Sky and Telescope - November 2017 - 33

mystery star so carefully noted by Wilkins!
And its sudden winking out, after shining
steadily for at least 5 minutes, would of course
have been the occultation itself.
Nevertheless, Maskelyne ruled out any
connection between the Aldebaran occultation and the sighting. He concluded,
I shall make no conjectures on the cause to
which this extraordinary phænomenon may be
attributed; but only remark, that it is probably
of the same nature with that of the light seen of late
years in the dark part of the moon by our ingenious and
indefatigable astronomer, Dr. HERSCHEL, with his powerful
telescopes, and formerly by the celebrated DOMINIC CASSINI;
although this has been so illustrious as to have been visible to
the naked eye, and probably equal in appearance to a star of the
third magnitude.

Those Disreputable TLPs!
Scientists in the late 18th century generally lacked the
sophisticated understanding of statistics that any good
scientist draws upon today. The statistician Thomas Bayes
had introduced Bayes Theorem two decades earlier; it is the
formal way to determine, among other things, what is "too
unlikely a coincidence." But even an astronomer as illustrious
as Maskelyne may not have considered his "priors." What was
the prior likelihood that, right at the time of such a remarkable and seemingly impossible sighting - if the witnesses
were wrong and the time was a few minutes before 7 instead
of 8 or 6 - Aldebaran would have been right there along with
the mystery "star"? What was the chance of such a coincidence given the rarity of bright-star occultations, much less
one at just the correct place on the Moon's limb?
And yet, Wilkins says

WIKIMEDIA COMMONS / MAGNUS MANSKE

I was very particular in my inquiries respecting the time, and
called purposely on a neighbour [a Mr. R. Bacon, publisher of
one of the Norwich newspapers] to ascertain it with certainty;
and found it a few minutes before eight o'clock, which I entered in
my pocket journal; and on inquiry of Mrs. WILKINS, she says I
left home at that time.
Castle Hill was just 50 yards from his home, so he would
have wasted little time getting there and back.
At 1st magnitude, Aldebaran is six times brighter than the
3rd-magnitude light that Maskelyne estimated for the lunar
event. The glare of the Moon makes a star close to it look
dimmer than it really is. But it's suspicious that none of the
eyewitnesses seems to have mentioned a second star at the
Moon that was at least as bright and eye-catching.
The dark limb would have been invisible to the naked eye;
the Moon was a day short of ﬁrst quarter (41% illuminated).
So where exactly was the line between on the Moon and off
the Moon? If the Moon's brilliance made it appear a little

t NEVIL MASKELYNE (1732-1811) was England's fifth
Astronomer Royal, holding that illustrious office from
1765 to 1811.

larger than reality to the naked eye, as often
happens, might one misjudge that Aldebaran
was inside the dark limb, rather than on it?
Quite possibly. In 1860 the famed observer
Rev. T. W. Webb published a note in the Monthly
Notices of the Royal Astronomical Society
describing this effect and saying that it accounted
for the famed Wilkins lunar volcano:
On the same evening there was an occultation of Aldebaran,
which Dr. Maskelyne thought a singular coincidence, but
which would now be acknowledged as the cause of [Wilkins's]
phenomenon. . . . [T]he effect of irradiation [expansion of
glare] upon an object out of focus is greater than might be
supposed by those whose vision is perfect; of this I have been
made aware through my own near sight, in consequence
of which luminous spaces are enlarged at the expense of
adjacent dark ones, to an extent which might hardly have been
anticipated.

The term Transient Lunar Phenomenon (TLP) would not be
invented for many years to come. Most reports of them refer
to particular lunar features appearing unusually bright or
hazy in a telescope. Their reality has always been questioned,
and lunar astronomers today take essentially all such reports
to be misobservations of features under changes of lighting
(see the August issue, page 52). But the decade leading up to
Wilkins's sighting was a heyday for TLP sightings, which were
sometimes described outright as active lunar volcanoes. By
1794 astronomers were primed for them.
The TLP controversy, long considered nearly dead, gained a
new gasp of life with a 2009 paper in The Astrophysical Journal
titled "Transient Lunar Phenomena: Regularity and Reality"
by Arlin Crotts. He wrote that judging from a 1968 NASA
catalog of 579 TLP reports since 1540, statistics suggest that
80% were real, since 50% were seen near Aristarchus (the
Moon's brightest white spot) and approximately 16% in Plato.
Nowadays, low-light video monitoring through telescopes
occasionally catches actual, brief pinpoint ﬂashes on the
Moon's night side. These are small meteoroid strikes, and
they appear at rates consistent with Earth's known meteoroid environment. Larger ones must also happen from time
to time. But these ﬂashes of white-hot vapor are gone in a
moment in the lunar vacuum. The star that Wilkins saw
shone unchanged for minutes - before winking away into
more than two centuries of astronomical lore and legend.
¢ ANDREW LIVINGSTON sees TLPs - Toronto's light-polluted
skies - all the time. Alan MacRobert, Don Olson, David Dunham, David Herald, Leslie Morrison, and Tony Cook contributed to the Aldebaran investigation.
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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Sky and Telescope - November 2017

Table of Contents for the Digital Edition of Sky and Telescope - November 2017