Sky and Telescope - May 2018 - 7

needs the same protection in certain
circumstances.
David Stoltzmann
Prescott Valley, Arizona

Jerry Oltion replies: You're correct

that the corneal illumination can
exceed retinal illumination under certain circumstances, but after doing some math and
conﬁrming it with another optical engineer
I've concluded that the danger is minimal
under any normal viewing scenario.
The intensity of light per unit area at
the exit pupil is completely independent of
aperture - it depends solely upon magniﬁcation. As aperture increases, the irradiated
exit pupil grows in diameter, but the intensity
of the light per unit of area doesn't change;
it changes only with magniﬁcation.
Solar ﬁlter material designed for telescopic use typically has an optical density of
5, which means it reduces the light throughput by 10-5, or 1/100,000. For a telescope to
concentrate enough light to counter the
attenuation of a solar ﬁlter, its magniﬁcation

must concentrate the light by a factor of 105.
This 100,000-fold intensity increase is
simply the magniﬁcation squared, which
means a magniﬁcation of 316×. So it's probably a good rule of thumb to not look at the
Sun telescopically at greater than 300×.

Seeing Mercury in Daylight
In his article about observing the poles
of Mercury (S&T: Jan. 2018, p. 52),
William Sheehan states that Giovanni
Schiaparelli observed Mercury "in broad
daylight." How was that done?
Dick Marti
Tifton, Georgia

William Sheehan replies: Be-

fore Schiaparelli, observers were
conﬁned to viewing Mercury in twilight -
through the thickest and most turbulent part
of Earth's atmosphere. Schiaparelli realized
the seeing might be better when Mercury

1968

1993

FOR THE RECORD

* The events listed in the table for the total
lunar eclipse (S&T: Jan. 2018, p. 49) are
given in standard (not daylight) time.

* In the caption below the Periodic Table
of Cosmic Origins (S&T: Feb. 2018, p. 37),
promethium should be identified as Pm
rather than Pc.

* The discussion of returning a 2006
balloon-borne experiment to Earth (S&T:
Feb. 2018, p. 19) should note that engineers had to send a second radio signal to
separate the parachute from the payload
(not the balloon) once it had landed.

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75, 50 & 25 YEARS AGO by Roger W. Sinnott
1943

was high in the daytime sky. Of course, it's
impossible to see the planet then with the
naked eye. However, because his telescope
had setting circles and a clock drive, Schiaparelli could point it to the right place in the
sky and study Mercury for hours at a time.

º May 1943
Oregon Meteorite "'I was coming
home," said Mr. Hughes, 'from
where I had been cutting wood for
the Willamette school. I noticed this
big rock for the first time but didn't
think anything of it. . . . I sat down
on the rock which was very flat and
about 18 inches above ground. . . .
Then I picked up a white stone and
started to hammer on the rock. It
rang like a bell. . . .'
"After months of urging by his
wife, Mr. Hughes went to work on a
crude truck with which to move the
huge mass to a spot near his house
three quarters of a mile away. He
trusted no one to help him aside
from his wife, his 15-year-old boy,
and his horse. He made the truck
of logs and mounted it on ungainly
wheels sawed from a tree trunk."
The 16-ton Willamette meteorite
found in 1902 by Welsh woodsman
Ellis Hughes attracted people from
far and wide, each paying 25 cents
for a look. J. Hugh Pruett goes on

to tell how Hodges lost a court
case regarding ownership of the
massive ﬁnd - and how it ended
up at the American Museum of
Natural History in New York City.
º May 1968
Relativity Test "Irwin I. Shapiro
and his colleagues at Massachusetts Institute of Technology's Lincoln Laboratory [have] announced
promising results from a novel
radar astronomy test. . . . Einstein's
general relativity theory says that
the speed of a light wave (or radio
wave) depends on the strength of
the gravitational field along its path.
On this basis, when a radar pulse
is sent past the sun to Mercury
or Venus at superior conjunction,
the round-trip delay should be
increased by about 0.0002 second.
"[Shapiro reports that the]
anticipated 'excess delay' was
confirmed to within the limits of
observational error, or about 20
percent. Further measurements
. . . may reduce this uncertainty to
five percent."

º May 1993
Opposites Merge "Many amateur
astronomers know the spiral galaxy
M64 in Coma Berenices as the
Black-eye galaxy because of a dark
lane of dust . . . that arches over its
bright nucleus. However, what has
intrigued Robert Braun (Netherlands Foundation for Research in
Astronomy) and his colleagues . . .
is the surprising presence of two
independent disks of gas rotating in opposite directions, the first
such system to be discovered. . . .
"[Braun and his team suggest]
that the unique configuration of
M64 resulted from the merger of
two counterrotating galaxies and
that the odd behavior will end in
about a billion years. By that time
all the gas in both disks will have
fallen toward the nucleus and
fueled star formation, though the
resulting stars may 'remember' the
counterrotation of the two disks."
Some other galaxies show
counterrotating populations of
stars, or of stars vs. gas. The
causes are far from settled.
s k y a n d t e l e s c o p e . c o m * M AY 2 0 1 8

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Sky and Telescope - May 2018

Table of Contents for the Digital Edition of Sky and Telescope - May 2018