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An instrument used to measure the angle between two visible objects. It is most commonly used at sea for navigation, to measure the angular distance between a celestial object and the horizon.

Also known as the North Star, it is nearly aligned with the Earth's axis of rotation. This means it appears almost stationary in the sky, making it an excellent fixed point for navigation in the Northern Hemisphere.

The angular measurement in a spherical coordinate system. In navigation, it is the direction of a celestial body from the observer, typically measured in degrees from true north.

The great circle on the celestial sphere whose plane is perpendicular to the zenith-nadir line. It's used as a reference for defining the altitude of celestial bodies.

The mean solar time at the Prime Meridian, which is 0 degrees longitude, located at Greenwich, England. It's used as a reference time for all points on Earth.

An annual publication that provides daily data on the positions of celestial bodies, useful for celestial navigation.

A method of estimating one's current position by using a previously determined position, and advancing that position based upon known or estimated speeds over elapsed time, and course.

An imaginary sphere of arbitrarily large radius, concentric with the observer. All celestial bodies can be thought of as projected upon the inner surface of the sphere, which means their positions can be defined by coordinates on the sphere.

The point in the sky directly above an observer on the Earth. It is the highest point on the celestial sphere.

A line on which an observer is known to be positioned, usually determined from the observation of a celestial body.

An imaginary line on the Earth's surface running from the North Pole to the South Pole, cutting through a given position of the observer. In celestial navigation, it's used for timing the transit of stars to estimate longitude.

The angle between an object in the sky and the observer's local horizon. In celestial navigation, it is typically measured using a sextant.

A highly accurate clock used by sailors to determine longitude by means of celestial navigation.

The shortest path between two points on the surface of a sphere. In navigation, it's the most economical path between two points on Earth.