Sky & Telescope - November 2023 - 24

Astronomy History
a pointer making one circuit every 76 years - precisely four
resets of the Metonic stylus. That fixed a flaw in the Metonic
cycle caused by the fact that a year is about a one-quarter
day longer than 365 days. Nineteen isn't divisible by four, so
the Metonic cycle falls one-quarter day short of the nominal
6,940 days, requiring one day to be removed from every
fourth Metonic cycle to stay in sync with the seasons.
On the right was a Games dial (appropriately for sportsmad
ancient Greece). Its pointer rotated once every four
years, indicating selected major sporting events for each year
of the Panhellenic (all-Greek) Games cycle. The Panhellenic
Games had near-sacred status because they were among the
few institutions that united the Greek city-states. The founding
of the Olympic Games, purportedly in 776 BC, was widely
considered to be Year One of Greek civilization.
The bottom spiral showed what eclipses could be expected
in each month of the 223-month saros cycle. Only a few
months have eclipses, so most of the cells were empty. For
months when one or more eclipses were expected, the cells
stated the possible eclipse types and expected times. To see
why a single 223-month spiral is sufficient, consider how
eclipses work. (Greek astronomers well understood all of this
by the time the Mechanism was built.)
Lunar eclipses can happen only at full Moon, when our
satellite is opposite the Sun's position in the sky. Likewise,
solar eclipses can happen only at new Moon. The fact that
the Moon's orbit is inclined 5° with respect to Earth's path
around the Sun means that in addition to being full or new,
the Moon must also be passing through the plane of Earth's
orbit, which is sometimes called the ecliptic plane for that very
reason. The time it takes for the Moon to cross up, down,
and up again through the ecliptic is called a draconic month,
after the mythical dragons that supposedly swallow the Sun
or Moon during an eclipse. Eclipses repeat every saros cycle
due to the fact that 223 synodic months equal 241.999 draconic
months and 238.992 anomalistic months. (More about
anomalistic months later.)
What that means in practice is that if the Sun, Moon, and
Earth line up to produce an eclipse, they will (except in marginal
cases) produce a very similar eclipse 223 months later.
A sequence of eclipses separated by intervals of 223 months
(18.03 years) is called a saros series. Saros series only last about
12 or 13 centuries because the various periods involved don't
line up perfectly. A saros series starts and ends with a thin partial
eclipse, with the longest and fullest eclipse near the center.
The saros period is about 8 hours longer than an integral
number of days, so every third eclipse in a saros series occurs at
roughly the same time of day (3 × 8 = 24 hours). That's another
way of saying that Earth is oriented roughly the same with
respect to the Sun, meaning that every third eclipse is visible
from roughly the same locations. For instance, the total solar
eclipse on March 7, 1970 peaked over Mexico, as will the one
on April 8, 2024, three saros cycles (54.1 years) later.
So just as four passes through the Metonic spiral have
special significance for calendars, three full passes through
the saros spiral have special significance for eclipses. And
sure enough, there was a little dial inside the Mechanism's
bottom spiral whose pointer rotated once every 54.1 years.
It was divided into thirds marked empty, 8, and 16, showing
the number of hours to be added to the time of day listed in
each eclipse cell.
All five dials on the back rotated at fixed ratios, just like
the hour and minute hands on a clock, though with more
complicated periods. The gearing is conceptually straightforShipwreck
location
GREECE
Ionian
Sea
Aegean
Sea
Sea
of Crete
Mediterranean Sea
Antikythera Island
TREASURE ISLAND Left: The tiny island of Antikythera, which lies in the middle of the shipping lane from the Adriatic Sea to the western Mediterranean,
has been the site of numerous shipwrecks. Right: X-ray tomography revealed that Fragment A, the largest of the 82 surviving fragments of
the Antikythera Mechanism, contains part or all of at least 29 gears. The big gear prominently displayed on the front is about 125 cm (5 inches) in
diameter and originally had 223 or 224 teeth.
24 NOVEMBER 2023 * SKY & TELESCOPE
MAP: BEATRIZ INGLESSIS / S&T; MECHANISM FRAGMENT:
WIKIMEDIA COMMONS / CC BY 2.5

Sky & Telescope - November 2023

Table of Contents for the Digital Edition of Sky & Telescope - November 2023