Sky and Telescope - November 2017 - 31

FACING PAG E: ENG R AVING: AU THOR'S COLLECTION; M OON: STELL A RIU M
THIS PAGE: PHILOSOPHICAL TR ANSACTIONS OF THE ROYA L SOCIE T Y (1794, P. 440)

n the evening of March 7, 1794, William Wilkins
took a stroll up Castle Hill in Norwich, England,
to see if he could spot Mercury in the evening sky.
Wouldn't you know it! Cloudy in the west. Then he looked up.
What he saw was a shock and surprise, one that stoked a
mystery that has sputtered on for more than two centuries,
helping to fuel tantalizing speculations about active events
on our supposedly dead Moon.
Wilkins, 44, was an amateur astronomer. He had been
inspired by William Herschel's discovery of Uranus - the
ﬁrst new planet since antiquity, putting astronomy in the
headlines (Britain triumphs over French astronomers!) -
just 13 years earlier. Like Herschel, Wilkins owned a reﬂecting telescope rather than the more common refractor. But
unlike the famous ex-musician, Wilkins hadn't quit his day
job. He was an architect, busy constructing a lighthouse with
Argand lamps (oil lamps with big cylindrical wicks, the latest
in high-intensity lighting). During the course of this work,
he wrote "the goodness of my sight has often been remarked,
in discovering vessels with the naked eye, which my companions could not discover but with the telescope." So he
described himself in a letter to the astronomy professor Rev.
Samuel Vince, lest there be any doubt about what followed:

Enter the Astronomer Royal
Wilkins's letter went straight to the top. It appeared in
the Royal Society's Philosophical Transactions as part of an
investigation by England's Astronomer Royal, Nevil Maskelyne, titled "An Account of an Appearance of Light, like a
Star, Seen in the dark Part of the Moon, on Friday the 7th
of March, 1794." Here Maskelyne writes that soon after he
learned about Wilkins, his own relation Sir George Booth
and his wife paid him a visit at the Royal Observatory
grounds in Greenwich. Lady Booth remarked that their servant, "who is curious for a person in his situation, and fond
of looking at the stars, had some time before seen something
extraordinary in the moon." When questioned by Maskelyne
himself, the servant recalled that "some time ago, about six
in the evening, he saw a light like a star, and as large as a
middle sized star, but not so bright, in the dark part of the
moon. He continued looking at it for a minute or more, dur-

. . . respecting the phænomenon I saw in the moon, on Friday
the 7th of March, a few minutes before eight o'clock . . . I had
been looking for that planet [Mercury] from the Castle-hill in
Norwich, but was disappointed by a clouded horizon. I mention
this merely as the reason of my being led to a more particular
notice of the moon . . . having lost the ﬁrst object of the evening's
attention. When I saw the light speck, as shewn in the sketch [at
right], I was very much surprised; for, at the instant of discovery
I believed a star was passing over the moon, which on the next
moment's consideration I knew to be impossible . . . I was, as
it were, rivetted to the spot . . . and took every method I could
imagine to convince myself that it was not an error of sight; and
two persons, strangers, passed me at the same time, whom I
requested to look, and they (may be, a little more ignorant than
myself) said it was a star. I am conﬁdent I saw it ﬁve minutes
at least . . . The whole time I saw it, it was a FIXED, STEADY
LIGHT, except the moment before it disappeared, when its
brightness INCREASED; but that appearance was instantaneous
... I mentioned this soon afterwards to a gentleman of my
acquaintance, who . . . conjectured this phænomenon to be some
great volcanic convulsion in the moon, which induced me at that
time to assist my memory with a sketch like what I have here
sent you. I shall be obliged to you, Sir, if you will favour me with
Dr. MASKELYNE'S opinion . . . .
t OBSERVING SITE Castle Hill in Norwich, England, as drawn in 1785, nine years before William Wilkins's fateful sighting (negative image).
Moon and Aldebaran: Thanks to modern software, we can re-create the Moon's appearance from Norwich (with Aldebaran shown greatly exaggerated) at 6:57 p.m. local mean time March 7, 1794, as Aldebaran was just about to vanish from view. What's the chance that Wilkins really made his
sighting one hour later? Above: Nevil Maskelyne published two people's drawings of their best recollection of where on the Moon they witnessed
the naked-eye point. Wilkins, an amateur astronomer, was particularly careful; Fig. 1 is from a sketch in which he attempted to specify the position
of the "star" on the Moon by noting that the line BA = AD = DC. Fig. 2 is what Wilkins drew for Maskelyne, "copying the moon's appearance from
your scheme." Fig. 3 is the same by another witness: Lady Booth's unnamed servant, the one "curious for a person in his situation."
s k y a n d t e l e s c o p e . c o m * N O V E M B E R 2 0 17

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