**Angle: **The distance between two lines from where they meet along an arc

**Angular Distance: **The (arc) distance between two points on the surface of a sphere

**Arcseconds: **A unit of distance, not time, that refers to 1/3600th of 1 degree of a circle used to denote a very small angle

Arcminute (‘) – 1/60th of 1 degree

Arcsecond (“) – 1/3600th of 1 degree

Arcmillisecond – 1/3,600,000 of 1 degree

**Celestial Sphere: An abstract sphere with a large radius that is centered on the Earth. All objects in the sky can be seen as projected onto the surface of the sphere, which may be centered on Earth/observer. **

**Circumference: **The distance around the edge of the circle

**Circumpolar: **A celestial feature (e.g. star) that remains above the horizon at all times in a given latitude because of their proximity to the celestial pole

**Degree (****°)****: **A measurement unit for angles that describes 1/360th of the circumference of a circle

**Diameter: **Length of a straight line that passes through the center of the circle

**Proper Motion ( μ): **Components of right ascension (

*μ*

*) and declination (*

_{α}*μ*). Their combined value is total proper motion (

_{δ}*μ*). Contains a position angle and a magnitude. Position angle refers to direction of motion on the celestial sphere (0 degrees = north, 90 = east). Magnitude is expressed in arcseconds per year

**Radian (rad****)****: **A measurement unit for angles that describes a arc with a length equal to the radius of the circle (180/π degrees = ~ 57.3°)

**Radius****: **Length of a straight line segment between the center and the perimeter of the circle