Sky & Telescope - December 2023 - 65

BOOK REVIEW by Peter Tyson
Up Close and Personal
UNDER ALIEN S IES: A Sightseer's Guide to the Universe
Philip Plait, PhD
W. W. Norton & Co., 2023
336 pages, ISBN 978-0-393-86730-5
US$30.00, hardcover
PHIL PLAIT, well-known author of
the blog, newsletter, and book Bad
Astronomy, has wondered since he was
a kid what celestial objects would look
like up close. What if you were nearby
in a spaceship or - if the object had a
solid surface - standing on it? After he
earned his PhD in the 1990s and began
giving public talks on astronomy, he
found that many others had the same
question. So he wrote this book.
The Moon
Plait starts with our nearest neighbor.
The Apollo landings gave us Earthlings
some notion of what it's like to be on
the lunar surface, but Plait seasons our
understanding. He reminds us that even
when the Sun blazes at midday, the sky
would remain black, because there's
little atmosphere to light up the sky.
When the Sun sets, the light disappears
instantly. " Just bang, night, " he writes.
And we'd have to contend with the temperature
change, which can swing from
250°F at noon to -200°F before dawn.
Thanks again to Apollo, we've seen
Earth from the Moon. But have we
considered how bright earthshine would
be on our satellite? As Plait observes,
Earth is about 14 times bigger and three
times more reflective than the Moon, so
reading by Earthlight when our planet is
" full " would be a cinch. And how about
watching a total solar eclipse from the
Moon? Earth would be much larger in
the sky than the Sun, so you wouldn't
see as much of the solar corona during
totality. " That's okay, " Plait writes,
" because this eclipse has something no
Earth-bound stargazer
could ever see: a world
with an atmosphere blocking
the Sun. "
Saturn
Plait then shuttles you to
Mars and the asteroids
before heading out to
Saturn. The Cassini mission
gave us breathtaking
closeups of the rings, but
Plait takes you even closer.
If you were in a spacecraft
right at the outer edge of the outermost
ring, he says, that edge wouldn't appear
curved when you looked along it but
perfectly straight. " The rings are circles,
but they're circles 170,000 miles across, "
he writes (he likes emphasis). They're
also incredibly thin - in most spots,
only 40 feet thick.
If your hypothetical spaceship
arrived at that outer rim edge-on to
Saturn, the rim would appear as a onedimensional
line in the sky. But rise just
up over that rim and you'd see a vast
plain of icy boulders stretching towards
the gas giant. " Perspective plays havoc
with your eyes and brain, " Plait writes,
noting that the boulders shrink with
distance toward Saturn, but the lack of
air means they're just as sharp far away
as nearby. Imagine if your ship flew into
Saturn's shadow, with the Sun on the
opposite side of the planet from you.
The rings would fill the sky in a grand,
whitish arc - except where the world's
75,000-mile-wide shadow erased them,
leaving pure black.
Far-away Worlds
As an astronomer, Plait makes use of
the latest scientific thinking, of course.
But since we have no close-up images of
deep-sky objects, he has no choice but
to speculate on near views
of objects beyond our solar
system. Readers will have
to judge for themselves
how successful he is.
Plait steers us to the
TRAPPIST-1 system of
exoplanets, to planetary
nebulae, and to starforming
regions like
the Orion Nebula. He
explains how someone on
a planet orbiting two stars
would enjoy " sunsrise "
and " sunsset, " and how traveling near a
black hole would be " trippy to see, one
of the weirdest sights in your life. " And
how about stargazing on a planet orbiting
a star near the core of a globular
cluster? After dark, Plait says, instead
of the several thousand stars visible at
a dark-sky site on Earth, you'd see tens
of thousands of suns overhead, some of
them so bright as to make you squint.
On the surface around you, multiple
shadows of different lengths would
shoot out on every side.
It's a delightful prospect, but globulars
formed so long ago, Plait notes, that
it's possible they have no rocky planets
at all. In providing such caveats, he
enhances our trust in him as our guide.
Altogether, Plait does a fine job taking
you right up to wondrous objects. As
he writes, " We may not know precisely
what it's like to fall into a black hole
or visit a star as it's forming, but our
understanding of astronomy, physics,
mathematics, and more can feed our
minds, allowing us to imagine with
great accuracy and remarkable detail
what it would be like if we were there. "
¢ PETER TYSON wishes the galaxycrossing
spaceships Plait imagines were
real and he had a seat on one.
sk yand tele scope .o r g * DECEMBER 2023 65
https://skyandtelescope.org/
Sky & Telescope - December 2023

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