The coordinates in spherical geometry are angles. We can choose to represent these angles in a number of different ways. Degrees are probably the most familiar to you, but degrees can be written as decimals, or in sexagesimal notation. In sexagesimal notation, each degree can be broken into 60 minutes of arc, or arcminutes ('). Each minute in turn can be divided into sixty arcseconds ("). For more accuracy, fractions of seconds can be quoted using a decimal point.

For example, the location of the Robotic Observatory for Sheffield Astronomy (ROSA) is 53°22’50” N, 1°29’9” W. That can also written in decimal degrees as 53.3806° N, 1.486° W. To convert between decimal and sexagesimal notation, divide the minutes by 60, the seconds by 3600 and add the degrees, minutes and seconds together. The calculator below can do this conversion for you

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Because the Earth rotates, it is also possible to express longitude in units of time. The Earth rotates by 360° in about 24 hours. Hence, the Earth rotates 15° of longitude in 1 hour, 1° of longitude in 4 minutes, 1´ of longitude in 4 seconds and 1" of longitude in 1/15 second. The longitude of ROSA can therefore also be written in hours, minutes and seconds as 1°29’9” = 0h5m56.6s.

It is a bit confusing that both time and sexagesimal degrees use the terms minutes and seconds! In this course, I shall always refer to arcminutes and arcseconds when talking about sexagesimal degrees.