Angular Measurement

In order to express distances between points on the celestial sphere, such as the length of a constellation or the distance between two stars, astronomers use a system known as angular measurement. Basically, angular measurement expresses the angle that would be subtended at the eye by two lines, each of which is lined up with one of the two points in the sky.

hand_2If you hold your outstretched hand at arms length, the angle between the tips of your little finger and thumb would be around 20° and that across the end of a finger around 1° as shown in the diagram.

A more star-orientated example is the angle subtended by Merak and Dubhe, the two stars at the end of the Plough, which is about 5°, as shown here. In other words, we say that Merak and Dubhe are 5° apart. The same diagram shows the total length of the Plough to be around 25°, and the distance between the Pointers and Polaris, the Pole Star, as 30°. Smaller distances (not illustrated here) Angular_Measurement_3include the apparent diameters of the Sun and Moon as seen from Earth, each of which is around ½°.  In other words, you could fit ten full moons between the two pointers in the Plough.

Other examples of distances expressed in this way include that between the horizon and the zenith, or overhead point, which is 90°, (a right angle); that from one point on the horizon, through the zenith and down to the directly opposite point on the horizon, which is 180°; and the altitude (height above the northern horizon) of the Pole Star as seen from London, which is around 52°.

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