Sky and Telescope - July 2017 - 6

FROM OUR READERS

The Schröter Eﬀect: A Closer Look

George Gladfelter
Rapid City, South Dakota

William Sheehan replies: Clearly,

Venus is a special case, and it is only
because its orbit is the most nearly circular of all the planets' that observers in the
1950s and 1960s (and presumably Schröter
himself) were able to ignore, with relative
impunity, the small discrepancy between the
times of greatest elongation and dichotomy.
As mathematician Jean Meeus points out,
the range of illuminated fraction varies at
the time of greatest elongation by no more
than 1.1%, which visual observers would not

Threads Laid Bare
I enjoyed Johnny Horne's review of the
ASI 1600MC camera (S&T: April 2017,
p. 58) but noted one slight ambiguity:
Both the article and the manufacturer's
specs state that the camera has "male
T-threads (often called M42 threads)."
However, readers should be made
aware that M42 and T are not compatible when used in lens mounts, even
though both have the same throat
diameter of 42 mm. M42 (sometimes
called P-thread) has a thread pitch of 1
mm, whereas the T-thread has a pitch
of 0.75 mm.
The confusion no doubt arises from
the use of M42 as metrological short6

J U L Y 2 0 1 7 * SK Y & TELESCOPE

Mercury
Sun

Earth

p The large eccentricity of Mercury's orbit, 0.21,
can cause the planet to appear more than half lit
during its greatest elongations from the Sun.

have noticed. Presumably, this accounts for
some of the variation in the estimates of the
Schröter effect. Likely more signiﬁcant are
seeing conditions, sky background (light vs.
dark sky), and experience of the observer.
In any case, it's interesting that during
January, as George Gladfelter points out,
Venus reached its geometric dichotomy
two days after its greatest elongation east
of the Sun. And yet, because of the play
of light in the planet's upper atmosphere
(and what might be called the Mallama

effect), Venus still appeared half lit to visual
observers before greatest elongation.
It's worth noting that similar observations involving Mercury would amplify
the kinds of discrepancies that Gladfelter
describes (as is evident in the illustration
above). I have often noticed, even in small
telescopes, that Mercury does not in
general appear exactly half lit at the time
of its greatest elongations - yet another
demonstration, were one needed, of the
large eccentricity of its orbit.

hand for "metric, 42 mm." But this
abbreviation has also come to mean a
42-by-1-mm lens mount.
Because these pitches are so similar, one might be tempted to connect
M42-mount and T-thread parts - they
will apparently "catch" when you try to
couple them together. But this will ultimately damage one or both as they're
tightened. Thus the manufacturer's
website repeated a common discrepancy
conﬂating the two, and I'm pleased
to see that the specs have since been
changed to read "Adaptor: 2″ / 1.25″ /
M42×0.75."

"Adorable" Black Holes
Although I have been reading and
enjoying Sky & Telescope since the mid1960s, I am hard-pressed to recall an
article as personable, literate, and informative as Camille Carlisle's "The First
Black Holes" (S&T: Jan. 2017, p. 24).
Thank you for some pleasurable reading.
I can well believe, as was noted at
the article's very end, that she ﬁnds
black holes "adorable," as her devotion
to them becomes clear. I look forward
to reading more of Carlisle's articles
as well as to her continuing efforts as
S&T's Science Editor.

Aldo Cugnini
Long Valley, New Jersey

Gary Lazich
Elk Grove Village, Illinois

LE AH TISCIONE / S&T, SOURCE: JE A N MEEUS

Another complication in gauging exactly when Venus appears half sunlit (S&T: Jan.
2017, p. 52) is that the geometrically calculated moment of dichotomy can vary
from the time of greatest separation by up to about 2.6 days regardless of the direction of elongation. This is due to the eccentricity of that planet's orbit.
Thus, observers should also be comparing the predicted and observed times of
dichotomy itself. During the event earlier this year, for example, I calculate that
greatest separation occurred at 13h Universal Time on January 12th, whereas
dichotomy occurred at 13h on January 14th - two full days later (not earlier, as the
Schröter effect would predict). Venus will have its next greatest separation on June
3rd at 12:31 UT with dichotomy the next day at 6:16 UT.
Meanwhile, the term "greatest elongation" is itself ambiguous, as this might
mean either the greatest difference in right ascension or the greatest angular separation. For Venus the time difference between the two can be up to 15 hours. Modern observers usually assume it refers to greatest separation in ecliptic longitude,
but it's unclear what was used historically.

Sky and Telescope - July 2017

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