I'm not sure I understand your questions. Do anomalies you describe apply only to the Moon (and planets)? Or, are you asking about all celestial objects including remote things like stars, galaxies, etc.?
If you are only talking about nearby things, such as the Moon, then perhaps all that needs to be said is that locating the Moon's position on the sky in a computer program is complicated by the fact that the Moon orbits the Earth. Its orbit is a little bit eccentric and is tilted somewhat relative to the ecliptic plane. So, the Moon's position is constantly even in equatorial coordinates. If you are asking about all celestial objects including stars, then let's talk about stars. The equatorial coordinates of the stars do not change significantly over, say, 100 years. For practical purposes we can think of them as fixed. But, of course, their alt-az coordinates change moment to moment because the Earth is rotating. For example, Vega rises in the northeast about 6 hours later is overhead and then 6 hours after that is in the northwest. When it rises, depends on the time of the year and how far north of east depends on ones latitude. In other words, given a particular location on the celestial sphere, i.e., given particular RA and Dec coordinates, it is fairly straightforward to compute the corresponding Alt-Az coordinates. But, this calculation depends on the latitude and longitude of the observer as well as the time of day and the date. The horizon is given by Alt=0. The various values for Az determine the direction you are looking. Since CdC already knows how to compute Alt/Az coords for any location/time, providing a local horizon is easy. You just need to say how high the horizon is in each direction (for each Az). More comments below... On Jul 21, 2010, at 11:57 AM, luzius.thuerlemann wrote: > Thanks Robert for this answer. > Ok, than the world is again as it should be :) . Yes, the ecliptic changes > with precession and nutation over 25800 years, right. > But WHY does ist pretend to do a daily 25800-cycle? I mean, what needs to be > different from the polar projection except the coordinates which would be > given in Alt-Az instead of Ra/Dec? Why this tumbling around. > > Ok, here my problems: > 1. when I want to see if there are other coordinate data in the information > board for the moon in Alt-Az and Ra-Dec, and let the software search for the > moon in the toolbar, the coordinates are centered +00°00' and 24h00m00,0s (in > polar projection). But the moon is on the other end of the sky. How to fix > this. I don't understand this question. I just now fired up CdC and I typed Moon into the search box on the toolbar. The Moon has just set here and so CdC shows me the SW horizon and the center of the field of view is slightly below the horizon (I have CdC configured to start in Alt-Az mode). I currently have CdC configured to by default hide things below the horizon so I don't see the Moon. To see it, I click "Configure the Program-->Observatory" and check the "Horizon" tab followed by the "Show Object below Horizon" box. Okay, now I see the Moon. I click on its label to see "Details". The RA/Dec coords are 16h59m, -25d34m. The Alt/Az coords are -11d30m, +246d34m. The Alt/Az coords are completely consistent with the reality that the Moon has set in the SW about an hour ago. I'm not sure what you mean by "the coordinates are centered +00°00' and 24h00m00,0s (in polar projection)". I guess you mean that you are in Alt/Az mode looking straight up, right? If I click on the "Z" to give the zenithal view, I don't see the Moon in the default zoom level. But, if I zoom out, the Moon appears in the SW a bit below the horizon exactly where it is expected. > 2. on the 21. July 2010 at 00:15.00 the moon completely disappeared below the > south-western mountain-horizon. When I simulate this time then the moon in > Alt-Az-projection (because in polar alignment I can't see the mathematical > horizon) is already displayed below the mathematical horizon. What is wrong > here? It is not possible to simulate your what you see without also knowing your latitude/longitude. > > The difficulty is that I can't imagine to create a proper local horizon with > these obstacles - I mean, one evening the moon rises above a mountain and on > the following night the moon rises above the same horizon but is displayed > much below or above the already created horizon line from the previous night. Could it be that you have not set your longitude/latitude and/or timezone in the "Observatory" window? Lastly, I should point out that the Moon is currently about 4 degrees from the closest point on the ecliptic. > > Thanks very much for help! > > Luzius > > > > --- In [email protected], Robert Vanderbei <r...@...> wrote: >> >> The ecliptic does not move with respect to the celestial sphere (the >> background stars) over short time scales. Hence in equatorial mode it >> remains fixed. But the ecliptic and the stars do move from moment to moment >> and from day to day (at a given time of day). Hence, in alt-az mode you >> will see changes. Not sure if this is the source of confusion. Hope. It >> helps. >> >> Sent from my iPhone >> >> On Jun 28, 2010, at 11:05 AM, "luzius.thuerlemann" <luzius.thuerlem...@...> >> wrote: >> >>> Hi John >>> >>> Yesterday I realised that the ecliptic moves during a single day. Sounds >>> stupid, but I've never realised that before. I thouhgt that it changes only >>> during long periods over months and seasons etc. But over a single day?! I >>> know the change of the seasons and the motion of the sun over a year etc., >>> but I really cannot explain this motion over 24 hours. When I animate the >>> daily motion it looks to me as if it was an exaggerated motion of the >>> earth's precession. I just don't get it. >>> The seasonal changes etc. are no problem, but these not obvious >>> short-term-things seem to be. >>> >>> But this should not affect the creation of a local horizon in CdC. When I >>> write down the time when the moon just rises above or sets below the >>> horizon, and subtract the moon's radius from the local altitude, then I >>> should get the horizon altitude at the moon's azimut at that time and I can >>> simulate the horizon altitude there. >>> But my problem is that the setting moon was displayed much closer to the >>> mathematical horizon compared to the already simulated >>> rising-horizon-mountain in the south-east (it was as if the western >>> south-mountain is half or a third as high as the south-eastern one) - >>> although the two mountain's altitude would not have created such an obvious >>> difference...they're pretty much of the same height, and the observing >>> position was also the same. >>> >>> It's really strange. And all because of this strange >>> daily-eliptic-precession. I don't get it. >>> >>> >>> Thanks for your help! >>> >>> Luzius >>> >>> >>> >>> ------------------------------------ >>> >>> Yahoo! Groups Links >>> >>> >>> >> > > > > > ------------------------------------ > > Yahoo! Groups Links > > > Robert J. Vanderbei Princeton University http://www.princeton.edu/~rvdb "I'd trade all my tomorrows for a single yesterday." -- K. Kristofferson [Non-text portions of this message have been removed]
