Hello Rod,
The time zone meridians are spaced at 15deg increments (starting at
Greenwich, England) and their boundaries (in theory) are on meridians
7.5degs to each side. These "boundary meridians" are really not so well
defined however, they typically follow population centers and political
borders. For example, the time zone meridian for Mountain Time is at
105d West and if theory were put into practice its boundaries would be
at longitudes 112.5d West and 97.5d West respectively. By examining the
map (URL below), the boundaries for the standard time zones, mountain
time included, are defined quite differently, sometimes spectacularly
so. Mountain Time, for example is weighted heavy towards the West.
http://aa.usno.navy.mil/AA/faq/docs/world_tzones.html
Regarding the Sun's noon position, you might picture the MEAN sun as
being at its due South position on the local time zone meridian when
your watch says its noon, the real sun is something different. Since the
mean sun is a mathematical construct, i.e., you can't see it, the
apparent sun is really positioned some arc distance away from the
meridian as defined by the equation of time. So when its noon where you
are the mean sun is on the time zone meridian but the apparent sun is
not (except, of course, when the EoT = 0).
There are quite a few design approaches towards having a gnomon perform
equation of time correction. Some appear on page 33 of Sundials by
Albert Waugh. In general, the gnomon can be shaped to provide the
outline of the analemma (solid or open) or a point can be projected onto
the analemma. The gnomon can also itself be the dial surface with a
compound curve as its surface. Lots of possibilities but they are
chiefly the first two.
I believe Polaris is approx. 1 deg. off from the true pole. If you were
able to record its movement over the course of a day (sidreal day) it
would describe a circle, half the time being above true North half the
time being below. Therefore, since Polaris will be on the meridian twice
a day compute when this will happen and if you can view it then simply
compensate for the difference in declination (between Polaris and the
pole) and voila, you've found true North and wasted an inordinate amount
of time doing so. But it was fun, so who cares. I've used a spotting
scope on a polar aligned gnomon with pretty good success. I imagine a
parallel sighting line (mounted some distance off the back of the
gnomon) would work just fine too.
Best,
Luke
> rodandkellyheil wrote:
>
> Hello all,
>
> As a newcomer to this list, I beg your forgiveness if my questions
> have already been hashed out on this list dozens of times. I have not
> been lurking long enough to see what kinds of questions are
> appropriate.
>
> I am what you might call an "enthusiast." My wife calls me
> "obsessed." I just have a very intense interest in sundials.
>
> My first question is What meridians require no longitudinal
> adjustment? In other words, are time zones constructed so that the
> sun is directly over the center of the time zone when the whole time
> zone is said to be at 12:00? I live in the Rocky Mountain time zone,
> which seems to run from about 100 deg. W. to 115 deg. W longitude.
> When it's noon here, is the sun directly over 107.5 deg. W.?
>
> Secondly, how does one construct a gnomon for an equitorial sundial
> which adjusts for the equation of time?
>
> Third, what clever ways have you discovered for pointing a gnomon at
> the north star? I am wondering how I can aim it with precision when I
> can't sight down the edge of it, as it is mounted on the base of my
> horizontal sundial.
>
> Last, is there any significant difference between the north star and
> true north, and do I need to adjust for this?
>
> Thanks in advance,
>
> Rod Heil
> Enthusiast
