Everything about Celestial Mechanics totally explained
Celestial mechanics is a division of
astronomy dealing with the
motions and
gravitational effects of
celestial objects. The field applies principles of
physics, historically
classical mechanics, to astronomical objects such as
stars and
planets to produce
ephemeris data. It is distinguished from
astrodynamics, which is the study of the creation of artificial satellite
orbits.
History of celestial mechanics
Although modern analytic celestial mechanics starts 400 years ago with
Isaac Newton, prior studies addressing the problem of planetary positions are known going back perhaps 3,000 or more years.
Classical Greek writers speculated widely regarding celestial motions, and presented many geometrical mechanisms to model the motions of the planets. Their models employed combinations of uniform circular motion and were centered on the earth. An independent
philosophical tradition was concerned with the physical causes of such circular motions. An extraordinary figure among the ancient Greek astronomers is
Aristarchus of Samos (310 BC - c.230 BC), who suggested a
heliocentric model of the universe and attempted to measure Earth's distance from the Sun.
Claudius Ptolemy
Claudius Ptolemy was an ancient astronomer and astrologer in early Imperial
Roman times who wrote several books on astronomy. The most significant of these was the
Almagest, which remained the most important book on predictive geometrical astronomy for some 1400 years. Ptolemy selected the best of the astronomical principles of his Greek predecessors, especially
Hipparchus, and appears to have combined them either directly or indirectly with data and parameters obtained from the
Babylonians. Although Ptolemy relied mainly on the work of Hipparchus, he introduced at least one idea, the
equant, which appears to be his own, and which greatly improved the accuracy of the predicted positions of the planets. Although his model was extremely accurate, it relied solely on geometrical constructions rather than on physical causes; Ptolemy didn't use celestial mechanics.
Johannes Kepler (December 27, 1571 - November 15, 1630)
Johannes Kepler was the first to closely integrate the predictive geometrical astronomy, which had been dominant from Ptolemy to
Copernicus, with physical concepts to produce a
New Astronomy, Based upon Causes, or Celestial Physics.... His work led to the
modern laws of planetary orbits, which he developed using his physical principles and the
planetary observations made by
Tycho Brahe. Kepler's model greatly improved the accuracy of predictions of planetary motion, years before
Isaac Newton had even developed his law of gravitation.
See
Kepler's laws of planetary motion and the
Keplerian problem for a detailed treatment of how his laws of planetary motion can be used.
Isaac Newton (January 4, 1643 – March 31, 1727)
Isaac Newton is credited with introducing the idea that the motion of objects in the heavens, such as
planets, the
Sun, and the
Moon, and the motion of objects on the ground, like
cannon balls and falling apples, could be described by the same set of
physical laws. In this sense he unified
celestial and
terrestrial dynamics. Using Newton's law of gravitation, proving Kepler's Laws for the case of a circular orbit is simple. Elliptical orbits involve more complex calculations, which Newton included in his
Principia.
Joseph-Louis Lagrange (January 25, 1736 - April 10, 1813)
After Newton,
Lagrange attempted to solve the three-body problem, analyzed the stability of planetary orbits, and discovered the existence of the
Lagrangian points. Lagrange also reformulated the principles of
classical mechanics, emphasizing energy more than force and developing a
method to use a single polar coordinate equation to describe any orbit, even those that are parabolic and hyperbolic. This is useful for calculating the behaviour of planets and
comets and such. More recently, it has also become useful to calculate
spacecraft trajectories.
Simon Newcomb (March 12, 1835 – July 11, 1909)
Simon Newcomb was a Canadian-American astronomer revised
Peter Andreas Hansen's table of lunar positions. In
1877, assisted by
George William Hill, he recalculated all the major astronomical constants. After
1884, he conceived with A. M. W. Downing a plan to resolve much international confusion on the subject. By the time he attended a standardisation conference in
Paris, France in 1896-May, the international consensus was all ephemerides should be based on Newcomb's calculations. A further conference as late as
1950 confirmed Newcomb's constants as the international standard.
Albert Einstein (March 14, 1879 - April 18, 1955)
After
Albert Einstein explained the anomalous
precession of Mercury's perihelion, astronomers recognized that
Newtonian mechanics didn't provide the highest accuracy. Today, we've binary
pulsars whose orbits not only require the use of
General Relativity for their explanation, but whose evolution proves the existence of
gravitational radiation, a discovery that led to a Nobel prize.
Principles of celestial mechanics
Examples of problems
Celestial motion without additional forces such as
thrust of a
rocket, is governed by gravitational acceleration of masses due to other masses. A simplification is the
n-body problem, where we assume n spherically symmetric masses, and integration of the accelerations reduces to summation.
Examples:
- 4-body problem: spaceflight to Mars (for parts of the flight the influence of one or two bodies is very small, so that there we've a 2- or 3-body problem; see also the patched conic approximation)
- 3-body problem:
In the case that n=2 (
two-body problem), the situation is much simpler than for larger n. Various explicit formulas apply, where in the more general case typically only numerical solutions are possible. It is a useful simplification that's often approximately valid.
Examples:
A binary star, for example Alpha Centauri (approx. the same mass)
A binary asteroid, for example 90 Antiope (approx. the same mass)
A further simplification is based on the "standard assumptions in astrodynamics", which include that one body, the orbiting body, is much smaller than the other, the central body. This is also often approximately valid.
Examples:
Solar system orbiting the center of the Milky Way
A planet orbiting the Sun
A moon orbiting a planet
A spacecraft orbiting Earth, a moon, or a planet (in the latter cases the approximation only applies after arrival at that orbit)
Either instead of, or on top of the previous simplification, we may assume circular orbits, making distance and orbital speeds, and potential and kinetic energies constant in time. This assumption sacrifices accuracy for simplicity, especially for high eccentricity orbits which are by definition non-circular.
Examples:
The orbit of the dwarf planet Pluto, ecc. = 0.2488
The orbit of Mercury, ecc. = 0.2056
Hohmann transfer orbit
Gemini 11 flight
Suborbital flights
Perturbation theory
Perturbation theory comprises mathematical methods that are used to find an approximate solution to a problem which can't be solved exactly. (It is closely related to methods used in numerical analysis, which are ancient.) The earliest use of perturbation theory was to deal with the otherwise unsolveable mathematical problems of celestial mechanics: Newton's solution for the orbit of the Moon, which moves noticeably differently from a simple Keplerian ellipse because of the competing gravitation of the Earth and the Sun.
Perturbation methods start with a simplified form of the original problem, which is simple enough to be solved exactly. In celestial mechanics, this is usually a Keplerian ellipse, which is correct when there are only two gravitating bodies (say, the Earth and the Moon), or a circular orbit, which is only correct in special cases of two-body motion, but is often close enough for practical use. The solved, but simplified problem is then "perturbed" to make its starting conditions closer to the real problem, such as including the gravitational attraction of a third body (the Sun). The slight changes that result, which themselves may have been simplifed yet again, are used as corrections. Because of simplifications introduced along every step of the way, the corrections are never perfect, but even one cycle of corrections often provides a remarkably better approximate solution to the real problem.
There is no requirement to stop at only one cycle of corrections. A partially corrected solution can be re-used as the new starting point for yet another cycle of perturbations and corrections. The common difficulty with the method is that usually the corrections progressively make the new solutions very much more complicated, so each cycle is much more difficult to manage than the previous cycle of corrections. Newton is reported to have said, regarding the problem of the Moon's orbit "It causeth my head to ache."
This general procedure — starting with a simplified problem and gradually adding corrections that make the starting point of the corrected problem closer to the real situation — is a widely used mathematical tool in advanced sciences and engineering. It is the natural extension of the "guess, check, and fix" method used anciently with numbers.
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