Dear All
All hours are unequal but some
hours are more unequal than others
I am not sure whether George Orwell knew that
but my new toy is a good illustration of the
the truth of this aphorism...
Given that the length of the solar day (transit
to transit) varies, the hours ticked off by an
ordinary sundial necessarily vary too.
Babylonian and Italian hours show greater variation
because the interval between successive sunrises
(or sunsets) varies more than the interval
between successive transits.
Unequal hours (one-twelfth part of the period
from sunrise to sunset) are even more unequal!
Even clock hours are not wholly exempt since
they occasionally last 3601 seconds.
Saturday was cloudy but Sunday offered some
hazy sunshine. I took the opportunity to
make some checks. I did some homework before
I set out...
I calculated when (by the clock) I could
expect to see the shadow of the nodus
fall on successive Babylonian and Italian
hour-lines. I was slightly surprised by
the figures.
Here are the clock times for five Babylonian
Hours in Cambridge on 21 March:
Babylonian Clock (UTC)
4 10:05:26
5 11:05:20
6 12:05:14
7 13:05:08
8 14:05:02
My Babylonian instrument appears to be
ticking off each hour about 6 seconds
more quickly than would be indicated on
a stop-watch.
This is mainly because the solar declination
is changing so rapidly at this time of year,
about one arc-minute per hour.
The sun rises earlier each day. At a given
moment, Babylonian hours don't refer to the
time the sun actually rose but to the time
the sun would have risen had the declination
been what it is now.
Each successive Babylonian hour is ticked
off relative to a moving reference.
Is it the same for Italian Hours? Well, not
quite. Here are the clock times for five
Italian hours in Cambridge on 21 March:
Italian Clock (UTC)
16 10:08:11
17 11:08:15
18 12:08:19
19 13:08:24
20 14:08:28
My Italian instrument appears to be ticking
off each hour about 4 seconds more slowly
than a stop-watch.
Sunset is getting later and each successive
Italian hour is ticked off relative to this
other moving reference.
So I end with a little puzzle for you all to
enjoy...
Why are the two rates not equal and opposite?
Why 6 seconds per hour for Babylonian hours
and just 4 seconds per hour for Italian hours?
I don't know whether I am educating the locals
but I'm certainly educating myself :-)
Frank King
Cambridge, U.K.
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