Originally from ticket #4821.
I am using SNP 4.5.2. on an older iMac. How is the image of the
crescent moon determined and displayed when the moon is about 12 hours
from new phase. When I calculate the position angle of the bright
limb, should that point coincide with the center of the illuminated
edge of the moon? I am considering topocentric, and measuring from
the grid in "Surface Guides".
-Jim
Jim,
Not sure I understand the question. We calculate the position of the Moon using standard astronomical algorithms. Are you concerned that it's reporting an incorrect position?
The position of the moon is not the question. The "illuminated" edge
of the moon is what I am questioning. How do you "illuminate" that
edge? Is the center of the "illuminated" part coincident with the
position angle of the bright limb? It is hard to visually discern
where that center is when the moon is really close to the sun. Would
I be safe in assuming that my calculated PABL would be at the center
of the "illuminated" edge?
-Jim
I guess this is tough to go through without something to look at. Can you post a screenshot that you have annotated in some way to clarify?
- Dave
Look at ANY 12 hour old moon. You will see a thin white edge on the
side facing the sun. I simply want to know how you determined 1)
Where to put that white edge, 2) How long the arc is, and 3) Where the
center point of the edge is.
-Jim
It would really help if you sent a screenshot and described what your end goal is.
Are you asking for a description of how this would be done in general? Do you want me to explain our source code? If you need these values for something you're working on, maybe I can help you get them, rather than explain how we calculate them.
We determine the position of the sun, the position of the earth, then calculate what part of the moon will be illuminated based on the relative positions.
- Dave
On Apr 5, 2012, at 1:20 PM, Simulation Curriculum Corp. wrote
> It would really help if you sent a screenshot and described what
> your end goal is.
>
Picture 2.png
The end goal is to determine where the center of the illuminated part
of the moon is.
> Are you asking for a description of how this would be done in general?
Since I can't tell by looking at the screen, I wanted to know how you
determined this. Then I could duplicate. For instance, if you assume
that the center of the illuminated crescent is at the position angle
of the bright limb, and you use the standard formula to determine
that, then I can find it. If you use spherical geometry to line up
the illuminated edge for the observer, then if you give me those
equations, I can find it. If you use something else, and you explain
it to me, then I might be able to find it.
> Do you want me to explain our source code?
This would not be necessary, but if you can't explain what method you
are using, then it might help.
> If you need these values for something you're working on, maybe I
> can help you get them, rather than explain how we calculate them.
The values will change with time and location, so only the method for
obtaining the values would be helpful.
> We determine the position of the sun, the position of the earth,
> then calculate what part of the moon will be illuminated based on
> the relative positions.
>
This much I assumed, but since you are illuminating part of the moon
when it has an elongation with the sun of less than 7 degrees, and the
geometry of Danjon's Limiit does not permit ANY illumination below
that elongation, you must be making other assumptions. It would help
me if I knew what they were. You are correct in illuminating the moon
below Danjon's Limit, since there have been at least 3 sightings below
that limit, but what criteria are you using (or ignoring)?
-Jim
Jim,
I think the assumption that we're making is that the Moon is a smooth sphere, and so we show the idealized view that is Danjon's starting point:
http://the-moon.wikispaces.com/Danjon+Limit
We setup the 3D solar system, the Sun is considered a point source with parallel rays, and the amount of illumination is calculated via spherical geometry knowing the positions of the Earth, Moon, and observer. I'm not sure I can be more clear than that without walking you through the hundreds of lines of source code that do those calculation.
Knowing that, I'm not sure there's an easy way to use Starry Night to find the exact "centre" (although you still haven't defined what that actually means) of the illuminated part of the limb. Do you want this in terms of "sky coordinates" (e.g. where it is in RA/Dec) or in "Moon Coordinates" (e.g. lat/long on the moon).
All that being said, if you need this number precisely, just google "moon limb position" and there are several formulae out there that can get you what you want.
Jean Meeus has a great chapter on it in his book "Astronomical Algorithms" that provides several formulae to help.
There is also this paper although it's pretty high-level: Computation of the Quantities Describing the Lunar Librations in The Astronomical Almanac
