Clarification of Question by
lcn2-ga
on
28 Jul 2005 17:27 PDT
Dear welte-ga:
You are on the right initial track, but simply posting the
(totality sky brightness / daytime sky brightness) ratio
won't yield an answer of the form that I need.
The answer needs to relate totality sky brightness to
the brightness of a cloudless/moonless zenith during
twilight by way of the Sun's position relative to the
horizon.
I believe this is one path to a successful and acceptable answer:
1) Determine the following:
1/V < (totality sky brightness / daytime sky brightness) < 1/W
NOTE: Perhaps excluding ultra short eclipses is
reasonable. Having been on the center line of a
very short totality, it was my perception that the
totality sky was brighter due to daylight scatter
up from nearby areas of the Earth that we outside
of totality.
NOTE: Assume that you are measuring totality brightness along
the centerline of the shadow so to maximize eclipse
duration and to minimize daylight scatter mentioned
above.
Here are some potential assumptions:
* brightness is being measured over the visible wavelengths
(this is a required assumption, actually!)
* the eclipse is of moderate to long duration (say > 2 minutes)
* the eclipse occurs far enough above the horizon so that
horizon effects may be safely ignored
* that observations are being taken from the centerline
of the shadow to minimize daylight scatter from areas
outside of the shadow.
Actions required: post 1/V, 1/W, site the references,
and
state any assumptions made
NOTE: Regarding references: Use your best judgment on how many
are needed. At least one good source is required.
Adding more good sources, up to a point, generates
a better answer tip! :-)
2) Determine if "4.5 to 10 degrees away from the Sun along the
ecliptic" and "general sky totality brightness" are essentially
equivalent
NOTE: If for some reason they are not, then you need to
somehow determine how much brighter or fainter one
is to the other.
NOTE: It may be safe to assume that the area 4.5 to 10 degrees is
far enough away from the typical visible corona. I
have never seen the visible corona extend much beyond
2-2.5 degrees. My own observations are supported
by figure 1 from:
http://homepage.oma.be/david/pub/2003_05_MIRA_LASCO/Brightness%20of%20the%20Solar%20F-Corona.htm
where they show the Eclipse sky dominating beyond
about 1.5 degrees.
So I'm guessing that one can determine that "4.5 to
10 degrees away from the Sun along the ecliptic" is
equivalent to the "general sky totality brightness".
NOTE: If we exclude any eclipse where 10 degrees from the
Sun would has significant horizon effects then we can
ignore daytime scatter from areas outside of the shadow
(i.e., where, during an eclipse, one often can often
observe a general fade from "night to day" -- the so
called 360-degree sunset effect) and we can ignore
brightness extinguishment near the horizon.
So in addition to the previous assumptions, we might add:
* 4.5 to 10 degrees from the Sun is far enough away
from the Sun to ignore the visible corona
* that the normal total eclipse sky dominates at 4.5 o 10
degrees from the Sun
* there is nothing special about the direction
of the ecliptic at the 4.5 to 10 degree range
Actions required: State assumptions (such as the above).
NOTE: Adding references to back up those assumptions, while
not required, would be a plus (and would yield
better tip :-)).
3) Determine the twilight conditions when:
1/V < (twilight zenith brightness / daytime brightness) < 1/W
Now we begin to equate twilight conditions to a range
of totality sky brightness conditions (see step 1 above).
So in addition to all of the previous assumptions, here are
some more potential assumptions:
* cloudless sky
* a moonless sky or that the moon's crescent is thin enough
to be safely ignored
* there are no very bright stars or planets near the zenith
* The level of light pollution is similar to that of the
eclipse site.
NOTE: Because people often select good dark eclipse sites,
one might assume both the Sunset site and the eclipse
site are relatively free from light pollution. So
one could restate the assumption as:
* Both the eclipse site and the twilight sites are
dark sites
* The elevation of the eclipse site and the twilight site
are similar
* The latitude of the twilight site is moderate (say 45N to
45S) so that arctic circle and antarctic circle effects
on path of the Sun (no midnight Sun, etc.) may be ignored
* Assume the the horizon in the direction where the Sun
is/has set is relatively clear (i.e., no major mountains,
trees, buildings, or other significant objects)
Actions required: state the twilight conditions, site the
references, and state the assumptions made
NOTE: Regarding references: Use your best judgment on how many
are needed. At least one good source is required.
Adding more good sources, up to a point, generates
a better answer tip! :-)
4) Translate the twilight conditions into the position of the
Sun relative to the horizon and generate a statement of the
form:
To simulate sky brightness conditions 4.5 to 10 degrees
from the Sun during totality, one may use the zenith of
twilight when the Sun is X to Y degrees below the horizon.
NOTE: I am suggesting "degrees below the horizon" to factor
out things like latitude, time of the year, etc.
NOTE: I am guessing that the conditions occur after Sunset.
If my guess is wrong, then give me a negative number
for X and/or Y! :-)
NOTE: If you have a better way to translate the twilight
conditions into a "mostly site independent statement",
feel free to ask me about your method.
NOTE: You most likely need to make assumption about what
"position of the Sun relative to the horizon" means.
It is the apparent position (i.e. due to Atmospheric
effects, when the center of the Sun is on the horizon,
it is physically about 90.5 degrees from zenith)?
It is the geometric position (ignoring the atmosphere)?
The point I am making here is that someone needs to
use an ephemeris / planetarium program (XEphem, Kstars,
TheSky, etc.), plug in their site parameters (Lat,
Long, Time, Elevation, etc.) and determine when the
Sun is between X and Y degrees below the horizon.
Actions required: post the statement, site references (if any
additional are needed), and state new
assumptions (if any additional are needed)
I hope this helps you to provide me with an answer. Pardon me
if I stated things that are obvious. Questions, suggestions and
corrections about the above are both encouraged and appreciated.
