Ch 15 summaryI have learnt that in past people know the relationship between earth events and position of heavenly bodies. They knew that season and flood occur when moon and sun reaches certain position in heaven. Ancient Chinese, Egyptian and Babylonians kept record like location of moon, sun and five planets. Greeks develop geometry and trigonometry and measure the things like size and distance of bodies like sun and moon. Greeks thought that earth is motionless. They thought that there is celestial sphere to which stars were attached and that sphere orbits around the earth. Greek use geometry to determine distance from earth to sun and moon and then determine their sizes. Ptolemaic system is the model of universe. Modern astronomy was developed by Nicolas Tycho, Johannes, Galileo and Sir Isaac Newton. Nicolaus convinced that earth is a planet and it also rotates. He said that sun is the center of planets and planets like Mercury, Venus, Earth, Mars, Jupiter and Saturn orbit around sun. Tycho Brahe invented telescope called pointers which were used to determine location of heavenly bodies. Johannes Kepler derived the basic law of planetary motion. He observed that speed of Mars vary as it approaches sun its speed increases while decreases as it moves down. Galileo Galilei did experiments and explain the behavior of moving objects. Sir Isaac Newton introduces the concept of inertia and law of universal gravitation.I have learned that nebular theory states the formation of solar system. He said that sun and planets are formed from interstellar gases and dust. Due to contraction of gravity most of the material collected in center while remaining material form disks and gradually cool and form rocky material and then due to repeated collisions causes to material to clump together and become asteroid sized object called plantesimals. Planets are classified into terrestrial planets which are like Mercury, Venus, Earth and Mars while Jovian planets are Jupiter, Saturn, Uranus and Neptune. Jovian’s planet have thick atmosphere are composed of hydrogen and helium whileterrestrial planets have meager atmosphere which consists of carbon dioxide or nitrogen. Moon isformed due to the collision between Mars size body and youthful Earth. Moon consists of dark regions which are called Maria and bright parts which are called lunar highlands. I have learned that Mercury is the innermost and smallest planet which completes its revolution around sun in 88 days it rotates slowly around its own axis. Venus obits around the sun in 225 Earth days. Mars is one-half of Earth in diameter. It completes its circle around sun in 687 days. Jupiter is heaviest planet and its mass is 2.5 times more than combined mass of all planets. Jupiter have 67 moons. Saturn completes its circle in more than 29 Earth years and Saturn have 62 moons. Uranus and Neptune are equal in diameter. They have methane in their atmosphere. Uranus completes its circle around sun in 84 Earth years while Neptune in 165 Earth years. Uranus have 5 moons while Neptune have 14 satellites. There are billions of small bodies in universe. Some are small, for example asteroids, comets while some are dwarf planets for example Ceres. Asteroids are smaller and are 4.6 billion years old. Comets consists of dust, waterand ice. I have also study about Meteors, Meteoroids and Meteorites and there types.I have learned about solar system. Solar system is very large. It consists of different planets, Meteoroids, moon and stars. As we know that population of Earth is increasing day by day but the space available in earth is shortening. So there is need of moving toward other planets. Weshould research about other planets that which is suitable for life. Scientists are researching aboutMars. We should also research on other planets so that at the time of need we could move to other planet.I have a question for that what things will be needed for life at a planet other than