The Mariner's Astrolabe was used to determine the latitude of a ship at sea by measuring the noon altitude of the Sun or the meridian altitude of a star of known declination.

It was not possible to determine longitude at sea in the early days of transoceanic navigation, but it was quite easy to determine latitude. To go to a place of known latitude, the ship was sailed to that latitude and then sailed east or west along the latitude line until the place was reached.

To find the latitude of the ship at sea, the noon altitude of the Sun was measured during the day or the altitude of a star of known declination was measured when it was on the meridian (due north or south) at night. The Sun's or star's declination for the date was looked up in an almanac.

The latitude is then 90° - measured altitude + declination.

A number of devices were used to measure the Sun's noon altitude. Among them were the quadrant, cross staff and, later, the back staff and the mariner's astrolabe. All these devices had a single use; to measure the altitude of a celestial body above the horizon.

The Mariner's Astrolabe, which was popular in the late 15th and early 16th centuries, was a simple brass ring, graduated in degrees with a rotating alidade for sighting the Sun or a star. The ring was cast brass, quite heavy and cut away to keep it from blowing around in the wind. It was not a very good instrument and errors of four or five degrees were common.

Related Images

Early navigational instrument for determining the altitude (declination or angular height over the horizon)) of a celestial object

Image ID: 2101

Record ID: 367