Astrolabe: A navigational instrument


The mariner’s astrolabe was a navigational instrument used to determine the sun’s or stars’ height at sea. Its history extends back to the Ancient Period and continues through the Middle Ages. Its name translates to “to take a star” or “star thief.” More than 200 years ago, the astrolabe was the chosen instrument. During their voyages across the oceans, sailors like Columbus and Magellan relied on this equipment.

History of Astrolabe

The astrolabe was an essential navigational instrument for determining latitude. It is a simplified version of the old astrolabe, which could be used to tell time, determine height, and determine space. The mariner’s astrolabe measures the elevation of the sun or a star over the horizon. An observer can determine their latitude using star and planet charts and tables. The history of astrolabes began in ancient Greece. Hipparchus, an ancient Greek astronomer and mathematician, is credited by many experts with inventing the astrolabe.

Claudius Ptolemy, an ancient astronomer, was the first significant author to describe and construct astrolabes. His Planisphaerium publications provided extensive information regarding the instrument’s first applications. Even though ancient Greek astronomers created a form of the astrolabe, by the 9th century, the instrument was highly refined and widely employed in the Islamic world.

The astrolabe was of great value to the Islamic faith. It assisted in determining the astronomically determined prayer hours and in locating Mecca, the holiest site in Islam. Initially, the astrolabe was utilised for land travel, astronomy (the study of the stars), and timekeeping, but not necessarily for water navigation.

In the early 12th century, Islamic Spain (al-Andalus) brought the astrolabe to Europe. As previously stated, early astrolabes were mainly utilised for terrestrial travel. Europeans began to travel greater distances over oceans and seas, so they invented a marine variant of the astrolabe. Thus, the astrolabe for seafarers was formed. It became prevalent in Europe throughout the late Middle Ages and Renaissance, reaching its zenith in the fifteenth and sixteenth centuries. Astronomical knowledge was deemed essential to schooling.

According to current knowledge, approximately one-third of all known astrolabes were produced in Portugal between the 16th and 17th centuries. The astrolabe was regarded as one of the most fundamental astronomy instruction instruments for centuries. Brass and wood were used to construct the first instruments. Astrolabes utilised by mariners would generally be composed of brass or iron. This rendered them weighty but durable. Better for use on moving vessels, allowing for more precise latitude measurements.

Some were as little as 4 inches in diameter, while others were as huge as 24 inches in diameter. In the early days of navigation, sailors could not determine longitude but could determine latitude. Navigators could then locate the latitude line and sail east or west along it to reach their destination. For this purpose, the mariner’s astrolabe proved extremely valuable.

Despite being a helpful instrument, the astrolabe had its flaws. It was not always a precise instrument at sea due to the difficulty of keeping it stable on a rocking ship, especially in strong gusts. This might lead to degree mistakes that could cause a ship to veer off course. Until the end of the seventeenth century, the mariner’s astrolabe was the most widely used astronomical tool. Devices supplanted it with more precision, such as quadrants and sextants.

How Astrolabe Operates

The astrolabe determines the angle between a star and the horizon. Typically, sailors would use the sun during the day and Polaris (the north star) at night to determine the angle. The astrolabe of a mariner is comprised of several essential components. The navigator would use the ring at the top to suspend or dangle the astrolabe. The alidade was equipped with two tiny openings for seeing the sun or a star. And the wheel with a degree scale showing. So how does it work?

During the day, measurements would be obtained using the midday sun. In order to accurately calculate the angle, the astrolabe must hang perpendicular to the water. Next, the navigator aligns the two holes using the alidade such that the sun’s rays pass through both holes. The navigator then reads the measured angle from the circumferential scale of the astrolabe. On the degree scale, the alidade would show the sun’s height.

This angle would be compared to star charts and tables, and height could be utilised to calculate latitude. Nighttime measurements were conducted differently. The gadget was brought to the eye to see the stars. The user would align the alidade’s pinholes so they could observe the star via both holes. On the degree scale, alidade would denote the height. This angle would be compared to star charts and tables, and height could be utilised to calculate latitude.

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