Sky and Telescope - July 2015 - 86

Focal Point

Alan Hirshfeld

Reading History in the Heavens
The night sky oﬀers a visual narrative of astronomical advances.

NASA / DAMIAN PEACH, AMATEUR ASTRONOMER

As director of a small college observatory, I have long treated community
residents to the wonders of the night
sky. Like many outreach astronomers,
I've compiled a roster of celestial objects
to display, featuring the usual descriptors for each one: category, brightness,
distance, and so on. But a purely datadriven tour of the night sky disregards a
highly inspirational aspect of the cosmic
landscape - the human dimension.
Astronomers should present the wonders
of the cosmos, as the Nobel Prize-winning physicist Isador Rabi said about
teaching science, "in the light of the history of human thought and human eﬀort,
rather than as the geography of a universe
uninhabited by mankind."
Over the millennia, our species has
engaged in a heroic enterprise to measure, analyze, and ultimately comprehend

the cosmos. From a state of complete
bewilderment about the dark province
above our heads, we have advanced to a
deep understanding of the universe and
its components. How we garnered that
knowledge is as important as the knowledge itself, for it endows humanity with a
reason for being as well as a lofty incentive to sustain our collective exploration.
Tying the history of astronomical
advancement to a tour of celestial objects
gives viewers a palpable sense of that
time when the camera and the spectrograph turned the ﬁrmament's enigmatic
specks and wisps into actual physical
entities scattered through space. In a historical context, the night sky becomes a
visual chronicle of scientiﬁc achievement
and technological inventiveness.
During early autumn, for example, I
point out the cross-shaped constellation

A mini Copernican cosmos: Jupiter and two of its Galilean moons, Io (left) and Ganymede.

July 2015 sky & telescope

of Cygnus the Swan and recite the stock
superlatives that characterize its major
stars. But then I direct visitors' attention
to a 5th-magnitude star tucked between
the swan's tail and its eastern wing. This
nondescript dot, 61 Cygni, gleams in the
annals of science history: in 1838, astronomer Friedrich Bessel measured 61 Cygni's
distance, making it the ﬁrst star outside
the solar system to be pinpointed in space.
The famous double star Mizar and
Alcor, in Ursa Major, bears multiple
historical imprints. It is the ﬁrst double
star detected in a telescope (by Benedetto
Castelli in 1617) and captured in a photograph (by George Bond in 1857). And the
brighter of this Mizar pair, too, is double,
as Edward Pickering announced in 1889.
It was the ﬁrst stellar binary discovered
by its spectroscopic signature.
The Moon evokes that perilous era
in Western intellectual history when
Galileo ﬁrst cast his telescopic eye over
the lunar landscape and pronounced it
Earthlike. His measurement of the height
of a lunar peak, in 1609, planted a virtual
ﬂag to human ingenuity centuries before
astronauts walked on the Moon. History
likewise imbues Jupiter and its satellites
with human drama: Galileo's clash with
church authorities, which was sparked by
the sight of what appeared to him to be a
diminutive Copernican cosmos.
In sum, as awe-inspiring as the night
sky is to the eye and to the mind, its
reﬂection of humanity's quest for knowledge heightens its inspirational power. ✦
Alan Hirshfeld is an astronomer at UMass
Dartmouth and author of, among other
books, Starlight Detectives: How Astronomers, Inventors, and Eccentrics Discovered the Modern Universe.

Sky and Telescope - July 2015

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