As recently as few centuries ago, earth was believed to be the center of universe. All other observable celestial bodies were assumed to revolve around earth. Developments in geometry and mechanics made this belief irrational. Attempts to depict paths of even the nearest celestial bodies were unsuccessful or illogical, until Johannes Kepler formulated his first and second laws on planetary motion (by analyzing observations by earlier astronomers) in year 1609 AD.
First law states that ‘All planets move about the Sun in elliptical orbits, having the Sun as one of the foci’. This law gives the shape of the orbital path and the second and third laws give mathematical properties of this path. Second and third laws depend on the first law. Shapes of planetary orbits were categorically stated as elliptical. (Circle is a special ellipse). Neither reason why such motions should take place nor the mechanisms of planetary motions were proposed by these laws. Choice of location of sun, out of two foci of the ellipse, was also not explained. In short, Kepler’s laws were formulated on the basis of empirical evidences only. They had no scientific base. Planetary orbital paths were depicted as they would appear to an observer on static sun. These were assumed as true paths of the planets in space.
While formulating his laws on planetary motions, Johannes Kepler used observations only for few planets. Although, the moon (a satellite of earth) and its orbital path were much easier to observe, they were left out. Probably, due to the realization that the moon, a satellite, could not execute an elliptical orbit around the moving earth. His laws are applicable only to the observed orbits of planets around a static sun. Observed orbital paths are what the observer sees, without considering his own state of motion. An observer on a static sun will see all planets orbiting around the sun. Similarly an observer in any of the planets will observe all other planets and the sun orbiting around him. Standing on earth, we see that sun, planets and moon orbit around us. All these orbital motions are mere appearance. They are apparent planetary orbits.
Apparent planetary orbits can be around any reference point. Since we consider instantaneous parameters of planetary bodies, for most of all practical purposes of predictions (of annually re-occurring phenomena), apparent orbits provide accurate results. This seems to have satisfied the curiosity of astronomers. Although most astronomers are aware of apparent nature of elliptical orbital paths, they still consider apparent orbit as true orbital path of a planet. Kepler’s laws on planetary motion and the elliptical planetary orbits are routinely used in conjunction with many of multi-body problems, including moon’s orbital path, which was not considered in the original laws. Although mathematical treatments of apparent actions may produce results, which suit apparent phenomena, they cannot describe real facts.
We must consider that Kepler’s laws of planetary motions were formulated at a time, when the phenomenon of gravitation or the phenomenon of central force were unknown. At that time, even the heliocentric nature of solar system was not an accepted fact. What Kepler has done is to formulate laws to suite the observed locations of planets about the sun, which was
considered to be static in space. No interactions or forces between central body and the planets were considered to cause relative motions of planets.
Even the relativistic mechanics subscribe to planetary orbital paths around central bodies. It suggests curvature of space near an attracting body as the cause of planetary orbits rather than an attractive force.Although a planetary body appears to move in orbital path around a central body, in reality, it has independent motion of its own. Apparent gravitational attraction towards the central body causes a planetary body’s path to deviate from straight line, to move about and along with the central body in its motions. Since a planet is a very small body, compared to the central body, deviations in planet’s path are more prominent. When these deviations are observed about a static central body, orbital path of the planet appears to be around the central body. This is the apparent orbit of the planet, which we observe in everyday life. Similarly, relative to an assumed static planetary body, apparent direction of motion of the central body is around the planet. Few centuries back, when an earth-centered universe was in prominence, this apparent motion was considered true. Later as the science progressed, a heliocentric universe came into prominence. Earth, orbiting around the sun, is considered true in a heliocentric universe. Although we now know that, the sun is no more at the centre of universe, our view of planetary orbits in a heliocentric solar system has not changed.
By simple mechanics it is impossible for a free body to orbit around another moving body, in any geometrically closed path. Both a circle and an ellipse are closed geometrical figures. Contemporary astronomers recognize the sun as a moving body and it is impossible for a planet to execute a closed orbital path around the moving sun. Hence, elliptical planetary orbital paths (closed geometrical figures) around the sun are false. Yet, no text books, atlases or any other type of literature agrees to this fact that planetary orbital paths are not circular or elliptical. Circular or elliptical planetary orbits around the sun are apparent structures. They are what the observer on the sun would notice. They do not exist in reality.With respect to an absolute reference (or an observer outside the planetary system and static in space), a planet does not orbit around the central body. Motion of the planet is wave-like along the central body’s path, periodically moving to the front and to the rear of the central body. In this sense, it can be seen that a planet (or a satellite) orbits around the center of the central body’s curved path (galactic center) and the wave pattern in its path is caused by the presence of the central body. Such changes in the path of a free body may be attributed to perturbations caused by the presence of nearby bodies. These perturbations look like orbital motion, only when they are referred to a relatively small system of bodies. Real planetary orbital paths about the sun are wavy in space. Astronomers may privately agree to the fact that planetary orbital paths are wavy. But in public, they are offended by such suggestion.
Both, a planet and its central body move in the same direction about the same median path in space. Since the orbital motion is an apparent phenomenon, either of the bodies can be considered as the central and the other as its planet. Planetary laws are equally valid in either case. Although it is generally stated that the earth orbits the sun in eastward direction, it is equally valid to state, ‘the sun orbits the earth in westward direction’. However, when more than two bodies are considered as a single system, it is more convenient to take the common and most prominent body as the central body and to take other bodies as planetary or satellite bodies.A group of large bodies in space forms a planetary system. Bodies of this group move together along a median path, while individual bodies have independent relative motions within the group. A planetary system is essentially a part of a galaxy. All stable galaxies are static in space (author has strong reasons to support this view). Galaxies are rotating system of macro bodies. Hence, a planetary system in it, traces a circular path around galactic centre. Median path of the planetary system is a very large circle around galactic centre. Largest body in the group has its path nearest to the median path and its path is least perturbed. This body acts as the leader of the group and it is the central body of the planetary system. All other bodies in the planetary system move along with the central body, while their paths are perturbed by the presence of all other bodies in the system.
Larger orbital path of a planet (and all bodies in a galaxy) is around the galactic centre. It is very large and contains many points of similar appearance in relation to the central body of a planetary system. Hence, it is convenient for us to use a much smaller structure ‘the apparent orbit’ with unique reference points on it for all practical purposes. Apparent orbit is a small part of the larger orbital path, between two identical appearances of the central body, looking from the planet (e.g.: one solar year). It is an imaginary concept, where shape of the path, speed of the planet and directions of motions are manipulated to suit observations. As such, it has no logical basis. It depicts the appearance of a system, where it is assumed that the central body by some imaginary mechanism (change of reference frame), is held stationary at the centre of the apparent orbit. While doing this, (much greater) motions of the planetary body, before it
became a planet and motion or path of the central body are ignored.