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Astrophysics History of Exploring the Universe Done by Zheng Han & Sun Bangjie celestial sphere Definition: In astronomy and navigation, the celestial sphere is an imaginary sphere of arbitrarily large radius, concentric with the Earth and rotating upon the same axis. In the Aristotelic and Ptolemaic models, the celestial sphere was imagined as a physical reality rather than a geometrical projection system of coordinates Definition: A coordinate (or co-ordinate) system is a system for assigning an n-tuple of numbers or scalars to each point in andimensional space Constellations- in northern part Newton's Law of Gravity Each object in the universe attracts each other body. If object A has mass Ma and object B has mass Mb, then the force F on object A is directed toward object B and has magnitude F = G Ma Mb / r2 So, considering the force between an Sun and the Earth The force exerted on the Earth by the Sun is equal and opposite to the force exerted on the Sun by the Earth. If the mass of the Earth were doubled, the force on the Earth would double. If the mass of the Sun were doubled, the force on the Earth would double. If the Earth were twice as far away from the Sun, the force on the Earth would be a factor four smaller. Notes on the meaning of r: If the two objects are very small compared to the distance between them, then the force is given by Newton's formula with r being the distance between the objects. If one of the objects is very small and the other is spherically symmetric, then the same formula holds with r being the distance from the small object to the center of the big object. Point two is a consequence of point one. Newton proved it by adding up the forces. Kepler's third law The average distance of a planet from the Sun cubed is directly proportional to the orbital period squared. Newton found that his gravity force law could explain Kepler's laws. Since Newton's law of gravity applies to any object with mass, Kepler's laws can be used for any object orbiting another object. Let's look at satellites orbiting a planet. If you have two satellites (#1 and #2) orbiting a planet, Kepler's third law says: (period #1/period #2)2 = (distance #1/distance #2)3, where the distance is the average distance of the satellite from the planet---the orbit's semimajor axis. The satellites must be orbiting the same planet in order to use Kepler's third law! Kepler found this law worked for the planets because they all orbit the same star (the Sun). If you have measured the orbital period of one satellite around a planet, you can then easily find how long it would take any other satellite to orbit the planet in any size oribt. Kepler's third law can be simplified down to Structure and behavior of the solar system The solar system is the star system we are now living in. It consists of the sun, eight planets, sixty six satellites and countless asteroids, comets and meteorites. The order of the planets from the sun to the outside is Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune. The planets that are near the sun like Mercury, Venus, Earth, Mars are called terrestrial planets. The same characteristics of them are of higher density, with smaller volume, slower rotation, fewer satellites and solid shell. The planets that are far away from the sun like Jupiter, Saturn, Uranus and Neptune are called Jovian planets. They all have thick atmosphere and the characteristics of theirs surface are difficult to know. Between Mars and Jupiter, there are more than 100000 asteroids. Meteorites are between the planets. These planets are moving around the sun on the elliptical orbits. Nature of Moon's motion and eclipses The Moon orbits the Earth roughly once a month. Looking down on the Earth and Moon from above the Earth's north pole, we see that its revolution is in the same direction as the Earth's rotation (and also the Earth's revolution around the Sun). At new moon, we can't see any of the illuminated half of the Moon; At full moon we can see all of it. Halfway in between new and full moon, we see half of the illuminated half of the Moon, or a quarter of the Moon. First quarter occurs as the Moon moves from new to full; third or last quarter occurs as the Moon moves from full to new. Between the new and quarter moons, only a small fraction of the Moon is illuminated; we call this a crescent moon. Between the quarter and full moons a larger fraction of the Moon is illuminated; we call this a gibbous moon. When the Moon moves from new to full, it becomes more illuminated, and we say that it is waxing. When it moves from full to new, it becomes less illuminated, and we say that it is waning. Ecliptic telescope In fact, the telescope just makes the object become nearer to our eyes. The image is lessened, inverted and virtual. microscope In fact, the microscope is used to twice magnify the object in order to get a bigger image. The image is magnified, upright and virtual.