Horologium the Pendulum Clock
Originally it was Horologium Oscillatorium but has since been shortened.  It was introduced by La Caile in the 1750's.  It is one of the fainter constellations, lying forgotten alongside the end of Eridanus the River near Achernar.  Unlike some of the other items of technology that were placed in the sky Horologium could be said to have earned it's place in astronomy. 

The Dutchman Christiaan Huygens was the first to make a working pendulum clock in 1656.  Galileo had noticed that a pendulum regulated mechanism would keep good time but was not able to complete one before his death.  For Huygens, well trained in mathematics and machinery, is was a relatively simple task.  His invention was quickly taken up by other clock makers and by 1671 you could obtain a pendulum clock accurate enough that it only lost a second or two per day.  Huygens produced a lot of work in his time: he made better telescopes that enabled him to discover Titan, the largest moon around Saturn, and to divine the true nature of Saturn's rings (although it took some time for other astronomers to recognise him for it) and invented the Huygen's eyepiece for telescopes, a style that is still made today.  NASA has honoured him by naming the probe that will be dropped into Titan's atmosphere when the Cassini mission reaches Saturn in 2004 after him.

Now that astronomers had good clocks at their disposal it was no matter to regulate the timing just a little further so that the clock kept sidereal time, the time of the stars.  A sidereal day was the time it took a star to return to a particular point in the sky and because of the orbital motion on the earth this is approximately 4 minutes less than a solar day, the time we use for every day schedules.  Astronomers would place the clock in the observatory with telescope they used and then scan the sky in a north-south strip.  When they found an object of note they read the time off the clock to get the Right Ascension (celestial longitude) of the object and read the Declination (celestial latitude) from another fixed circle connected to the telescope mount.  

Pendulum clocks are still seen today, but mainly as pieces of decorative furniture.  The advent digital and atomic clocks have the made the pendulum clock obsolete for astronomical work.  

This article is ©2002 Stargazers Astronomy Shop

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