of the moon's longitude and the node of her orbit than any that could
be obtained from modern observations made with instruments of the
highest precision.

So again, Mr. Hind [3] was enabled to trace back the period during
which Halley's comet has been a member of the solar system, and to
identify it in the Chinese observations of comets as far back as 12
B.C. Cowell and Cromellin extended the date to 240 B.C. In the same
way the comet 1861.i. has been traced back in the Chinese records to
617 A.D. [4]

The theoretical views founded on Newton's great law of universal
gravitation led to the conclusion that the inclination of the earth's
equator to the plane of her orbit (the obliquity of the ecliptic) has
been diminishing slowly since prehistoric times; and this fact has
been confirmed by Egyptian and Chinese observations on the length of
the shadow of a vertical pillar, made thousands of years before the
Christian era, in summer and winter.

There are other reasons why we must be tolerant of the crude notions
of the ancients. The historian, wishing to give credit wherever it may
be due, is met by two difficulties. Firstly, only a few records of
very ancient astronomy are extant, and the authenticity of many of
these is open to doubt. Secondly, it is very difficult to divest
ourselves of present knowledge, and to appreciate the originality of
thought required to make the first beginnings.

With regard to the first point, we are generally dependent upon
histories written long after the events. The astronomy of Egyptians,
Babylonians, and Assyrians is known to us mainly through the Greek