San Diego Astrolabe

san diego astrolabe
San Diego Astrolabe | @museumxst0ries

San Diego Astrolabe

Have you ever wondered how sailors in early times navigate their way through the sea?

Astrolabe comes from the Greek word astrolabos which means “to take a star”. It is a navigational instrument used to determine the ship’s latitude by measuring the angle between a star and the horizon. So how does it work?

First, the navigators suspend the instrument by a rope or hold it out at arm’s length using the ring at the top. Next, the pivoting pointer called alidade is oriented to precisely align the sun’s rays through the axis holes. When used at night, the navigators hold the astrolabe up to the eye, aligning the alidade so they could see the star through both holes. The alidade would indicate the altitude of the star through the wheel’s degree scale. The calculated angle would be compared to star charts and tables to determine the ship’s position in degrees of latitude.

The San Diego astrolabe is made of bronze and is relatively well preserved. It measures 18.3 centimeters in diameter, 1.7 centimeters thick at the top, and 1.8 centimeters thick at the bottom. Weighing 2.4 kilograms, it is heavy but sturdy which is efficient for use on moving vessels. There are no date or signature marks found in San Diego’s astrolabe making it hard to identify its geographic origin and manufacturing date. However, its strong similarities with other astrolabes suggest a Portuguese origin. One artifact it resembles is the Valencia astrolabe in the Greenwich Maritime Museum and the other is the Barlow astrolabe which was also found in the port of Manila.

When more practical navigational devices were developed, it is likely that most astrolabes were melted down so their valuable material could be reused for a variety of purposes. This may be the reason why only a few survive today. In fact, there are only 104 known historical astrolabes existing in the world. Due to its rarity and significance to the Philippines’ maritime heritage, San Diego’s astrolabe was declared a National Cultural Treasure.

It is currently displayed at the National Museum of Anthropology.

National Museum of the Philippines (Facebook Page)

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