© Duane A. Cline 1999
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Navigation: The Nocturnal Nocturnals, or nocturlabes as they were sometimes called, are devices for telling the time of night. Their operation is based
on the fact that the stars, while remaining fixed relative to one another, appear to rotate about the North Star. It is, of
course, the earth which is rotating, and the North Star remains fixed because it lies along the Earth's axis of rotation. As the
other stars appear to rotate, their position at any moment indicates the time. A nocturnal consists of several pieces of metal or wood, which are attached at the center so they can rotate relative to one
another. At the axis of rotation there is a hole. When in use, the nocturnal is held upright by the handle until the North Star can be sighted through the hole. The long arm
of the device is then turned until it lies along the line made by the two brightest stars in the constellation known as the Great
Bear -- also known as the Big Dipper. These two stars are often used as "pointers" because they are easily seen and they lie
along a line which passes close to the North Star. The bright star in the Little Bear -- or Little Dipper -- can be used in the
same way. The sample nocturnal included in this packet is one that was a favorite of London instrument-makers a little after the time of
the Pilgrims. However, the nocturnals in the early 17th century were very much like this one. This nocturnal can be used
with either the Great Bear or the Little Bear, and thus it is labeled "Both Bears" on the handle. If the Little Bear is used, the
inner dial would be turned so that the pointer marked "LB" would lie against the date on which the observation is being
made. By doing this, we automatically make the correction from sidereal time to solar time. Remember that the "solar
time" is an average of four minutes longer that "sidereal time." After setting the inner dial to the correct date, the North Star is sighted through the hole and the long arm is turned until the
bright star in the Little Bear lies on it. The time is then read off from the scale on the central dial -- just as if the long arm
were acting as the hand of a clock. If the Great Bear is chosen as a reference, the procedure is the same, except that the small pointer marked "GB" is set
against the date. Thus, the navigators on the Mayflower were able to tell the time at night -- if the weather was clear. The problem was that
in foul weather when the sky was obscured, they had no way of knowing the exact time except by the running of the
sand-glasses. Spring-wound watches were being made by this time in history, but we have no actual reference to any on the Mayflower.
That is a question which may never be answered. A pattern for a nocturnal will be found in the enclosed worksheets. It is very important to be sure you have the following
materials readily at hand when beginning the project. MATERIALS NEEDED: Assemble the three parts of the nocturnal in the sequence as numbered on the pieces.
Last modified October 7, 1999
by
Webmaster Dave Lossos