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Predicting
Eclipses in the Ancient World:
Eclipses
were of Enormous importance
in the ancient world. If you were an Astronomer, there were even
places where it was a matter of Life and
Death!
Most
early cultures were able to predict Eclipses with surprising
accuracy, despite not understanding many
of the underlying orbital principals.
This
was no mean feat. Consider that to predict an eclipse you need know.
-
That
the Line of Nodes is aligned with the Sun at new or full Moon.
-
That
the Line of Nodes is regressing along
the path of the Moon's orbit.
To
predict an Eclipse you need to account for both of these factors.
It turns out that you can!
-
Realize
that an opportunity for an Eclipse can occur every 29.5 days (a Synodic
Month) if
the Line of nodes points toward the Sun.
-
After
each Eclipse you must wait until the next time the Line of Nodes comes
into alignment.
-
Recall
that the Nodes are Regressing,
which means that the time spacing between Node alignments is complex.
It takes 346.6 days between Node alignments.
This is called the Eclipse Year.
-
Unfortunately
346.6 is not evenly divisible
by the 29.5 day Synodic Month. And since you can only
have an Eclipse if both the Line of Nodes and
the phase of the Moon work together, you must wait for some
number of Eclipse Years to have another
Eclipse where you live.
-
You
need to use the smallest common factor
for both time scales...(in other words
some number of Synodic Months equals Some number of Eclipse Years).
223
x 29.5 = 19 x 346.6 = 6585.3 days
In
the ancient world, this was called the Interval
of Saros. At any given place on
the Earth, an Eclipse will occur once every
3 interval's of Saros (that 0.3 days moves
the location of the Eclipse!). With this technique, Astronomers could
predict Eclipses to within a day.
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