Trivia 8

The Earth globe is going to help us understand how we may locate objects in the Celestial sphere. A brief review of how we locate places (cities, lakes, etc.) on the Earth, will help us. This is elementary for most of you, but a review does not hurt. The sphere is divided in two hemispheres by a great circle called the Equator. Parallel to the equatorial circle there are five more. As the circles move from the Equator to the North Pole, they a getting smaller. These circles are called parallels or lines of latitude. We will measure from the Equator to the North, North Latitudes; to the Southern hemisphere, South Latitudes. You will find the same circles drawn parallel to the Equator towards the South Pole. Between the circles there are 6 divisions of 15^o each; it is just 1/4 of the whole circle: from the Equator to the Pole there are 90^o. Each degree has 60′ (minutes) and each minute has 60″ (seconds). These are minutes and seconds of arc, not of time.

There are also lines of Longitude, also called Meridian lines, or simply meridians. Those lines are not parallel to each other like the latitude lines. They are traced from the north pole to the south pole. There are 24 of them. The Earth rotates around its axis in 24 hours (close enough). If we divide the equatorial circle in 24 parts, that is 360^o by 24, we also get 15^o between two meridians. Let me point out a very important meridian, that is the Greenwich Meridian. It is the zero meridian, from which we can count degrees, minutes, seconds to the East or West. The Greenwich meridian is the meridian that passes through Greenwich observatory in England. It goes through western France, eastern Spain, Algeria, Mali, and Ghana, before entering the Atlantic Ocean and crossing the equatorial line, or simply, the Equator. Before watching the video, try to guess the terrestrial coordinates of Lisbon, capital of Portugal.

Video: http://www.showme.com/sh?h=yrWmfEO