Weather Permitting - GPS Declassified: From Smart Bombs to Smartphones - page 30

one-sixth of a circle, or 60 degrees, enabling measurements of larger angles
up to 120 degrees. 12 A sky chart shows the relative latitude of the sun over the
earth for each day of the year and is useful for determining latitude, if the sun
can be observed at noon. The compass appears to have been invented in China
around 800 ad. It allowed ships to sail year-round in the Mediterranean and
greatly increased trade.
Lighthouses are useful for sea navigation. However, they are subject to dan-
gerous misinterpretations, as occurred with the ss Atlantic . On March 31, 1873,
the ship was headed with 814 passengers and 143 officers and crew toward
Halifax, Nova Scotia, to replenish its coal. Lost in the dark, it hit a rock early
the next morning. The officers and crew rushed to the deck and were able to
free ten lifeboats, but the currents washed the boats away. Twenty people were
killed on the deck when the bow on the foremast came loose. Ropes were
brought ashore and some people managed to clamber across them to safety.
Many married men refused to leave their wives behind and died with them.
Five hundred forty-ive people died in this catastrophe, which may have been
caused by the crew confusing Sambro Light for Devil's Light, which was far-
ther to the west. None of the 138 women on board the Atlantic survived;
undoubtedly, they were handicapped by their heavy clothing. This included
one woman disguised as a man who served as a crewmember. 13 It was the
worst loss of life in a marine disaster prior to the Titanic .
Lunars, Jovian Moons, and Clocks
As mentioned earlier, the Board of Longitude was charged with finding a reli-
able way of estimating longitude. The largest award was £20,000 for a method
that could determine longitude to an accuracy of half a degree of a great circle.
The astronomical approach used celestial objects such as the Jovian moons or
the lunar distances method, also called lunars. Galileo discovered four moons
of Jupiter when he viewed the planet with a telescope in 1610. Their orbits
range from 1.7 days for Io to 16.7 days for Callisto. Astronomers can predict
their orbits and when they will pass behind Jupiter (be eclipsed by Jupiter).
With four moons, there are many such eclipses. Observatories established
tables showing the time of the eclipses at a well-known longitude such as Lon-
don. Comparing the time on the published table with the time established by
the observer's local noon gives the difference and the longitude compared to
London. Measuring the local time of Jovian eclipses worked well on land; how-
ever, the eclipses could be seen only at night when the sky was clear and Jupi-