Sky & Telescope - October 2020 - 6

FROM OUR READERS
Jupiter and its three kindred giants have
always been defined by their cloudtops.
Why is Venus treated differently?
Mark Gingrich
Mendocino, California

"

I have found my doppelgänger. When I
showed the picture of Rod Mollise in
"Exploring the Deep Sky With Video"
(S&T: June 2020, p. 66) to my wife, she
asked when had I sent in the article,
and what telescope was I using. My
sister-in-law was also fooled!

S Left: Donald Watts replicates the image of
Rod Mollise from the June issue. Right: Rod
Mollise prepares for a night of video astronomy.

A Cable Solution

own video gear until I had a solution.
I put the imager in a toolbox. All the
cables are contained within the box -
the only exception is the one connecting
the camera to the telescope. I hope this
picture inspires readers who are also
intimated by the tangle of wires involved
in video astronomy.

"Exploring the Deep Sky With Video"
was of great interest to me. I've owned a
Revolution Imager System R2 for almost
two years. I use the video camera on
my Celestron NexStar 8SE SchmidtCassegrain, Astro-Tech 72-mm refractor,
and Coronado SolarMax II 60-mm solar
telescope. So I was fascinated by all the
images in that article.
The "Wired For Video" picture on
page 67 shows the "seemingly endless
number of cables" that are a staple of
video astronomy. The wire situation was
so intimidating that I refused to use my

S A toolbox and some ingenuity solve the
cable situation.

6

OCTOBER 2 02 0 * SK Y & TELESCOPE

So, we tried to duplicate the picture
as closely as we could. Thanks for such
a superb publication.
Donald W. Watts * Boise, Idaho

Archinal and colleagues give 6,051.8
km for Venus's semidiameter in their paper,
https://is.gd/Archinaletal, which we use
to get 8.34 arcseconds as Venus's semidiameter (without clouds) at a distance of 1
astronomical unit. I've been using this value
to calculate S&T's almanac table since 1984!
Before that, I used 8.41 arcseconds, which
includes the clouds. In fact, that traditional
value goes back to a paper by A. Auwers in
Astronomische Nachrichten in 1891.
I was simply following the Astronomical
Almanac. That way, we've always agreed on
the published values. I'm sure Archinal's solid-Venus diameter has been refined several
times since 1984, but not enough to change
the adopted constant of 8.34 arcseconds.

Martian Waves

Greg Cisko
St. Charles, Illinois

I'm interested in the News Note,
"NASA's Insight Detects Marsquakes"
(S&T: June 2020, p. 8). How are the
depth, distance, and direction of the
sources of these quakes calculated? Also,
is the magnitude scale for Mars the same
as that for Earth?

The Cloudtop Debate

Don Herron
Sugar Land, Texas

According to June's Planetary Almanac
(S&T: June 2020, p. 44), Venus began
that month with an angular diameter of
57.6 arcseconds, a value consistent with
most authoritative annuals. Pardon my
nitpicky nature, but it's not true!
That value holds if the bare Venusian surface could be glimpsed. But our
bedazzling empress of planets is swaddled in thick clouds, which, on June 1st,
added 0.6 arcsecond of girth. Trifling as
this may seem, it's a discrepancy measurable with a telescope and micrometer.
The scant few who discern with hazepiercing infrared or radar have good
reason to abide by the status quo. Yet,
in deference to the far more prevalent
visual observers, shouldn't a planet's
predicted size conform to what the eye
perceives? Besides, the angular spans of

"

Camille Carlisle replies: The News

Note is an abridged version of the
longer story https://is.gd/marsquakes,
which goes into more detail, though I didn't
dive deeply into the method. The team
did use the same magnitude scale for the
marsquakes as we do for earthquakes - so
these are quite small temblors.
In terms of how they did it: Well, it's not
easy, because they only have one seismometer. On Earth, seismologists can triangulate
using multiple seismometers at various
locations. So, the team used a fair amount
of extrapolation. However, the same principles hold on both planets: Different kinds
of seismic waves, p and s waves, come
from the same event but travel at different
speeds. Therefore, the time between one
type reaching a detector and the other tells

CA BLES: G REG CISKO

Matching Astronomers

Roger Sinnott replies: Brent


https://www.is.gd/Archinaletal https://www.is.gd/marsquakes

Sky & Telescope - October 2020

Table of Contents for the Digital Edition of Sky & Telescope - October 2020

Contents
Sky & Telescope - October 2020 - Cover1
Sky & Telescope - October 2020 - Cover2
Sky & Telescope - October 2020 - 1
Sky & Telescope - October 2020 - Contents
Sky & Telescope - October 2020 - 3
Sky & Telescope - October 2020 - 4
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