An astrolabe illustrates how the sky looks at any given time of day and throughout the year. This is accomplished by drawing the sky (star patterns) on the astrolabe with moveable parts to adjust for the date and time. The traditional sky map astrolabe allowed people to locate celestial objects in the sky similar to how a place is located using a map. A mariner’s astrolabe is different in that it was used to measure the height or altitude of the Sun or of a star above the horizon. The altitude of a star is the angular height above the horizontal from 0 to 90°. Mariners needed to measure the altitude of the Sun or stars in the sky in order to determine their position. Knowing the altitude of the Sun at noon or of the pole star at night told them their latitude (their distance, north or south, from the equator.

The azimuth of a star is its angular position from north, measured clockwise. For example a star located directly to the north of Earth has an azimuth of 0°, while a star directly to the east has an azimuth of 90° and a star to the northwest has an azimuth of 315°.

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