On Fri, 06 Oct 2006 05:30:57 -0500, Geoff Schultz
wrote:

Does the hours of sunlight follow the rule of 12ths? I suspect that it
does, but I'm too busy to figure it out by myself and I thought that
someone might now the answer off the top of their head.

-- Geoff

Hi Geoff,
I am not quite sure what you are wanting but this may be of help. It is in a way
similar to the estimation provided by the rule of 12ths.

When you wish to roughly ascertain the remaining hours of daylight at sea, hold
your hand at arm's length towards the setting sun and bend it so that your
fingers and palm are at 90 degrees to your arm with your fingers tightly
together side by side and are horizontal.

Bring your hand down beneath the sun so that the bottom of your little finger
lies along and just touches the horizon. The distance in finger widths between
the horizon and the bottom of the sun disc is the number of 15 minutes periods
before sunset.

e.g. four fingers is one hour of daylight left. You can easily make a rough
estimate of the time until sunset this way. I know that one could look up tables
or even look up the page in the GPS that shows sunset at destination, but in mid
passage or when making landfall, it works well enough for me and is surpisingly
accurate. One could possibly get into arguments about degrees from the equator
but I don't care to go down that route.

I hope that my explanation is easy to comprehend.

cheers
Peter Hendra
N.Z. yacht Herodotus

Peter Hendra wrote:

e.g. four fingers is one hour of daylight left. You can easily make a rough
estimate of the time until sunset this way. I know that one could look up tables
or even look up the page in the GPS that shows sunset at destination, but in mid
passage or when making landfall, it works well enough for me and is surpisingly
accurate. One could possibly get into arguments about degrees from the equator
but I don't care to go down that route.

Well, you might not want to, but I couldn't resist!

Using the following page:
http://aa.usno.navy.mil/data/docs/AltAz.html

I checked out the altitude of the Sun one hour before sunset at the
Equator and 60 North, for both Sept 20 and Jun 20.

At the equator, the altitude is about 13 degrees around the Solstice,
and 14.6 near the Equinox. And at 60 North its 4.7 and 6.7 degrees,
respectively. On July 30, at 45 North, its 9.6 degrees. So there's a
huge range here.

As for the angle subtended by the hand. My "eye to finger" distance
is 23 inches, and the four fingers are about 3 inches. This results
in an angle of about 7 degrees.

I know this is a bit geeky, but last year I was asked to figure out a
similar problem - the amount of time before sunset that the Sabbath
must be observed, as implied by the specifications in one of the Dead
Sea scrolls, as opposed to the normal Jewish Traditions. The DDS Cult
folks added an extra "Sun Diameter," or several minutes, depending on
time of year. They, like many others down through the ages, knew that
the End of the World was near, and if you wanted salvation, extra care
was needed to maintain purity.