The longitudinal line which passes through Greenwich, England is given a longitude value of zero degrees and is termed the prime meridian, an arbitrary starting point for the longitude lines that run around the Earth. Time in Greenwich, England is called Greenwich Mean Time (GMT) or more commonly now Universal Time (UT). Hence, one’s longitude could be determined by knowing the angle at which you are viewing a star, what your local time is (readily determined by the sun) and what time it is in Greenwich, England. These results could then be extrapolated to any location and any time around the Earth. Why was Greenwich, England used? Because that was where the British Royal Observatory was and from where the initial observations were made. Using meticulous observations and calculations, tables were created listing where and at what angles commonly observed celestial bodies should be seen at a given time and a given location-Greenwich, England. Since one day is 24 hours in length and the Earth is a sphere of 360 degrees, it follows that the Earth rotates at a rate of 15 degrees every hour (360/24 = 15).Īgain, if for practical purposes we think of the Earth as being fixed in position, then another way of saying the same thing is that the stars rotate above the Earth, or across the sky, at a rate of 15 degrees every hour. Celestial navigation is based on the fact that at any specific point in time, a given celestial body is located directly over one point on the Earth’s surface, i.e. Rather than the Earth rotating, think of the Earth as being fixed in position and the celestial bodies rotating across the sky above us. Longitude was different and far harder to determine.Ĭelestial navigation, the method sailors in Cook’s day used to find their way around the globe, refers to measuring the angle between a celestial body (the sun, moon or star) and the horizon and using this data to determine one’s position on the Earth’s surface. Sailors could readily determine their latitude, how far above or below the equator they were, by measuring the sun’s angle at its highest point above the horizon, or by measuring the angle of the North Star or other stars above the horizon. Lines of longitude are the lines that run north-south and determine how far around the globe you are. As you may remember, lines of latitude are the lines that run east-west on a map or globe and determine how far north or south of the equator you are. One of the biggest problems facing early European navigators including Captain Cook, who was the first European to map New Zealand, was determining one’s longitude while at sea.
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