DK Eyewitness - Astronomy
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Argo, the<br />
Ship<br />
A celestial globe<br />
The celestial globe records the figures and stars of all the constellations<br />
against a grid of lines representing longitude and latitude. During the<br />
17th and 18th centuries, all ships of the Dutch East India Company<br />
were given a matching pair of globes—terrestrial (p.10) and celestial.<br />
Calculations could be made by comparing the coordinates on<br />
the two different globes. In practice, however, most navigators<br />
seemed to use flat sea-charts to plot their journeys.<br />
Hydra, the water snake<br />
Meridian<br />
ring<br />
Celestial globe<br />
1618<br />
The Southern<br />
Cross<br />
Shadow vane lined up<br />
with horizon vane<br />
Southern<br />
Triangle<br />
Centaurus,<br />
the Centaur<br />
Sun<br />
The great navigators<br />
Explorers of the 16th century had no idea what they would find<br />
when they set out to sea. Their heads were full of fables about<br />
mermaids and sea monsters. Even though this engraving of the<br />
Portuguese navigator Ferdinand Magellan (1480–1521) has many<br />
features that are clearly fantastical, it does show him using a pair of<br />
dividers to measure off an armillary sphere (p.11). Beside the ship,<br />
the sun god Apollo shines brightly; it was usually the Sun’s position<br />
in the sky that helped a navigator find his latitude.<br />
Horizon<br />
vane<br />
Scale in degrees<br />
Holder<br />
Sight<br />
vane<br />
Navigator with his<br />
back to the Sun<br />
Horizon<br />
Scale in degrees<br />
Using a backstaff<br />
The backstaff allowed a navigator to measure the height of<br />
the Sun without having to stare directly at it. The navigator<br />
held the instrument so that the shadow cast by the shadow<br />
vane fell directly on to the horizon vane. Moving the sight<br />
vane, the navigator lined it up so he could see the horizon<br />
through the sight vane and the horizon vane. By adding<br />
together the angles of the sight and shadow vanes, the<br />
navigator could calculate the altitude of the Sun, from<br />
which he could determine the precise latitude of his ship.<br />
Two angles<br />
give the Sun’s<br />
altitude<br />
Doing the mathematics<br />
To work out latitude at sea, a navigator<br />
needs to find the altitude of the Sun at<br />
noon. He doesn’t even need to know<br />
the time; as long as the Sun is at its highest<br />
point in the sky, the altitude can be<br />
measured with a backstaff or other<br />
instrument (p.12). Then, using nautical<br />
90° angle tables of celestial coordinates, he can find his<br />
latitude with a simple equation using the<br />
Horizon angle of altitude and the coordinates<br />
of the Sun in the celestial sphere (p.13).<br />
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