Sky and Telescope - February 2018 - 6

FROM OUR READERS

Finding "Planet X" - Then and Now
I enjoyed immensely Scott Sheppard's
recent article (S&T: Oct. 2017, p. 16)
about solar-system resonances and the
methods being used by him and others
to locate a hypothesized distant, massive
planet. But I couldn't stop thinking that
I had read this story before. So I did a
search in my library and located a copy
of Tony Simon's The Search for Planet X, a
book about Clyde Tombaugh's discovery
of Pluto, which I'd read in 1965. Will
history repeat itself - ﬁrst in February
1930 and perhaps very soon, some 88
years later? Thanks for reawakening a
52-year-old memory.

John F. Fay
Mary Esther, Florida

"

Camille Carlisle replies: We asked
science writer Renjiang Xie to track
down the answer. According to Chinese
space ofﬁcials, the rover's expected lifetime
is about 92 days - three months on Earth,
but as experienced on Mars.

The Other Southern Sky

u The Search for Planet X, published in 1962,
recounts the events that led to Pluto's discovery.

After reading "Three Cosmic Chirps &
Counting" (S&T: Sept. 2017, p. 24), I
paused to reﬂect on this question: Do
gravitational waves really travel at the
speed of light - or, instead, do electromagnetic waves travel at the speed of
gravity? Perhaps they both travel at the
speed that waves propagate through
spacetime, and we need a better way to
talk about them both.
Paul Gallez
Mariposa, California

"

Camille Carlisle replies: I suppose
we could say that both electromagnetic and gravitational waves travel at the
maximum speed possible in spacetime:
300,000 kilometers (186,000 miles) per
second. We ﬁrst knew this value, called c,
as the speed of light in a vacuum, so we're
accustomed to calling it by this name. But
your question makes me wonder if talking
about c in a different way would bring new
insights. On the other hand, many other
holdover terms in astronomy continue to
endure. For example, astronomers once
referred to spirals like M31 in Andromeda
as "late-type galaxies," because they were
thought to evolve from elliptical galaxies. We

6

FE B RUA RY 2 018 * SK Y & TELESCOPE

now know that's not true, yet astronomers
still use the term. So we might be stuck with
"light speed" for awhile.

A Month on Mars?
Your excellent article "The Race to
Mars" (S&T: Nov. 2017, p. 14) included
the confusing term "three Martian
months" as the expected operational
lifetime of a planned Chinese rover. So I
must ask: What is a "Martian month"?
Here on Earth, our notion of the
month began as a single revolution of
the Moon. But Mars has two moons,
one of which revolves more quickly
than the planet rotates. So this deﬁnition doesn't seem likely. Alternatively,
the modern month on Earth is usually
30 or 31 days long. So a Martian month
could be 30 or 31 sols, the 24.66-hour
length of one Martian rotation. Yet the
difference between a day here and there
is small enough that the adjective "Martian" would seem superﬂuous.
Meanwhile, the Gregorian calendar's
12 months suggests that three Martian
months could be deﬁned as one quarter
of a Martian year. This seems likeliest,
though conﬁrmation would be nice.
A fourth alternative is the Darian

Brian Ventrudo's history of NicolasLouis de Lacaille (S&T: Oct. 2017, p. 34)
should at least have mentioned that the
indigenous peoples of southern Africa -
as well as aboriginal Australians, Paciﬁc
Islanders, and South Americans - had
their own traditional maps of the sky
long before European colonization.
Lacaille's vague and uninspired constellations only provide a window into the
impoverished imagination of one 18thcentury nerd.
Learning about these ancient constellations helps us to connect with
our ancestors of millennia past. Sky &
Telescope could express some respect for
cultural diversity by publishing one or
more feature articles on the sky lore of
people from the Southern Hemisphere.
Howard Ritter's beautiful photograph of
"the Emu" on pages 72-73 in that same
issue would be a ﬁtting illustration.
Anthony Barreiro
San Francisco, California

Worth the Wait
Mr. Tyson, I need to tell you how great
S&T is. I'm 85 years old now, and just a
few years ago I got interested in astronomy again. In the 1950s I was building
telescopes and into grinding mirrors.
When my kids were small, I delighted
in showing them Saturn and its rings.
Now my daughter is always bugging me
to tell her what I want for Christmas,
so a few years ago I asked for an S&T
subscription.

CURTIS UR BA N

Curtis Urban
Pueblo, Colorado

Spacetime Speed Limit

calendar, proposed in 1986 by Thomas
Gangale (and named for his son). It
divides the Martian year into 24 months
of 27 or 28 days each. With this scheme,
three Martian months would equal one
eighth of a Martian year.
So which is it?
Sky and Telescope - February 2018

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