Determining Latitude and Longitude (without a GPS!). Pendulum discussion Review definitions of lat and long and gauge prior knowledge Elaborate as needed Finding latitude using the stars Why longitude is so difficult to determine Ways to determine longitude
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Pendulum discussion • Review definitions of lat and long and gauge prior knowledge • Elaborate as needed • Finding latitude using the stars • Why longitude is so difficult to determine • Ways to determine longitude • Using a chronometer to find longitude • Practice • Off to the planetarium
What is latitude? • What might I be wearing if… • I am standing outside at 60ºN latitude, and it is January. • I am standing outside at 10ºN latitude, and it is February. • I am standing outside at 35ºN latitude, and it is July. • I am standing outside at 40ºS latitude, and it is August
How can you determine your latitude…without a GPS? • Use Polaris -the North Star • Animation • Ship view • At one time Polaris was exactly at the celestial pole. Precession of the earth's axis is causing the celestial pole to move very slowly away from Polaris.
What is longitude? • Lines of longitude, also called meridians, run north - south. • Farthest apart at the equator, and converge at the North and South Poles • Measure distance east or west of the prime meridian (located arbitrarily at Greenwich, England - has been located elsewhere throughout history) • Longitude of a particular location is the distance along a parallel, measured in degrees, between that place and the prime meridian.
How can you determine your longitude…without a GPS? • Need to know the time aboard the ship and home port (or other known longitude) at exactly the same moment • Two clock times permit conversion of time difference into geographic separation • 1 revolution of the Earth (a 360° turn), takes 24 hours, so 1 hour of spin = 1/24 of a turn or 15° • Every hour of difference between ship time and home port time is 15° of travel • Distance traveled will vary depending on your latitude (e.g. at the equator 15° = 1000 miles)
Quick Check… • While studying one summer at the British Museum in London, you have been kidnapped, drugged, and abandoned on a desert Island. When you come to, you find that your captors have left you with your Rolex watch (a fine chronometer) and a sextant. If you can only find out where you are, you might be able to find a way home. Using the sextant you discover that the North Star is 19° above the horizon at dusk. The next day you find that when the sun appears to reach its highest point, your watch reads 4:30 PM. Questions:Where are you? (What ocean, what latitude and longitude?)What is the nearest land?
Why was longitude a problem? • Time could not be measured accurately… • Methods… • Solar/Lunar Eclipses • Moons of Jupiter • Powder of sympathy (ha ha) • Magnetic variation • Celestial sphere • Cannon fire at ports • Ships anchored at regular distances • Build an accurate chronometer (clock!)