Astronomy Answers
From the Astronomical Dictionary

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the solid angle

The surface area of a constellation is a solid angle, for example measured in square degrees or in steradians. The length, width, and circumference of a constellation are (plane) angles, for example measured in degrees or radians. A small part of the sky with a length and width of 1 degree has a solid angle (surface area) of about 1 square degree. All directions put together (both above and below the horizon) cover 129600/π = (about) 41253 square degrees, or 4π = (about) 12.566 steradians.