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(3) To find the meridian line by means of the pole star is a simple and accurate method, as a value can be obtained at any time at night by a simple altitude, provided the time of observation is known.[8]

If these means of directly determining the astronomical meridian line are not available, then we have to do it indirectly by using the magnetic meridian in the first instance.

If we take a magnetic needle and balance it horizontally on a vertical pivot, its ends will be directed to two points on the horizon. By drawing a great circle through these two points and the zenith point of the place, we obtain the magnetic meridian.

The magnetic meridian line is the intersection of the plane of the magnetic meridian with the plane of the horizon. The angle between the astronomical and magnetic meridian lines is called the variation, E. or W. according as the needle points to the W. or E. of true—that is, astronomical—north at any particular place at any particular time. The variation may vary from place to place, and always varies from time to time.