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

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.

My first reaction is no, but perhaps I'm missing your intended
meaning.

The "Rule of 12ths" is commonly used to estimate tide height. How
does that relate to hours of sunlight?

Hours of sunlight depends on both latitude and time of year, largest
seasonal variation is at the poles, smallest variation near the
equator.

Wayne.B wrote in
:

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.

My first reaction is no, but perhaps I'm missing your intended
meaning.

The "Rule of 12ths" is commonly used to estimate tide height. How
does that relate to hours of sunlight?

Hours of sunlight depends on both latitude and time of year, largest
seasonal variation is at the poles, smallest variation near the
equator.

I just ran a spread sheet looking at the number of hours of sunlight per
month. If I apply the rule of 12ths to calculate the hours of sunlight, I
come up with a result that's within 5% of the actual result, well within my
range of error. The rule of 12ths deals with rate of change over half of a
6 hour tidal cycle and I assumed that it would also apply to daylight hours
in a 6 month cycle.

-- Geoff

The angle of incidence of sunlight on a stationary flat panel would be a
sinusoidal curve between sunrise and sunset but I don't think the power
output would quite follow the Rule of 12ths. For one thing the amount of
atmosphere the light has to pass through would squeeze the peak output
towards the center where both the filtering and the angle of incidence would
be least. If the panel were mounted so it tracks the sun the output curve
would be flatter.

--
Glenn Ashmore

I'm building a 45' cutter in strip/composite. Watch my progress (or lack
there of) at: http://www.rutuonline.com
Shameless Commercial Division: http://www.spade-anchor-us.com

"Geoff Schultz" wrote in message
.. .
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

I'm not thinking about solar panels. I was just watching the daylight
disappear while thinking having to move BlueJacket from the Chesapeake to
Ft. Lauderdale at the end of the month. I was thinking about how quickly
we're loosing daylight and wondered if rate of change was the same as tidal
change and thus the rule of 12ths. I couldn't see why not.

-- Geoff

"Glenn Ashmore" wrote in
news:fHsVg.23885$rg1.377@dukeread01:

The angle of incidence of sunlight on a stationary flat panel would be
a sinusoidal curve between sunrise and sunset but I don't think the
power output would quite follow the Rule of 12ths. For one thing the
amount of atmosphere the light has to pass through would squeeze the
peak output towards the center where both the filtering and the angle
of incidence would be least. If the panel were mounted so it tracks
the sun the output curve would be flatter.

On Fri, 06 Oct 2006 08:44:21 -0500, Geoff Schultz
wrote:

I was thinking about how quickly
we're loosing daylight and wondered if rate of change was the same as tidal
change and thus the rule of 12ths. I couldn't see why not.

The daily rate of change is highest at the spring and fall equinox
since that is where the sine curve has the steepest slope (when it
passes through zero).

Short days are definitely an issue when you bring a boat south in the
fall but it gets somewat better as you get further along towards lower
latitudes.

I assume you are comfortable running at night with all the cruising
you have done. If so, wait for a good weather window and head
offshore for a few days at a time. That's what we did last year and
it worked out well. As long as we are not in the ICW, or coming into
strange harbors, running at night is not a problem.

In that case you will probably get pretty close. The Rule of 12ths is
actually a way to estimate the slope of a sine wave and except for a few
minor variations the seasonal variation in day length is sinusoidal.
--
Glenn Ashmore

I'm building a 45' cutter in strip/composite. Watch my progress (or lack
there of) at: http://www.rutuonline.com
Shameless Commercial Division: http://www.spade-anchor-us.com

"Geoff Schultz" wrote in message
.. .
I'm not thinking about solar panels. I was just watching the daylight
disappear while thinking having to move BlueJacket from the Chesapeake to
Ft. Lauderdale at the end of the month. I was thinking about how quickly
we're loosing daylight and wondered if rate of change was the same as
tidal
change and thus the rule of 12ths. I couldn't see why not.

-- Geoff

"Glenn Ashmore" wrote in
news:fHsVg.23885$rg1.377@dukeread01:

The angle of incidence of sunlight on a stationary flat panel would be
a sinusoidal curve between sunrise and sunset but I don't think the
power output would quite follow the Rule of 12ths. For one thing the
amount of atmosphere the light has to pass through would squeeze the
peak output towards the center where both the filtering and the angle
of incidence would be least. If the panel were mounted so it tracks
the sun the output curve would be flatter.

On Fri, 6 Oct 2006 12:14:25 -0400, "Glenn Ashmore"
wrote:

In that case you will probably get pretty close. The Rule of 12ths is
actually a way to estimate the slope of a sine wave and except for a few
minor variations the seasonal variation in day length is sinusoidal.
--
Glenn Ashmore

Glenn, what's the latest estimate for splashing noises coming from
your direction?

Glenn, what's the latest estimate for splashing noises coming from
your direction?

I keep saying that if I can keep my ass in gear I can be in the water in the
Spring. The problem is keeping it in gear. :-)

I am down to the picky and painful parts now. Laying the teak on the side
decks, setting plumbing fixtures, final fairing and painting the topsides
and a whole bunch of interior sanding and varnishing. The big plus is that
everything but the electronics, spars, rigging and sails is on hand and paid
for so no more time wasted waiting for the boat kitty to refill. :-)

You know you are getting close when you go to the Annapolis show and don't
look at the boats. Spent all my time in the tents looking at parts and
discovered a downside to gradually acquiring things as bargains are
presented. I picked up 350' of NE Ropes 3 strand last year and they just
introduced a new nylon plait anchor rode with a soft hand that stacks in
about half the space of 3 strand. I will be setting my two Atlantes heads
this weekend and Raritan has a new model that is simpler and uses less amps.
Fortunately I have not bought any electronics yet. There are some amazing
things happening in autopilots and nav systems.

--
Glenn Ashmore

I'm building a 45' cutter in strip/composite. Watch my progress (or lack
there of) at: http://www.rutuonline.com
Shameless Commercial Division: http://www.spade-anchor-us.com

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