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
18th 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 (16931776) 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