1. Finding your longitude The trickier part of celestial navigation

2. Latitude was easy Altitude of celestial pole Declination of a star at the zenith You can also use the sun, but you need to know the declination of the sun at the time

3. Longitude is harder Arbitrary starting point Longitude 0 degrees at Greenwich, England

4. Calculating your longitude You can use stars or you can use the Sun But in addition to making observations you need to know the time for some location of known longitude, local time alone is not enough

5. The problem of finding longitude at sea To the middle of the 18 th century, no mechanical clock would keep accurate time in a sea-tossed ship

7. Natural clocks Motion of the moon against the background of the stars Motions of the moons of Jupiter But these were hard to observe from a ship, although they could be observed from land

8. Odder Solutions Anchor a series of ships across the ocean that would shoot off flares and guns at set times Telepathic connection between animals on ship and those ashore

9. Harrison’s chronometer John Harrison (1693- 1776) invented clocks that would keep good time at sea

10. Watch out for the difference between local time and local mean time The Equation of Time

11. Finding local time “Culmination” of the Sun at noon RA of a star on the meridian

12. Sextant