Finding your longitude The trickier part of celestial navigation

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

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

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

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

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

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

Harrison’s chronometer John Harrison ( ) invented clocks that would keep good time at sea

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

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

Sextant