 Back |
Next
|
Predicting
Eclipses in the Ancient World: The Saros
By
keeping accurate records of past eclipses, ancient astronomers could predict
future eclipses to within one day. The interval between eclipses
is
the Saros.
Let's
derive the Saros with what we now know about the moon's orbit. Start
from when an eclipse does occur. For another eclipse to occur...
-
Line
of nodes needs to be back in the same place
If
the line of nodes didn't move, this would happen exactly 1 year later.
But the line of nodes rotates clockwise (retrograde) every 18.61
years, so in a little less than one year, the line of nodes is back in
place
one
eclipse year = 346.6 days
-
Must
be an even number of Synodic Months
(29.53 days). Trial and error, 223 Synodic months is the same as
19 eclipse years
223
x 29.5 = 19 x 346.6
=
Saros
= 6585.3 days
-
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!).
| Back |
Next
|