the Sky was Plumbed, Then and Now
of constellations decorate the back of a bronze
mirror from China, ca. A.D. 600-900. A poem
inscribed around the rim discusses "the regularities
of the heavens."
by Phil and Phylis Morrison
from the print version of Exploratorium
Magazine on space
key facts of naked-eye astronomy had, we think, been realized
long before writing arose. We cannot say much of what people knew before
anyone could write it down, but that does not mean that they were ignorant
of their own rich experience. Seasons were noted and found to recur with
the motions of the luminaries. It seems to us that all the sky's visibly
transient features -- the clouds, fog, mist, rain, thunder and lightning,
rainbows and sun-dogs, even meteors and comets -- were studied many times
by the old ones and eventually set well apart from the truly repetitive
features -- the sun and moon, the night's starry points of light, and
the soft glow of the Milky Way.
All this plays out by day and
by night on the two-dimensional dome we call the sky. That it really has
three dimensions is harder to realize; but some sort of eclipse every
year or two will in the long run have persuaded people that the sky has
depth like the lands. Sometimes the moon might pass in front of a star
or planet, the earth cast a shadow to darken the full moon, or the new
moon's dark disk briefly obscure the sun.
These were long generations
of dance and These were long generations of dance and trance, no writing
nor much ciphering yet, but plenty of tales by wise women and reflective
hunters. Ages passed as small bands wandered the lands and watched the
night sky until both became familiar.
Triangulating the Heavens
Without written texts it is
hard to find much to cite from long-past astronomy, and the ubiquitous
ancient legends that surround sky events remain pretty cryptic. But after
writing had arisen in many places, once the sailors had regularly crossed
blue water, once the travelers had made their way by land over thousands
of miles, once the learned could calculate and measure shadows, angles,
and hours, the rich Greek legacy of texts flowed vigorously on to Euclid
and his celebrated successors. By then the round earth was a certainty
for scholars, its size set at some five or ten thousand miles in diameter.
What Babylon accomplished much earlier for meticulous arithmetic, Greece
achieved after 500 b.c. for a powerful geometry. It is there and then
that the physical magnitude of cosmic space finds its first known clear
expression. The recognition of continuity between far-off landscape and
the utterly trackless celestial domain was the key.
Anaxagoras, an articulate philosopher
who taught in Athens about 450 b.c., wrote that "the sun, moon, and all
the stars are flaming stones carried around by the revolution of the aether.
. . . It is the sun that endows the moon with its brilliance, and that
sun is larger than the Peloponnesus" (most of Greece southward of Athens).
Such ideas connect with what we believe still. His theory drew his imprisonment
under grave charges of atheism -- in a city where the Sun was a god --
and he was set free only after personal intervention by Pericles himself,
the leading Athenian statesman of the day. How could Anaxagoras have supported
his points at that time? "Flaming rock" is not a bad beginning; for the
heavenly lights are enduring, neither gods nor persons, animals nor plants,
and surely not human-made.
from pre-Columbian times, this Mayan manuscript
known as the Dresden Codex contains a numerical
record of the periods of the sun, moon, and Venus
as they were seen to circle the earth.
How large is the moon? Well,
we can see its disk, and, using geometry, we can conclude that its distance
from us is about a hundred times its diameter. Each month the moon circles
the sky. How far away is its circle? Certainly it never lies this side
of the hills, and it always clears the mountains. So we know it is at
least ten miles overhead. But were it as close as that, the full moon
seen upon rising would be farther away eastward, and therefore it would
appear smaller than when overhead.
We have also noticed for ourselves
a wonderful though simple rule: things nearby seem to move past more quickly
than distant ones. The rabbit hops across our field of view and is gone
-- poof -- while the airplane or the rain cloud takes enough time
for you to hum quite a snatch of song. Near motions seem fast, far ones
The far-off moon walks along
with you for a few hours, as no house or tree does: only the distant mountains
can come with you that way. As the visible diameter is the same at moonrise,
moon high, and moonset, the moon's orbit is understandable only if it
is very large compared to our earth. Then it cannot come close or go far
away as seen by any observer anywhere on earth. But if it is tens of thousands
of miles away as we have shown, then it must be hundreds of miles across.
The sun never appears in front
of the moon, though the moon can just block out the eclipsed sun. The
sun is therefore farther than even the moon, so it is bigger than the
moon, and hence bigger than the whole Peloponnesus! The sun is slower
to circle the sky than the moon, taking a year, not a month, to move through
the constellations of the zodiac, and so it is likely to be farther off
-- a helpful check on the conclusion.
bone engraving is over 30,000 years old. Alexander
Mershack, the archaeologist who studied this piece
of bone thinks it records the days that pass during
the cycling of the moon.
Once each month the moon globe
is half in shadow, half in light. Aristarchus, a century or two later
than the time of Pericles, still well before Euclid, saw that the sun
would need to be very much farther than the moon if that judgment of half
shadow were precise. He set the angle between the sun and the half-moon
at 87 degrees, and from the triangle he drew claimed that the sun was
thirty times farther than the moon. The sun is actually four hundred times
farther away; the sun-moon angle for the half-lit moon is 89.9 degrees.
But that's hard to verify even today from the rough shadow on that mountainous
It pleases us to see that the
Greek invention of geometry was not an intellectual abstraction
only, as it was taught when we went to school, but was a
powerful tool for enlarging our most basic understanding
of how the world is put together. Using geometry and their
eyes, the Greeks had reason to believe that cosmic distances
were much greater than earthly ones, cosmic space ampler
by far than our own lands. That was a major new conception
of the human place in nature; never mind the errors, even
to Hubble Prologue