Retrograde Planets

Chapter-01

Astronomical Background

The Milky Way and Solar System

1.1The Milky Way is the galaxy that contains our Solar System. The Milky Way is estimated to contain 100–400 billion stars. The Solar System is located within the disk, about 27, 000 light-years from the Galactic Center. The center of the Galaxy lies in the direction of the constellation Sagittarius; it is here that the Milky Way is brightest. The band divides the night sky into two roughly equal hemispheres. The Milky Way is rotating around its dense galactic centre, thus the Sun is moving in a circle within the galaxy’s gravity. The Sun (in fact our entire solar system) orbits around the centre of the Milky way at an average speed of 828000 Km/ hr. Even at this rate it takes about 2.30 million years to make one complete orbit around the galaxy.

1.2It is belived that the Solar System was formed some 4.6 billion years ago from the gravitational collapse of a giant interstellar molecular cloud. The vast majority of the system’s mass is in the Sun, with most of the remaining mass contained in Jupiter and other smaller planets. The Solar System is the gravitationally bound system comprising the Sun and the objects that orbit it, either directly or indirectly. The Sun is the centre and biggest heavenly body of our solar system. It is the hottest body and radiates light which is further reflected by various planets and their satellites.

1.3The Sun is the Solar System’s star and by far its most massive component. Its large mass (332,900 Earth masses) produces temperatures and densities in its core high enough to sustain nuclear fusion of Hydrogen into Helium, making it a main sequence star. This releases an enormous amount of energy, mostly radiated into space as electromagnetic radiation peaking in visible light.

Planets

2.1The term planet is ancient, with ties to history, astrology, science, mythology, and religion. Several planets in the Solar System can be seen with the naked eye. These were regarded by many early cultures as divine, or as emissaries of deities. The International Astronomical Union (IAU) officially adopted a resolution in August 2006, defining planets within the Solar System as follows:

A planet is a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, but its mass should not be more than 13 times that of the Jupiter to avoid thermo & nuclear fusion of deuterium, a heavier isotop of Hydrogen and (c) has clear neighborhood around its orbit.

2.2This definition is controversial because it excludes many objects of planetary mass based on where or what they orbit. Although eight of the planetary bodies discovered before 1950 remain planets under the modern definition, some celestial bodies, such as Ceres, Pallas, Juno and Vesta (each an object in the solar asteroid belt), and Pluto (the first trans-Neptunian object discovered), that were once considered planets by the scientific community, are no longer viewed as such. A different class of dwarf planets was created to include Pluto, Eris, Ceres, Haumea and Sedana. All these five bodies are located beyond Neptune. Depending on the accretion history and presence of solids, rocks and ice, the planets were classified as Terrestrial (Mercury, Venus, Earth and Mars), Giants (Jupiter and Saturn) and Icy-giants (Uranus and Neptune). In order of increasing distance from the Sun, first there are the four Terrestrials- Mercury, Venus, Earth, and Mars, then the two Giant planets- Jupiter and Saturn and finally the Icy-giant planets- Uranus, and Neptune. However, as per Vedic astrology, a planet is a celestial body/ point which is capable to seize and control natives, their karmas and fortune. Accordingly Sun a star, Moon a satellite of Earth and Rahu-Ketu important & sensitive points along ecliptic have also been grouped as planets.

2.3It is not known with certainty how planets are formed. The prevailing theory is that they are formed during the collapse of a Nebula into a thin disk of gas and dust. Through accretion (a process of sticky collision) dust particles in the disk steadily accumulate mass to form ever-larger bodies. Local concentrations of mass known as planet esimals form, and these accelerate the accretion process by drawing in additional material by their gravitational attraction. These concentrations become ever denser until they collapse inward under gravity to form Proto-planets. Proto-planets that have avoided collisions may become natural satellites of planets through a process of gravitational capture, or remain in belts of other objects to become either dwarf planets or small bodies.

2.4As of 23 May 2016, 3412 known extra-solar planets ranging in size from just above the size of the Moon to gas giants about twice as large as Jupiter have been discovered, out of which more than 100 planets are the same size as Earth, nine of which are at the same relative distance from their star as Earth is from the Sun, i.e. in the habitable zone.

2.4As of 23 May 2016, 3412 known extra-solar planets ranging in size from just above the size of the Moon to gas giants about twice as large as Jupiter have been discovered, out of which more than 100 planets are the same size as Earth, nine of which are at the same relative distance from their star as Earth is from the Sun, i.e. in the habitable zone.

Introduction to Planets in Solar System

3.1Mercury is the closest planet to the Sun. It orbits the Sun quickly, once every 88 days. It rotates slowly, however, only once every 59 days. Mercury is small, about 4850 kilometers (3000 miles) in diameter. Because Mercury is so close to the Sun, the side of its surface that faces the Sun is very hot, 800°K. The surface of Mercury is gray to orange in color, and is covered with craters. Mercury is named for a mythical god who ran very fast.

3.2Venus, the second planet away from the Sun, is Earth’s closest neighbor. It is about the same size as the Earth, a little over 12, 000 kilometers (7300 miles) in diameter. Venus has a very thick atmosphere, composed largely of sulphuric acid and CO2. We could not breathe on Venus, because the atmosphere would be very toxic to humans. This atmosphere gives Venus a brownish-yellow color. It also traps heat (the greenhouse effect) making the surface of Venus the hottest in the Solar System, about 900°K. Venus rotates very slowly, taking 243 hours to complete one turn. It is named for the Roman goddess of love.

3.3Earth is a little more than 12,000 kilometers in diameter. It differs from the other planets because it has liquid water on its surface, maintains life, and has active plate movement. The Earth has one Moon. The Earth is rotating or spinning around its axis, this is evidenced by day and night. At the equator the earth has an eastward velocity of 0.4651 km/s (1040 miles/hr). Earth is orbiting around the Sun in an orbital revolution. A complete orbit around the sun takes one year or about 365 days; it averages a speed of about 30 km/s (67, 000 miles/hr).

3.4Mars is a little more than half the size of the Earth, having a diameter of 6, 790 kilometers. It takes Mars 687 days to revolve once around the Sun. It rotates at about the same speed as the Earth, taking 24.6 hours. Mars has a very thin atmosphere which is composed largely of CO2. Its surface is very cold, and is covered with craters, volcanoes, and large canyons. Mars is reddish in color. Mars has two small moons. It is named for the Roman god of war.

3.5Jupiter is the largest planet in the Solar System, with a diameter of 142, 980 kilometers, more than 11 times wider than the Earth. Jupiter orbits the Sun once every 12 years. It rotates very fast, in 9 hours and 19 minutes. Its surface is made up of gas (mostly hydrogen), so that if you landed on the surface you would sink into it. Jupiter probably has a core of metallic hydrogen and rock, although evidence for this is theoretical. The outer gaseous part of Jupiter is broken into bands of white, yellow, red, and brown clouds. Jupiter has 4 rings mainly composed of dust. Huge oval-shaped storms also occur on the surface. Jupiter has 67 known satellites (as of 2016) including the four large Galilean moons (Io, Europa, Callisto, and Ganymede) plus many more small ones some of which have not yet been named. Jupiter is named for the Roman supreme god of heaven.

3.6Saturn is well known for its system of three rings. It is a large planet: at 120, 536 kilometers it is only a little smaller than Jupiter. It revolves around the Sun in 30 years, and rotates on its axis in a little more than 10 hours. Like Jupiter, Saturn is composed of mostly gas, and has a core composed of rock and metallic hydrogen. The surface of Saturn looks banded, and has a brown-yellow, butterscotch color. Saturn’s rings are probably composed of small particles of ice and rock. Saturn has 62 moons (as of 2016). It is named for the Roman god of agriculture.

3.7Uranus is 51, 118 kilometers in diameter, about 4.4 times the size of the Earth. It revolves around the Sun slowly, taking 84 years to complete one orbit. It rotates in about 17 hours. It is covered by a thick layer of gas, and has a fairly uniform blue-green color. Uranus has 27 moons (as of 2016) and is surrounded by a system of nine rings. It is named for another Roman god, the grandfather of Jupiter

3.8Neptune is slightly smaller than Uranus, with a diameter of 49, 500 kilometers. It circles the Sun once every 165 years, and rotates in 16 hours. Its atmosphere appears blue, and is marked by large dark blue storm systems. It is surrounded by a system of five rings and at least 14 moons. Neptune is named for the Roman god of the ocean.

3.9Pluto in 2006 was renamed as a dwarf planet. It has an eccentric, oval-shaped orbit, which is tilted with respect to the rest of the Solar System. Pluto revolves around the Sun in 248 years, and rotates in a period of 6.4 hours. Pluto is probably composed of rock. Its surface and color are unknown. It has one large moon that is almost like a twin with 2 smaller moons. Pluto is named for the Roman god of outer darkness.

Causes of Rotation/ Revolution of Planets

4.1The entire universe including our Milky Way and the Solar system are perpetually in motion, but their relative motion is almost static. Within our solar system, the Sun rotates along its axis anti-clock-wise from East to West. In addition to moving along their own axes, planets also revolve around the Sun in an elliptical orbit. Most of the rotational and orbital motions in the solar system are in the same eastward direction. Let us analyze as to why they do so.

4.2The revolution of any planet around the Sun is a result of the conservation of momentum. The entire Solar System was formed from a rotating gas hydrogen and other ionized particles. And when this gas condensed into the Sun and the planets, by virtue of the conservation law, all the planets also started going around the Sun. Planets revolve around the Sun (or more appropriately, centre of its mass) partly due to Gravity that keeps pulling planets towards itself and partly due to conservation of angular momentum. The planets have momentum and they want to move away from the star, but the star keeps pulling them back with gravitational force. The planet’s velocity is always in the perpendicular direction to that gravitational pull. As a result the planet keeps revolving around the star.

4.3Rotation on other hand is due to conservation of angular momentum. Since planets (as a matter of fact nearly everything in visible part of universe) is formed due to accretion of matter, the repeated collisions/ accretion of matters lead to condensation of matter in a very small space than it was before. Just like a ballet dancer pulls her hands in to increase speed, this condensation of matter in a smaller space causes bodies to rotate faster, because that prevented it from collapsing upon itself entirely and ending up into a black hole.

The regions closer to the center, which weren't going around with enough speed collapsed to the center and formed the Sun. But they still rotate, about the center of the Sun, to conserve the angular momentum, but just not fast enough to escape the Sun. Due to conservation of angular momentum, the particles that collided with each other to form the planet, had their own angular momentum and the planet gets the resultant. The gases which were further away from the center were rotating with enough speed to stay away from the Sun. But small irregularities in the gas caused some regions to be denser than the rest. By virtue of gravity, matter collapsed onto itself, and formed planets in the same way the Sun was formed. Though there were regions of higher density, there still was a lot of gaseous matter around too and after a lot of tug of war, the denser regions won (obviously). This tug of war is the one that resulted in the rotary motion, and hence the planets rotate. Almost all planetary systems are gyroscopic systems and hence the angular momentum of planet remains almost constant for indefinite periods of time. But actually it is not true due to many factors like the planets being not a perfect sphere and interaction of magnetic and gravitational fields of neighboring planets. It is not possible for a planet to not have a rotational motion. Any unbalanced torque about an axis will initiate a rotation of the planet along its axis. With time the rotating body settles in to a state of constant angular velocity as unbalanced torque is cancelled by other torques. One surprising effect of this kind of settling down is that the Moon only shows its one face to Earth.

4.4This rotary/ revolutionary motion is extremely important. It keeps the planets from falling into the Sun and from collapsing onto themselves as black hole. (note that the Sun uses the pressure from the fusion in its core too, to prevent itself from collapsing onto itself). Most of the rotational and orbital motions in the solar system are in the same eastward direction. Motions in this direction are referred to as direct motions, while motions in the opposite direction are referred to as retrograde.

Laws of Planetary Motion

5.1In 499 AD, the Indian astronomer Aryabhata propounded a planetary model that explicitly incorporated Earth's rotation about its axis, which he explains as the cause of what appears to be an apparent westward motion of the stars. He also believed that the orbits of planets are elliptical. Aryabhata’s followers were particularly strong in South India, where his principles of the diurnal rotation of Earth, among others, were followed and a number of secondary works were based on them. In 1500, Nilakantha Somayaji of the Kerala school of astronomy and mathematics, in his Tantrasangraha, revised Aryabhata’s model. In his Aryabhatiyabhasya, a commentary on Aryabhata’s Aryabhatiya, he developed a planetary model where Mercury, Venus, Mars, Jupiter and Saturn orbit the Sun, which in turn orbits Earth, similar to the Tychonic system later proposed by Tycho Brahe in the late 16th century. Most astronomers of the Kerala school who followed him accepted his planetary model.

5.2With the invention of telescope in 16th century, vast research work was done in the field of astronomy. J. Kepler of Germany (27-12-1571 to 15-11-1630) made major contribution formulating laws of planetary motion around the Sun, which are as under:

1.The orbit of a planet is an ellipse with the Sun at one of the two foci (and there being nothing at the second foci.

2.A line segment joining a planet and the Sun sweeps out equal areas during equal intervals of time (as the planet travels along its elliptical orbit).

3.The square of the orbital period of a planet is proportional to the cube of the semi-major axis of its orbit. (Thus a planet which is farther from the Sun moves at a slower speed than a planet which is nearer to Sun).

Figure-1: Illustrates Kepler's three laws with two planetary orbits.

(1)The orbits are ellipses, with focal points ƒ1 and ƒ2 for the first planet and ƒ1 and ƒ3 for the second planet. The Sun is placed in focal point ƒ1.

(2)The two shaded sectors A1 and A2 have the same surface area and the time for planet 1 to cover. segment A1 is equal to the time to cover segment A2.

(3)The total orbit times for planet 1 and planet 2 have a ratio a1³/²: a2³/².

5.3Kepler's work (published between 1609 and 1619) improved upon the heliocentric theory of Nicolaus Copernicus, explaining how the planets’ speeds varied, and using elliptical orbits rather than circular orbits with epicycles. Kepler in 1621 and Godefroy Wendelin in 1643 noted that Kepler's third law applies to the four brightest moons of Jupiter. Isaac Newton computed in his Philosophiæ Naturalis Principia Mathematica, the acceleration of a planet moving according to Kepler's first and second law.

1.The direction of the acceleration is towards the Sun.

2.The magnitude of the acceleration is inversely proportional to the square of the planet's distance from the Sun (the inverse square law).

This implies that the Sun may be the physical cause of the acceleration of