A total eclipse of the Sun provides an opportunity, rare though it may be, of obtaining an instantaneous fix from the Sun alone. Eclipses vary greatly in character, in position on the Earth, in the width of the path of totality, in the duration, and also in the direction of the path. However, the shadow of the Moon cast by the Sun is always a right circular cone which, in the case of a total eclipse, intersects the Earth's surface at some point before its vertex. Owing to the motion of the Moon in its orbit round the Earth the shadow moves at a speed of about 2000 m.p.h. from west to east (it varies considerably according to the distance of the Moon from the Earth).