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113 lines
5.6 KiB
113 lines
5.6 KiB
README.planetmath: Understanding Planetary Positions in KStars.
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copyright 2002 by Jason Harris and the KStars team.
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This document is licensed under the terms of the GNU Free Documentation License
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-------------------------------------------------------------------------------
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0. Introduction: Why are the calculations so complicated?
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We all learned in school that planets orbit the Sun on simple, beautiful
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elliptical orbits. It turns out this is only true to first order. It
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would be precisely true only if there was only one planet in the Solar System,
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and if both the Planet and the Sun were perfect point masses. In reality,
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each planet's orbit is constantly perturbed by the gravity of the other planets
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and moons. Since the distances to these other bodies change in a complex way,
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the orbital perturbations are also complex. In fact, any time you have more
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than two masses interacting through mutual gravitational attraction, it is
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*not possible* to find a general analytic solution to their orbital motion.
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The best you can do is come up with a numerical model that predicts the orbits
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pretty well, but imperfectly.
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1. The Theory, Briefly
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We use the VSOP ("Variations Seculaires des Orbites Planetaires") theory of
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planet positions, as outlined in "Astronomical Algorithms", by Jean Meeus.
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The theory is essentially a Fourier-like expansion of the coordinates for
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a planet as a function of time. That is, for each planet, the Ecliptic
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Longitude, Ecliptic Latitude, and Distance can each be approximated as a sum:
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Long/Lat/Dist = s(0) + s(1)*T + s(2)*T^2 + s(3)*T^3 + s(4)*T^4 + s(5)*T^5
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where T is the number of Julian Centuries since J2000. The s(N) parameters
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are each themselves a sum:
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s(N) = SUM_i[ A(N)_i * Cos( B(N)_i + C(N)_i*T ) ]
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Again, T is the Julian Centuries since J2000. The A(N)_i, B(N)_i and C(N)_i
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values are constants, and are unique for each planet. An s(N) sum can
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have hundreds of terms, but typically, higher N sums have fewer terms.
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The A/B/C values are stored for each planet in the files
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<planetname>.<L/B/R><N>.vsop. For example, the terms for the s(3) sum
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that describes the T^3 term for the Longitude of Mars are stored in
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"mars.L3.vsop".
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Pluto is a bit different. In this case, the positional sums describe the
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Cartesian X, Y, Z coordinates of Pluto (where the Sun is at X,Y,Z=0,0,0).
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The structure of the sums is a bit different as well. See KSPluto.cpp
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(or Astronomical Algorithms) for details.
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The Moon is also unique, but the general structure, where the coordinates
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are described by a sum of several sinusoidal series expansions, remains
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the same.
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2. The Implementation.
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The KSplanet class contains a static OrbitDataManager member. The
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OrbitDataManager provides for loading and storing the A/B/C constants
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for each planet. In KstarsData::slotInitialize(), we simply call
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loadData() for each planet. KSPlanet::loadData() calls
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OrbitDataManager::loadData(QString n), where n is the name of the planet.
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The A/B/C constants are stored hierarchically:
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+ The A,B,C values for a single term in an s(N) sum are stored in an
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OrbitData object.
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+ The list of OrbitData objects that compose a single s(N) sum is
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stored in a QPtrVector (recall, this can have up to hundreds of elements).
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+ The six s(N) sums (s(0) through s(5)) are collected as an array of
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these QVectors ( typedef QVector<OrbitData> OBArray[6] ).
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+ The OBArrays for the Longitude, Latitude, and Distance are collected
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in a class called OrbitDataColl. Thus, OrbitDataColl stores all the
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numbers needed to describe the position of any planet, given the
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Julian Day.
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+ The OrbitDataColl objects for each planet are stored in a QDict object
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called OrbitDataManager. Since OrbitDataManager is static, each planet can
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access this single storage location for their positional information.
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(A QDict is basically a QArray indexed by a string instead of an integer.
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In this case, the OrbitDataColl elements are indexed by the name of the
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planets.)
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Tree view of this hierarchy:
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OrbitDataManager[QDict]: Stores 9 OrbitDataColl objects, one per planet.
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+--OrbitDataColl: Contains all three OBArrays (for
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Longitude/Latitude/Distance) for a single planet.
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+--OBArray[array of 6 QVectors]: the collection of s(N) sums for
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the Latitude, Longitude, or Distance.
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+--QVector: Each s(N) sum is a QVector of OrbitData objects
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+--OrbitData: a single triplet of the constants A/B/C for
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one term in an s(N) sum.
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To determine the instantaneous position of a planet, the planet calls its
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findPosition() function. This first calls calcEcliptic(double T), which
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does the calculation outlined above: it computes the s(N) sums to determine
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the Heliocentric Ecliptic Longitude, Ecliptic Latitude, and Distance to the
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planet. findPosition() then transforms from heliocentric to geocentric
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coordinates, using a KSPlanet object passed as an argument representing the
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Earth. Then the ecliptic coordinates are transformed to equatorial
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coordinates (RA,Dec). Finally, the coordinates are corrected for the
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effects of nutation, aberration, and figure-of-the-Earth. Figure-of-the-Earth
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just means correcting for the fact that the observer is not at the center of
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the Earth, rather they are on some point on the Earth's surface, some 6000 km
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from the center. This results in a small parallactic displacement of the
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planet's coordinates compared to its geocentric position. In most cases,
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the planets are far enough away that this correction is negligible; however,
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it is particularly important for the Moon, which is only 385 Mm (i.e.,
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385,000 km) away.
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