Sky and Telescope - May 2015 - 51

Learn how to use detailed star charts to ﬁnd the faintest things
with your telescope: skypub.com/charts.

whereas a polished crystal ball would show a small,
bright specular reﬂection of the Sun. He correctly
attributed slight irregularities on the Moon's terminator
to mountains casting long shadows. As an analogy, he
described how Mount Athos in northern Greece casts
a shadow near sunset that extends 50 miles across the
Aegean Sea to the island of Lemnos. Plutarch may have
observed the shadow of the lunar Apennines, the 19thcentury lunar astronomer J. H. Mädler suggested. The
Apennines brieﬂy produce a deﬁnite naked-eye irregularity in the terminator around ﬁrst and last quarter.
Then in the 11th century, the Arab astronomer alHaitham, better known as Alhazen, argued that because
the Moon's markings never changed shape, position, or
size, they were permanent areas of diﬀerent materials.
He wrote that the darker ones should be denser rock. But
Plutarch and Alhazen were almost completely ignored
despite their renown for their other writings. Not until
the century before Galileo did the idea of the Moon having actual blemishes start gaining traction. (See "Lunar
Studies Before the Invention of the Telescope" by Joseph
Ashbrook, S&T: June 1962, p. 322.)
Meanwhile, anyone with eyes could see that the Man
in the Moon never changed, regardless of the weather or
the Moon's position in the sky.

Visual Tests
What's the smallest lunar feature you can see with the
naked eye? Look carefully, and you may be amazed.
About a century ago William H. Pickering, the
brother of Harvard Observatory director Edward C.
Pickering, drew up a list of 12 test features in order of
increasing diﬃculty. They're marked on the photo at left.
The large gray maria are easy for anyone with fairly
good (or corrected) vision. The easiest feature in Pickering's list is the bright splash of rays surrounding the
crater Copernicus. As Ashbrook wrote in his 1962 article,
Reasonably good eyes should be able to see No. 7, Mare
Vaporum, as a very dark spot, while really good vision can
reach No. 10, the faintly shaded area near Sacrobosco.
Persons with exceptional visual acuity might distinguish the
11th feature, a dark patch at the edge of Mare Imbrium, just
across the Apennine Mountains from Mare Vaporum. But
Pickering thought that his No. 12, the Riphaeus Mountains,
might be beyond even the keenest naked-eye vision.
Careful experiments of this kind reveal a surprising amount of
lunar surface pattern. The best results, I have found, are obtained by viewing the waning gibbous moon during the latter
part of morning twilight. Much less is visible by night, when
glare hampers, or in full daylight, when contrasts are diluted.

Alan MacRobert

The waxing gibbous Moon early in evening twilight
should be equally favorable, though the air then is less
often crystal clear.
It also matters whether the Moon is near perigee or
apogee. The size diﬀerence between a "supermoon" and
a "mini-moon," as they've recently come to be called, is
not obvious to the naked eye. But for detecting limit-ofvision details, I've found that features that are normally
borderline deﬁnitely become easier around perigee.
What else in our everyday lives, we may wonder, has
gone unnoticed for centuries because we assume there's
no reason to look at it critically?

No Alpha Comae Eclipse!
It was an epic goof. The 4th-magnitude visual
double star Alpha Comae Berenices, with a 26-year
period and an orbit almost exactly edge-on to us,
was supposed to undergo its ﬁrst observed selfeclipse around January 23rd, as told in the January
S&T, page 50.
But just days beforehand, as photometrists
around the world monitored the star, lead investigator Matthew Muterspaugh announced that his
prediction was mistaken. The eclipse had already
probably happened a couple months earlier, if it
happened at all.
The mistake wasn't exactly his. His calculated
orbit was based on 609 measurements of the binary
pair that astronomers had made across more than
a century. For three of these measurements, made
in 1896, 1911, and 1937, the observers accidentally
swapped the primary and secondary components
of the pair, which are nearly the same brightness.
These were enough to throw oﬀ the timing for the
predicted eclipse. See the full story at arxiv.org/
pdf/1501.05639v1.
If there's an upside to the story, it's that the corrected orbit opens a slim chance that Earth will see
a secondary eclipse of the pair: around January 11,
2026. The next primary eclipse should come in late
September 2040, when Alpha Comae will be unobservably close to the Sun. So mark your calendar for
the next one, in July 2066.

MAY ME TE O R S
The waning gibbous Moon on the morning of May
6th will make this year's Eta Aquariid meteor shower
a challenge to observe, even from the southern latitudes where its radiant is highest before dawn and
the most meteors therefore appear in the sky.

Sk yandTelescope.com May 2015

http://www.skypub.com/charts http://www.arxiv.org/pdf/1501.05639v1 http://www.arxiv.org/pdf/1501.05639v1 http://www.SkyandTelescope.com

Sky and Telescope - May 2015

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