The starry sky gives the illusion of being a huge sphere, with us as the observers at the center of the sphere. Consequently, any two celestial bodies in the sky seem to connected by a segment of a circle, with an angle representing the distance between them. It is thus common for astronomers to measure angular separations or distances between celestial bodies and angular sizes of the celestial bodies themselves.
Angular sizes can vary greatly depending on the object.
Constellations can be as large as 30 or 40 degrees and planets can be as small as a handful of arcseconds. By measuring angular sizes, astronomers have a convenient and useful system of comparing the apparent sizes of objects in the sky. It may not be reproduced in any form without permission . Or send e-mail to Sky Publishing .