Sky and Telescope - November 2016 - 16

Cosmic Relief
David Grinspoon

Thank Our Lucky Planets
Human history has always been linked to the inﬂuence of distant orbits.
Every astronomer has had the annoying

ISTOCKPHOTO / PETE DRAPER

experience of being introduced as "an astrologer." We
then have to explain that astronomy is the study of the
universe beyond Earth, while astrology is the belief that
this universe controls our lives. There's no good reason
to hold that the position of the planets at your birth
decided your personality or life path. But it is true that
planetary motions have strongly inﬂuenced human history and nature.
For several million years, climate changes in Africa
repeatedly shaped our evolution. In large part, these climate swings were forced by a complex series of rhythmic
oscillations in Earth's orbit and spin - oscillations that
stem largely from the perturbing gravitational inﬂuence
of Jupiter, Saturn, and the Moon.

Several evolutionary breakthroughs came about during such periods of extreme, modiﬁed climate. Upright
posture, which freed up our inventive hands; a rapid
increase in brain size; and the use of ﬁre, allowing a
meat diet that spurred further increase in brain size -
scientists have linked all these to episodes of rapid climate alteration. These great leaps forward transformed
us from just another species to one with the abilities that
enabled the science and technology through which we've
uncovered our own natural history.
Later we left Africa and peopled the world, our path
set by climate-driven changes in sea level such as the
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November 2016 sky & telescope

one that opened up the Bering Strait land bridge to
North America. Seven thousand years ago a phase of
stable sea level coincided with the ﬁrst large coastal
settlements and the rise of complex societies. The origin
of many sophisticated technologies and the symbolic
language to pass them down also seem to have arisen in
response to climate-caused survival threats.
Now, in a twist, some of our technology threatens
the climate we depend on to survive. Astrology will not
save us, but astronomy might. By widening our scope
of knowledge and improving our modeling capabilities,
planetary exploration is crucial for understanding climate
and responding eﬀectively to our current challenges.
Our exploration of the solar system owes its own path
to fortuitous planetary positions. Every 175 years the
outer planets arrange themselves perfectly for a "grand
tour" mission that can ricochet from one gas giant to the
next. One such rare alignment came in the 1970s, when
we'd just barely developed the necessary technology to
launch the pair of Voyager spacecraft. Another important lineup occurred soon after astronomers discovered
Pluto's moon Charon in 1978 (no doubt causing astrologers to redo their charts).
It's lucky we found Charon when we did. Just two
years later, the plane of its orbit lined up precisely with
Earth to create a 5-year-long season of Pluto-Charon
eclipses. This won't happen again for more than a
century. More importantly, these events and subsequent
studies, along with the puzzling nature of Neptune's
moon Triton, seen in Voyager's ﬁnal pass in 1989, helped
motivate those who agitated for a dedicated Pluto mission, culminating in last year's historic ﬂyby.
Maybe a species that has colonized its home world
would have emerged on Earth even if the solar system
didn't work the way it does. Once here, maybe we were
bound to explore our neighboring worlds. But route
and timing were dictated from above. The planets have
indeed always ruled us. ✦
David Grinspoon is an astrobiologist and senior scientist at
the Planetary Science Institute. His book Earth in Human
Hands is due out next month. Follow him on Twitter at @
DrFunkySpoon.

Sky and Telescope - November 2016

Table of Contents for the Digital Edition of Sky and Telescope - November 2016