modern writers on gnomonics was SEBASTIAN
MUeNSTER (q.v.), who published his _Horologiographia_ at Basel in 1531.
He gives a number of correct rules, but without demonstrations. Among
his inventions was a moon-dial,[1] but this does not admit of much
accuracy.

During the 17th century dialling was discussed at great length by many
writers on astronomy. Clavius devotes a quarto volume of 800 pages
entirely to the subject. This was published in 1612, and may be
considered to contain all that was known at that time.

In the 18th century clocks and watches began to supersede sun-dials, and
these have gradually fallen into disuse except as an additional ornament
to a garden, or in remote country districts where the old dial on the
church tower still serves as an occasional check on the modern clock by
its side. The art of constructing dials may now be looked upon as little
more than a mathematical recreation.

_General Principles._--The diurnal and the annual motions of the earth
are the elementary astronomical facts on which dialling is founded.
That the earth turns upon its axis uniformly from west to east in
twenty-four hours, and that it is carried round the sun in one year at
a nearly uniform rate, is the correct way of expressing these facts.
But the effect will be precisely the same, and it will suit our
purpose better, and make our explanations easier, if we adopt the
ideas of the ancients, of which our senses furnish apparent
confirmation, and assume the earth to be fixed. Then, the sun and
stars revolve round the earth's axis uniformly from east to west once
a day--the sun lagging a little behind the stars, making its day some
four minutes longer--so that at the end of the year it finds itself
again in the same place, having made a complete revolution of the
heavens relatively to the stars from west to east.

The fixed axis about which all these bodies revolve daily is a line
through the earth's centre; but the radius of the earth is so small,
compared with the enormous distance of the sun, that, if we draw a
parallel axis through any point of the earth's surface, we may safely
look on that as being the axis of the celestial motions. The error in
the case of the sun would not, at its maximum, that is, at 6 A.M. and
6 P.M., exceed half a second of time, and at noon would vanish. An
axis so drawn is in the plane of the meridian, and points to the pole,
its elevation