I'm new to the list too. But I'll attempt to answer your questions.
My responses are preceded by --
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.?
--- I think Luke Coletti answered this as well as it can be answered.
Short version: yes, if on that day the equation of time is zero.
Secondly, how does one construct
a gnomon for an equitorial sundial which adjusts for the equation of time?
-- I don't think anyone tackled this, so here
goes.
Since the equation of time causes the sun's apparent
rotation around the gnomon to vary in angular speed, there isn't anything
you can do to the gnomon to compensate for that in a standard equatorial
design. I assume you are attempting to make the sundials' display
of local solar time begin to approximate standard time like on your watch;
i.e. you could set your watch with it.
There are two ways (at least) to compensate for
the equation of time: One is to make an absolutely perfect dial with
15 degree increments (offset properly for your position within the time
zone), and simply subtract or add the equation of time values from what
the clock displays. Another is to make the equatorial dial moveable--with
a small index mark labeled with positions corresponding to the equation
of time on various dates, say one for every month or every two weeks.
The sundial instructions would then read:
"Rotate the dial clockwise or counterclockwise
until the index mark matches the date. Then the clock will read the
corrected time."
The pattern
of dates will resemble a squashed analemma of the correct size in relationship
to the circle--for example, a 15 minute offset would be 15 minutes*360
degrees/(24 hours x 60 minutes) or a little less than 4 degrees.
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.
-- Here's another couple of ideas on this topic.
Not having tried it I can't say if it would work --but you might also
try mounting a mirror at 45 degrees to the gnomon and sighting the north
star by looking in the mirror. Removes the gnomon being attached
to the base problem.
Or if you own a telrad (a device which amateur astronomers use to point
telescopes, sort of like a virtual gunsight), it has a flat bottom and
you could just set it on the gnomon and sight the north star off of that.
The telrad has a bullseye which can be used to estimate the position of
the north star relative to the true celestial pole.
Another way you might consider is doing everything else as carefully
as possible --using a level to make the base perfectly horizontal, compensating
for the equation of time, having the latitude precisely set--and simply
rotating the base of the sundial until it reads the correct time; therefore
it must be pointing north.
Last, is there any significant
difference between the north star and true north,
--already answered--
and do I need to adjust for this?
I would say that depends on how accurate your sundial is, and that
in turn depends on how large it is. For book-sized sundials I wonder
if they would be precise enough to worry about...for something the size
of a small table perhaps it would be.
Thanks in advance, Rod
HeilEnthusiast
