Astronomy Introduction PDF

INTRODUCTION M YAZDAN ZUHAD & M ANAS QURESHI  GLOBE A globe is a three-dimensional, spherical, scale model of Earth (terrestrial globe or geographical globe) or other celestial body such as a planet or moon. While models can be made of objects with arbi...

INTRODUCTION M YAZDAN ZUHAD & M ANAS QURESHI  GLOBE A globe is a three-dimensional, spherical, scale model of Earth (terrestrial globe or geographical globe) or other celestial body such as a planet or moon. While models can be made of objects with arbitrary or irregular shapes, the term globe is used only for models of objects that are approximately spherical. M. YAZDAN ZUHAD  GLOBE The word “globe” comes from the Latin word Globus, meaning round mass or sphere. Some terrestrial globes include relief to show mountains and other features on the Earth’s surface. There are also globes, called celestial globes or astronomical globes, which are spherical representations of the celestial sphere, showing the apparent positions of the stars and constellations in the sky. M. YAZDAN ZUHAD  HEMISPHERE A half of a sphere bounded by a great circle. A half of the earth, usually as divided into northern and southern halves by the equator, or into western and eastern halves by an imaginary line passing through the poles. The most common such divisions are by latitudinal markers: North-South Northern Hemisphere, the half that is north of the equator Southern Hemisphere, the half that lies south of the equator M. YAZDAN ZUHAD M. YAZDAN ZUHAD  POLE The axis of rotation of any rotating (spinning) object is called its Pole. Earth’s pole is tilted by about 23.44°. This axial tilt of Earth is also termed by astronomers as the obliquity of ecliptic; and is equal to the angle between the ecliptic and the celestial equator on the celestial sphere. M. ANAS QURESHI  EQUATOR An equator is the intersection of the surface of a rotating sphere with the plane that is perpendicular to the sphere's axis of rotation and midway between its poles. M. YAZDAN ZUHAD  EQUATOR The equator usually refers to the Earth's equator: an imaginary line notionally drawn on the Earth's surface equidistant from the North Pole and South Pole, dividing the Earth into the Northern Hemisphere and Southern Hemisphere and constituting the parallel of latitude 0°. The Equator is about 40,075 kilometers (24,901 mi) long, some 78.7% lies across water and 21.3% over land. Other planets and astronomical bodies have equators similarly defined. M. YAZDAN ZUHAD  Spherical Astronomy Spherical astronomy or positional astronomy is the branch of astronomy that is used to determine the location of objects on the celestial sphere, as seen at a particular date, time, and location on Earth. M. YAZDAN ZUHAD  GREAT CIRCLE A great circle, also known as an orthodrome or Riemannian circle, on the surface of a sphere is the intersection of the sphere and a plane that passes through the center of the sphere. M. YAZDAN ZUHAD  GREAT CIRCLE ARC The shortest distance between any two points on the surface of a sphere is actually a great circle arc, i.e. part of a great circle. M. YAZDAN ZUHAD  SMALL CIRCLE A circle on the surface of a sphere whose plane does not pass through the sphere's centre. M. YAZDAN ZUHAD  ZENITH Zenith refers to an imaginary point directly above an observer or a particular location on the imaginary celestial sphere. M. ANAS QURESHI  NADIR Nadir is the imaginary point directly below an observer or a particular location; i.e. in the direction exactly opposite to that of zenith. That makes zenith & nadir the two vertical directions (directly above and below respectively) at any specified location on the imaginary celestial sphere. M. ANAS QURESHI  HORIZON It is the circular line where the Earth and sky or sea & sky apparently meet. The horizon or skyline is the apparent line that separates earth from sky, the line that divides all visible directions into two categories: those that intersect the Earth's surface, and those that do not. At many locations, the true horizon is obscured by trees, buildings, mountains, etc., and the resulting intersection of earth and sky is called the visible horizon. When looking at sea from ashore, the part of the sea closest to the horizon is called the offing. M. ANAS QURESHI  HORIZON M. YAZDAN ZUHAD  CELESTIAL SPHERE An imaginary sphere depicting the sky. The observer is considered to be at the center of this sphere and all the objects in the observer's sky perceived as lying on the inside surface of this sphere, as if it were the underside of a dome or a hemispherical screen. M. ANAS QURESHI  CELESTIAL SPHERE In astronomy and navigation, the celestial sphere is an imaginary sphere of arbitrarily large radius, concentric with Earth. All celestial objects are then considered projected upon the inner surface of this sphere (without any consideration of their linear distances from the observer). The celestial sphere is a practical tool for spherical astronomy, allowing observers to plot positions of objects in the sky when their distances are unknown or unimportant. M. ANAS QURESHI  CELESTIAL EQUATOR The projection into space of the earth's equator; an imaginary circle equidistant from the celestial poles. The celestial equator is a great circle on the imaginary celestial sphere, in the same plane as the Earth's equator. In other words, it is a projection of the terrestrial equator out into space. As a result of the Earth's axial tilt, the celestial equator is inclined by 23.44° with respect to the ecliptic plane. M. YAZDAN ZUHAD  CELESTIAL POLES The points on the celestial sphere directly above the earth's geographic poles, around which the stars and planets appear to rotate during the course of the night. The north and south celestial poles are the two imaginary points in the sky where the Earth's axis of rotation, indefinitely extended, intersects the celestial sphere. The north and south celestial poles appear permanently directly overhead to an observer at the Earth's North Pole and South Pole respectively. As the Earth spins on its axis, the two celestial poles remain fixed in the sky, and all other points appear to rotate around them, completing one circuit per day (strictly per sidereal day). M. ANAS QURESHI  ECLIPTIC PLANE M. ANAS QURESHI  ECLIPTIC M. YAZDAN ZUHAD