Arthur Carlson wrote:
>
> Near sunrise and sunset, atmospheric refraction displaces the image of
> the sun by about half a degree, which should show up as an error of
> nearly 1-1/2 min, dial fast at sunrise and slow at sunset. I would be
> interested in hearing if you can measure this effect. I suppose the
> 20 sec accuracy you reported was for measurements taken when the sun
> was reasonably high in the sky.
>
> Art Carlson
I started to keep a log of dates and time readings taken from the
analemmic-equatorial park sundial after it was installed. I watched the
sundial until the most-sharply defined edge of the gnomon's shadow
intersected exactly with a minute mark on the dial, and then checked the
time reading against my watch to see how the sundial reading compared
with the U.S. Naval Observatory Master Timeclock's time. All of the
dates and time readings I recorded are shown below, along with how much
the sundial was fast (+) or slow (-) compared to my watch, and any notes
I made during the readings.
5/04/98 1:30 pm +- 0 secs (O.K., I got lucky once), date exact.
5/05/98 7:30 am + 60 secs (fast), date O.K. I was only able to take
this one reading as the sun disappeared behind a bank of clouds.
5:50 pm +- 0 secs to +20 secs, date O.K. I watched the time
indicated by the gnomon's shadow vary rapidly from 0 seconds under
bright sun to +20 secs fast as thin clouds darted across the sun. Since
this sundial uses an analemma opening, more clouds = wider gnomon shadow
= smaller observed analemma shape = faster time observed (on this date
from the left side of the analemma pattern.)
5/06/98 7:10 am +- 2 secs (I checked it twice!) Date exact. Clear
sky.
5/07/98 12:45 pm - 10 secs, date exact. Clear sky.
5/15/98 10:53 am - 15 secs, date exact. Clear sky.
5/17/98 12:17 pm - 6 secs, date exact. Clear sky.
5/23/98 9:25 am - 30 secs, 3/4 day slow. Sun behind cloud fringe.
6/04/98 (Forgot to record time) -20 secs, date exact. Some thin cloud
fringe.
6/07/98 1:47 pm - 8 secs, date exact. Clear sky.
6/09/98 5:44 pm - 6 secs, date exact. Clear sky.
6/14/98 7:52 am - 80 secs (slow), date exact. Clear sky with only a
minimum of high light haze. What happened here?
2:30 pm - 8 secs, date exact. Light cloud wisps.
So there were three different occasions when I observed time readings
before 8:00 am... 5/05/98 @ 7:30 am, the sundial was 60 secs fast as the
sun disappeared behind thick clouds; 5/06/98 @ 7:10 am, the sundial was
within +- 2 secs under a clear sky; and 6/14/98 @ 7:52 am, the sundial
was 80 secs slow under clear sky with only a minimum of high light haze.
Bottom line, I have not yet been able to accurately see the atmospheric
refraction effect you described, caused by sunlight traveling obliquely
through an atmosphere of uniformly increasing density. However, I DID
see some large time and/or date inaccuracies, which I could best explain
by refraction of the sunlight through random cloud, temperature and
humidity layers. I think Boise has too much weather!
Pete Swanstrom.
