To locate a point on the Earth's surface, we use latitude and longitude. Latitude is measured, in degrees, along a line from the Earth's equator towards the North Pole or the South Pole. Longitude is measured, in degrees, along a line of constant latitude, East or West of the Prime Meridian. The Prime Meridian is a line of constant longitude that passes through Greenwich, England.
Using this scheme, the location of Providence is:
Latitude: N 41 49 26 (deg, min, sec)
Longitude: W 71 24 47 (deg, min, sec)
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To locate a celestial object, we introduce the idea of a celestial sphere. This is an imaginary sphere, with an infinite radius, on which we imagine that the stars and other celestrial objects are placed.
Where the extension of the Earth's axis of rotation outward from the Earth's North Pole meets the celestial sphere is known as the North Celestial Pole (NCP). Where the extension of the Earth's axis of rotation outward from the Earth's South Pole meets the celestial sphere is known as the South Celestial Pole (SCP). The extension of the Earth's equator outward to the celestial sphere is known as the Celestial Equator.
The angular distance along a line from the celestial equator to either the NCP or the SCP is called declination (DEC) and is measured in degrees, minutes, and seconds.
Since it appears to us that the sky rotates once in 24 hours, distance along the celestial equator is measured in hours, minutes, and seconds. This is called Right Ascension (RA). Note that 24 hours equals 360 degrees, and therefore that 1 hour of RA equals 15 degrees. It follows that 1 degree of arc equals 4 minutes of RA and that 15 minutes of arc equals 1 minute of RA.
Using this scheme, the location of the star Aldebaran is:
RA 4 h 36 m
DEC 16 d 31 m