Relativity Calculator
Against a very distant seemingly "fixed star" background, any relatively nearby star will display an apparent movement as the Earth makes its yearly orbital transit about the Sun. This apparent movement produces an angle of perception called "angle of Parallax" which is used to measure the distance to the star from Earth. The Method of Parallax for determining astronomical distances to nearby stars is therefore a heliocentric method. This phenomenon is similar as to when an object is held at arm's length and seems to move against a more distant background as each eye is opened and closed. However at extreme distances the angle of Parallax becomes infinitesimally small and hence unusable.
A short history of Parallax
More specifically why Parallax? The Earth-Sun distance AU had been a constant astronomical question since the time of Aristarchus (310-230 B.C.), Samos-Alexandria, who provided the first estimate and who also preceded Copernicus in proposing a Sun-centered heliocentric universe. The actual first measurement of AU is usually credited to Giovanni Cassini (1625-1712) in 1672 by simultaneous parallax measurements of Mars from Paris where he remained and by colleague Jean Richer sent to Cayenne, French Guiana. Knowing the distance between Paris and Cayenne as approximately 7,200 km (4,500 miles) [ how did Cassini know the Paris-Cayenne distance? ]and then employing the relative AUs [ not absolute ] of Johannes Kepler (1571-1630) who in turn enlisted the earlier solar data marshaled by Tycho Brahe (1546-1601), Cassini determined a parallax of 9.5 arc second for AU where the correct modern version is 8.8 arc second. Hence, Cassini's indirect determination of solar AU by way of Mars's parallax was d = 1/9.5 or 0.1052631 parsec instead of true d = 1/8.8 or 0.1136363 parsec, an error of only 7%!! The actual km for AU will be derived in another panel using some of Cassini's parallax mathematics. Interestingly however, Danish Olaf (Ole) Roëmer (1644-1710) working with Cassini at the Paris Observatory in December, 1676, published the first actual calculation for the speed of light based upon Cassini's own AU calculation and speculation that light travels with a finite speed.
Giovanni Cassini
[ note: aberration, aberration of (star)light, astronomical aberration, or stellar aberration is an astronomical phenomenon different from the phenomenon of parallax whereby small apparent motion displacements of all fixed stars on the celestial sphere due to earth's orbital velocity mandates that terrestrial telescopes must also be adjusted to slightly different directions as the earth yearly transits the sun. Stellar aberration is totally independent of a star's distance from earth but rather depends upon the transverse velocity of an observer on earth, all of which is unlike the phenomenon of parallax. For example, vertically falling rain upon your umbrella will appear to come from in front of you the faster you walk and hence the more you will adjust the position of the umbrella to deflect the rain. Finally, the fact that earth does not drag with itself in its immediate vicinity any amount of aether helps dissuade the concept that indeed the aether exists. ]
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