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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 />

15

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