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!)