b) Early Navigational Instruments
"Morski Sviat" magazine,
The quadrant is another instrument which seamen adapted for
navigation. Its name comes from the quarter-circle that it uses
as a scale. The simplest quadrants are made of a 90-degree protractor
with a plumb weight hanging from its vertex. Astronomers were
the ones who took full advantage of the instrument and the Arabs
were, again, among the peoples who were quite familiar with the
device. Instead of degree measures, some of the ancient quadrants
had names of major ports written on the appropriate spots around
the arc. When the cord hanging from the vertex cut the arc at
the name of a certain place, the sailor knew that he had to turn
east or west along that latitude line in order to reach that certain
port. The readings of the quadrant were taken by two people -
one to take the instrument and look at the celestial body of reference,
and the other to read the altitude from the arc. In rough weather
it was very hard to take a reading on this device since the instrument
had to remain steady for an accurate reading. However this is
slightly offset as in harsh weather visibility is usually poor
and astronomical instruments couldn't be used in the first place.
In daylight times, navigators need to be able to orient themselves
by the sun. The astronomical ring is yet another rather simple
device, relying on the sun, rather than the stars, for its readings.
It is simply a ring, as its name suggests. It consists of a circle
hung from a ring. Gravity aligns the ring with the zenith (the
highest point that the sun reaches in the sky). A tiny whole in
the ring (pinnule) allows a ray of sunlight to shine onto the
inside of the circle, which is graduated in a degree scale. Thus
the sun's altitude is recorded; from that, latitude can be calculated.
"Morski Sviat" magazine, permission obtained
The Vikings had no compasses or other accurate and complicated
devices for navigation, but simply a variation of a sundial.
They used these simple wooden instruments for their long open-sea
voyages to Iceland, Greenland, and probably even as far as the
North American continent. The sundial principle of keeping track
of time remained the main type of "clock" (along with
sandglasses) used on ships until the chronometer took over in
the second half of the 1700s. The sundial was used to keep track
of sailors' duty shifts, of celestial observations, and helped
determine ships' speed. Later when clocks became available for
ships to use aboard, sundials remained in use - to roughly check
the accurateness of the other clocks on board.
An old, very complex, and highly accurate clock was the
nocturnal. It was a star clock, meaning that it used the
position of the stars to determine the phases of the moon, the
lengths of days and nights, holidays, sunrises and sunsets, positions
of the sun relative to the zodiac, and even (some advanced nocturnal)
calculated tides. The first of these devices was developed in
the 1200s and was used by Europeans as well as Arabs. The nocturnal
consisted of a sight, a pointer, and date and hour disks. They
were often crafted in brass and some were very elaborate. They
remained the most accurate clocks at sea (and on land, for a long
time). They were in use up until the beginning of the 19th century
- long after the chronometer was perfected into a most accurate
time keeping device. The only downside of the nocturnal was that
it depended on the Polaris and could not function in the southern
hemisphere (where Polaris was below the horizon) or in any other
circumstances in which the star was not visible.
The first navigational instrument, whose creator's name is
known for sure, was the backstaff.
An English explorer of the 16th century, John Davis, was impressed
by the cross-staff, but wanted to improve upon it, to avoid the
error due to the "disorderly placing of the staff to the
eye." His 1590 device (also known as the Davis or English
quadrant) was so simple and accurate that it earned a place in
navigation for over 200 years.