Sky & Telescope - August 2021 - 24

THE MET / FLETCHER FUND, 1931
COMPUTATIONAL ASTRONOMY by Douglas MacDougal
Alas, Babylon!
When Mars Draws Near . . .
Mesopotamian astronomers pioneered astronomical computing.
W
henever Mars reaches opposition, as it did so
brilliantly last October, my thoughts turn to
Babylon. Why Babylon? Because incredibly precise
accounting of planetary positions started with the Babylonians
- indeed, computational astronomy itself originated
with them.
Babylonian astronomers consistently tracked and recorded
the positions of Mars and the other naked-eye planets from
about the mid-7th century BC and kept it up for another half
millennium. These records are inscribed on about 1,000 clay
tablets currently held mainly in the British Museum in London.
The Babylonians also developed mathematical methods
that allowed them to predict planetary positions and create
ephemerides, even correcting for variable orbital motion.
Their saros cycle of the Moon appears on the famed Antikythera
Mechanism, discovered on an ancient shipwreck.
A Synodic Clock
The Babylonians tracked Mars oppositions over many centuries.
Such alignments of the Sun, Earth, and Mars occur
about every 2 years and 50 days - a span of time known as
24 AUGUST 2021 * SKY & TELESCOPE
GUARDIAN BEAST The pacing lion is an iconic symbol of Babylon
and its once-fl ourishing, vibrant culture. This relief is from the famous
processional way leading into the city from Babylon's Ishtar gate.
Mars's synodic period or synodic cycle. The intricacies of the
synodic cycle can best be explained by imagining a unique
clock. Its two hands represent the movements of Earth and
Mars in their counterclockwise orbits (assuming uniform
circular motion for each). The shorter hand moves with the
mean speed of Earth around the Sun, completing a revolution
in 365¼ days. The longer hand represents Mars, which
takes about 687 days to complete an orbit. The Sun is at the
center of the clock where the hands are anchored, and the
clock's face is the plane of the ecliptic. Whenever the hands
are lined up with each other, we have an opposition.
If Mars were standing still, Earth would simply pass it
every year and its synodic cycle (the time between oppositions)
would be 365¼ days. If Mars were going half the speed
of Earth, the synodic cycle would be exactly two Earth years.
And in either scenario Mars would appear to return to the
same place in the sky - the same ecliptic longitude, in the
same constellation - at each opposition. But, of course,

Sky & Telescope - August 2021

Table of Contents for the Digital Edition of Sky & Telescope - August 2021

Contents
Sky & Telescope - August 2021 - Cover1
Sky & Telescope - August 2021 - Cover2
Sky & Telescope - August 2021 - 1
Sky & Telescope - August 2021 - Contents
Sky & Telescope - August 2021 - 3
Sky & Telescope - August 2021 - 4
Sky & Telescope - August 2021 - 5
Sky & Telescope - August 2021 - 6
Sky & Telescope - August 2021 - 7
Sky & Telescope - August 2021 - 8
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Sky & Telescope - August 2021 - Cover3
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