The Caldwell Objects

tube's front end. This meant that the observer's
head would not block incoming light. A "slider"
with an adjustable base was located at the lower
end of the tube, where it pointed directly at the
center of the great mirror. It carried a brass tube,
into which eyepieces or micrometers were
inserted.
Due to its massive size, Herschel seldom used the
great telescope, even though it was his most
celebrated instrument (and, at the time, the
world's largest). The telescope required much
time to ready for use and two workmen to move.
Because of its weight, the mirror had to be left
inside the telescope, so it tarnished quickly. (By
contrast, Herschel's two 18¾-inch mirrors
remained highly polished, as they were stored in
protective boxes and placed in their telescope
tube only when observations were about to be
made.) Herschel used the 40-foot reflector
sparingly and wrote that it should be used only
to examine objects that other instruments could
not reach.
Interestingly, Herschel never reported seeing
spiral structure in any "nebula" through the 40foot
telescope, even though such structure
should have been obvious in some galaxies with
so large an instrument. Equally strange is that
Herschel did not distinctly mention the fifth or
sixth stars of the Trapezium in M42, an object
that he observed repeatedly. On one occasion the
gigantic telescope even failed to show Saturn's
rings. Obviously it gave poor images, even with
its second mirror.
THE MOON AND PLANETS
Like many of his contemporaries, Herschel
observed the Moon and, among other tasks, he
carefully measured shadows cast upon its surface
by its mountains. He deduced a lunar diameter
of 2,180 miles, surprisingly close to today's more
accurate figure (2,160 miles), but he also made
some very controversial lunar
observations. On May 28, 1776, he used a 10-foot
reflector at 240x to view what he believed were
forests in Mare Humorum, complete with
shadows cast by trees at the edge of the woods.
The next night he could not see any "woods" and
seemed to have dropped the idea completely.
However, in a 1780 letter to Maskelyne he wrote
of his absolute certainty of the Moon being
inhabited, and if given the chance, Herschel
vowed,
I would not hesitate a moment to fix upon the Moon
for my habitation. What a glorious view of the
heavens from the Moon. Do not all the elements seem
at war when we compare the Earth with the Moon?
Air, water, fire, clouds, tempests, volcanoes; all these
are either not on the Moon or at least in much greater
subjection than here.
In 1783 and again in 1787, Herschel reported
a volcano in the Earthlit part of the new Moon, in
the crater Aristarchus, stating that it resembled a
red 4th-magnitude star. However, other
astronomers in Paris thought that the Aristarchus
region, which was relatively bright, might well
have produced this volcano-like apparition by
reflecting earthshine. Herschel may have been
persuaded of this also, as the whole issue was
quietly dropped.
Herschel's observations of Mars include
many references to "luminous spots" (ice caps)
that appeared to project beyond the edge of the
planet's disk. He deduced that the polar spots
were frozen, covered with mountains of ice and
snow that melted only partway when alternately
exposed to the Sun. He also noted dark spots on
the surface and determined a rotation period of
24 h 37 m 26.3 s — close to recent values. And he
was aware that Mars had an atmosphere, noting
an observation by Cassini that "a star in the water
of Aquarius, at the distance of six minutes from
the disk of
Appendix C 459