object (e.g. planet, moon, artificial satellite, spacecraft, or star) is the speed at which it orbits around either the barycenter (the combined center of mass) or, if one body is much more massive than the other bodies of the system combined, its speed relative to the center of mass of the most massive body.
The apsides refer to the farthest (2) and nearest (3) points reached by an orbiting planetary body (2 and 3) with respect to a primary, or host, body (1). An apsis (from Ancient Greek ἁψίς (hapsís) 'arch, vault'; pl. apsides / ˈ æ p s ɪ ˌ d iː z / AP-sih-deez) [1] [2] is the farthest or nearest point in the orbit of a planetary body about its primary body.
The original form of this law (referring to not the semi-major axis, but rather a "mean distance") holds true only for planets with small eccentricities near zero. [27] Using Newton's law of gravitation (published 1687), this relation can be found in the case of a circular orbit by setting the centripetal force equal to the gravitational force:
If the four giant planets were on a straight line on the same side of the Sun, the combined center of mass would lie at about 1.17 solar radii, or just over 810,000 km, above the Sun's surface. [ 7 ] The calculations above are based on the mean distance between the bodies and yield the mean value r 1 .
Figure 1. Typical elliptical path of a smaller mass m orbiting a much larger mass M. The larger mass is also moving on an elliptical orbit, but it is too small to be seen because M is much greater than m. The ends of the diameter indicate the apsides, the points of closest and farthest distance.
In orbital mechanics, Kepler's equation relates various geometric properties of the orbit of a body subject to a central force.. It was derived by Johannes Kepler in 1609 in Chapter 60 of his Astronomia nova, [1] [2] and in book V of his Epitome of Copernican Astronomy (1621) Kepler proposed an iterative solution to the equation.
All three planets (red, blue and green) are at the same distance r from the center of force C. It is required to make a body move in a curve that revolves about the center of force in the same manner as another body in the same curve at rest. [42] Newton's derivation of Proposition 43 depends on his Proposition 2, derived earlier in the ...