In anticipation of the TOTAL Solar Eclipse on 11 August 1999 (see the January/February
issue of Navigation News), it seemed very appropriate to repeat this short article by one of
the Institute's most respected Fellows. It was first published in Vol. VII, October 1954.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). The intersection of this cone with the Earth's surface is an ellipse, which moves
over the surface at speeds which are very high when the cone is nearly tangential (i.e. when the
Sun's altitude is low) and at speeds as low as about 1000 m.p.h., when the eclipse is central over
the equator at noon and the Earth's rotation has its maximum effect. The speed of the shadow
is generally low enough to give a position line of considerable accuracy from the observed time
of either second or third contacts, that is the beginning or ending of the total phase. An error
of 1 second corresponds, in the most favourable case, to about one-third of a mile. The position
line is, of course, the portion of the elliptic shadow corresponding to the observed phase and
time; these can be precomputed.
XXIX.—On the Red Prominences seen during Total Eclipses of the Sun. Part I