Sky & Telescope - March 2023 - 4

SPECTRUM by Peter Tyson
Down to Earth
OUR FOCUS at Sky & Telescope is typically well beyond our own
planet. In part that's because Earth is the only body in the solar
system, indeed in the universe, that we humans can't observe as a
whole (unless we're astronauts). So we commonly cast our gaze on
objects much farther away - on other worlds circling the Sun right
out to galaxies and other celestial treasures light-years away.
But now and then we aim closer to home. In this issue, you'll find more photos
than usual of our beautiful world. Next to all the night-sky images, it can be
refreshing - see, for example, the satellite shot on pages 34-35.
That striking photo opens our cover story this month, which asks a question
scientists have pondered for decades: Where did Earth get its water? While our
planet, as writer Javier Barbuzano notes, is 99.8% rock, we've got oceans and
oceans of H2O. And surface water is just part of it; lots more is likely locked up
in minerals deep underground. Where did all that water originate? Barbuzano
runs through the leading possibilities.
Tom Dobbins and Bill Sheehan's feature on page
A European Space Agency
Meteosat satellite captured this
image on March 23, 2022.
22 concerns sunlight reflected off the Moon, but
the article has everything to do with our planet.
Earthshine - sunlight reflected from Earth to the
Moon and then back again - is more than just
an observer's treat. As the authors explain, it can
actually help us study our planet. A key element
contributing to alterations in our climate is how
much sunlight Earth reflects back to space. Earthshine
can help scientists monitor these fluctuations
in our globe's albedo, or reflectivity.
That ghostly glow from the Moon might also aid
us in our search for signs of life on exoplanets. Flora on our land and phytoplankton
in our seas both have distinctive spectral signatures that, amazingly
enough, scientists have detected in earthshine. If they can spot such signals
in twice-reflected sunlight, why not in once-reflected starlight from distant
worlds? That's the hope, anyway.
Other pieces in this issue also exhibit an earthbound bent. In Celestial
Calendar starting on page 48, Bob King offers tips on how to spy Venus and
Jupiter in the daytime. As he writes, " Seeing other worlds in a blue sky makes
them seem closer somehow - like they're part of an earthly scene. " We wrap
up on page 84 with a look at phenomena in our own atmosphere: an update
from backyard astronomer Jay Brausch on his decades-long
watch for aurorae high over his home in North Dakota.
We hope you enjoy this quasi-geocentric issue.
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Sky & Telescope - March 2023

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