Sky & Telescope - December 2018 - 6

FROM OUR READERS
Starting Spectroscopy

Aerial View
Ralph Lorenz's article about viewing impact craters from commercial airplane
ﬂights (S&T: June 2018, p. 18) took me back to 1993, when I was ﬂying from Harare
(Zimbabwe) to Johannesburg (South Africa) and passed over the Pretoria Saltpan,
now known as the Tswaing Meteor Crater. The indigenous word tswaing, meaning
"place of salt," derives from a brine lake on the crater ﬂoor that was exploited as a
source of soda and salt up until the 1950s.
The crater is about 1,100 meters (3,600 feet) across and 120 meters deep, with a
rim height above the surrounding plain of 60 m. Its estimated age is 220,000 years.
The brine lake is about 7.5 hectares (19 acres) in area, and its depth is about 3 m. A
plaque there honors the late Gene Shoemaker, who pioneered the idea that craters like
Tswaing result from impacts.
I have walked around
and down into the crater
on several occasions - it
lies only 40 kilometers (25
miles) north of Pretoria,
where I live. It must be one
of the more easily accessed
impact craters!
Michael Poll
Pretoria, South Africa
u Tswaing crater in South Africa,
as seen from a plane

I loved the Test Report "Backyard Spectroscopy with RSpec" (S&T: Sept. 2018,
p. 68). The description of the author at
the end really piqued my interest: "Rod
Mollise enjoys getting some real science
out of his time at the telescope."
I recently retired and also would like
to do some observing that can really
contribute to science. I'm a member of
the American Association of Variable
Star Observers and the Association of
Lunar and Planetary Observers, belong
to a local astronomy group, and have
access to some very good telescopes.
I also have dabbled with some video
equipment to capture meteor trails and
calculate their trajectory.
I'm prepared to purchase some
relatively signiﬁcant equipment depending on the data to be collected - a very
good cooled camera, spectrograph, etc.
What I lack are some good ideas and
contacts with astronomy researchers
who can actually use what I collect.
Shawn Dilles
Vienna, Virginia

I was fascinated by Donald Bruns's
article on his replication of Sir Arthur
Eddington's famous experiment in
starlight deﬂection during a total solar
eclipse (S&T: Aug. 2018, p. 22). As a science teacher just starting my career, I'm
interested in using the history of this
experiment in an upcoming lesson, and
I have some questions.
First, in the "Angle of Light" illustration, what do you mean by "Newton's
angle = 0.88 arcsecond"? Wouldn't
this angle be zero if Einstein's theory
was invalid? Second, in the "Deﬂection Angle" graph, please explain what
the blue squares and the round yellow
circles speciﬁcally represent.
Richard Collins
Georgetown, Massachusetts

Monica Young replies: Newton

considered light to be "corpuscular."
In fact, he assumed light had mass, but this
really didn't matter. In his equations, which
consider a "test particle" with mass m in
the presence of a larger body with mass M,

DECE MBE R 2 018 * SK Y & TELESCOPE

the test particle's mass is often irrelevant.
For example, to calculate the acceleration
of a photon approaching the Sun, you can
write out the equation as F = GMm/r2 = ma.
As you solve for the acceleration (a), the
test particle's mass (m) cancels out. For a
more detailed explanation of the Newtonian
deﬂection angle, see https://arxiv.org/abs/
physics/0508030. As this article notes,
Newton never explicitly discussed a light ray
being deﬂected by a massive celestial body.
Einstein did, though, which is what spurred
Eddington to make his 1919 solar-eclipse
observations in the ﬁrst place.
Regarding the graph: The two sets of
points show how far stars appear deﬂected
from their true position relative to their distance from the solar limb. The yellow circles
represent data collected during an eclipse in
1973. Note that no circles are very close to
the Sun, so the researchers weren't able to
measure the deﬂection angle precisely. The
blue squares represent data that Dr. Bruns
collected during his experiment. He was
able to collect data on stars much closer
to the solar limb, so he got a more precise
measurement of the deﬂection angle.

Rod Mollise replies: While both
AAVSO and ALPO would likely have
interest in the spectroscopic data you collect, a good place to begin when ﬁguring
out what to do with your data once you
become adept at getting it, is the RSpec
website, rspec-astro.com, and the associated mailing list for RSpec. There you'll ﬁnd
some great ideas for projects you might
undertake once you're up and running.
I'd also urge you to start simply if you
haven't done any work in spectroscopy
before. For most of what you'll be doing
initially, a cooled camera, for example, isn't
required. I found it easier to get started with
the little (color) video-type camera described
in the article. Later, if you want to go after
the really dim stuff - quasars, perhaps - a
more sensitive camera might be in order.
However, I was amazed at what I could do
with my humble ZWO. Also, in the beginning a simple grating like I used for the Test
Report is quite adequate.

Galactic Wanderer
Greg Laughlin's article on `Oumuamua
(S&T: Oct. 2018, p. 20) leaves me, for

MICHAEL POLL

Eclipse Experiment

http://www.rspec-astro.com https://arxiv.org/abs/physics/0508030 https://arxiv.org/abs/physics/0508030

Sky & Telescope - December 2018

Table of Contents for the Digital Edition of Sky & Telescope - December 2018