Astronomy is a natural science which is the study of celestial objects (such as stars, galaxies, planets, moons, and nebulae), the physics, chemistry, and evolution of such objects, and phenomena that originate outside the atmosphere of Earth, including supernovae explosions, gamma ray bursts, and cosmic microwave background radiation. A related but distinct subject, cosmology, is concerned with studying the universe as a whole. A galaxy is a massive, gravitationally bound system consisting of stars, stellar remnants, an interstellar medium of gas and dust, and dark matter, an important but poorly understood component.[1][2] The word galaxy is derived from the Greek galaxias (γαλαξίας), literally milky, a reference to the Milky Way. Examples of galaxies range from dwarfs with as few as ten million (107) stars[3] to giants with one hundred trillion (1014) stars, each orbiting their galaxys own center of mass. Galaxies contain varying numbers of planets, star systems, star clusters and types of interstellar clouds. In between these objects is a sparse interstellar medium of gas, dust, and cosmic rays. Many galaxies are believed to have Supermassive black holes (SMBH) at their center. These are thought to be the primary driver of active galactic nuclei found at the core of such galaxies. The Milky Ways presumed SMBH, known as Sagittarius A*, has a mass four million times that of our sun The Average Motion of the Moon The Moon moves around the Earth in an approximately circular orbit, going once around us in approximately 27.3 days, or one sidereal period of revolution. As it does so, its position changes, relative to the stars. Since there are 360 degrees in a circle, the Moon moves 360 / 27.3 or 13.2 degrees per day relative to the stars, which is just over half a degree per hour, and approximately equal to its apparent size. This means that from night to night, the Moon moves a little more than one hand-width to the East (the direction of its motion around the Earth) relative to the stars, and from hour to hour, it moves about one diameter to the East, among the stars. An approximate representation of the motion of the Moon around the Earth. Moving once around in 27.3 days, its average movement is about 13.2 degrees per day, or 92 degrees per week. (As is usual in such diagrams, the sizes of the Earth and Moon are exagerrated, in comparison to their distance.) Motions of the Earth The Earth is constantly in motion, revolving around the Sun and rotating on its axis. These motions account for many of the phenomenon we see as normal occurrences: night and day, changing of the seasons, and different climates in different regions. With a globe ball properly mounted and rotating on its axis, the movements of the Earth around the Sun may be illustrated accurately. Rotation The Earth spins on its axis from West to East (counter-clockwise). It takes the Earth 23 hours, 56 minutes, and 4.09 seconds to complete one full turn. Day and night are produced by the rotation of the Earth. The speed of rotation at any point upon the equator is at the rate of approximately 1,038 miles per hour, decreasing to zero at the poles. Revolution While the Earth is spinning on its axis, it is revolving around the Sun in a counter-clockwise direction. It takes the Earth one full year to complete one full revolution around the Sun. This path is known as the Earths orbit. It is very near a circle. The mean distance of the Earth from the Sun is about 93 milling miles and the distance varies by 3 million miles, forming a slightly oval path. The revolution of the Earth around the Sun traverse a distance of 595 million miles in 365 days, 6 hours, 9 minutes and 9.5 seconds. This means a speed of 18 miles a second (or 66,000 miles per hour) while at the same time rotating once each twenty-four hours. The Solar System[a] comprises the Sun and the objects that orbit it, whether they orbit it directly or by orbiting other objects that orbit it directly.[b] Of those objects that orbit the Sun directly, the largest eight are the planets[c] that form the planetary system around it, while the remainder are significantly smaller objects, such as dwarf planets and small Solar System bodies (SSSBs) such as comets and asteroids.[d] The Solar System formed 4.6 billion years ago from the gravitational collapse of a giant molecular cloud. The vast majority of the systems mass is in the Sun, with most of the remaining mass contained in Jupiter. The four smaller inner planets, Mercury, Venus, Earth and Mars, also called the terrestrial planets, are primarily composed of rock and metal. The four outer planets, called the gas giants, are substantially more massive than the terrestrials. The two largest, Jupiter and Saturn, are composed mainly of hydrogen and helium; the two outermost planets, Uranus and Neptune, are composed largely of substances with relatively high melting points (compared with hydrogen and helium), called ices, such as water, ammonia and methane, and are often referred to separately as ice giants. All planets have almost circular orbits that lie within a nearly flat disc called the ecliptic plane.
Posted on: Thu, 20 Nov 2014 06:01:03 +0000
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