I think this modeling explanation is as close as you'll get to a conceptual view of the question (without mathematics or technical terminology). Here are some of the thoughts I've had about this type of visualization problem.
1. You could take a purely observational view and say, Haven't you noticed that the sun is higher in the summer than in the winter? If the sun is higher, aren't sundial shadows shorter? Then of course the EOT, which the position of a sundial shadow taken at the same (clock) time of day for a year, must have a vertical displacement to show this. This of course avoids the question of **why** the sun is higher in the summer; but that isn't necessary from a purely empirical view. 2. One thing I think is a big hurdle for people learning this for the first time is they have difficulty relating the physical and mathematical models to what they see personally in the sky (or on the ground--shadow-wise). It is important to note that the observer is "at the dot" and sees what a tiny person (like an ant on the globe) would see. This is the same kind of mental leap as was made by a student of mine several years ago when I explained that the illustrations of the solar system in textbooks are confusing because they are drawn as concentric circles, with the observer far above the plane of the solar system. The observer, however, is **in** the plane of the solar system, so the proper way to look at one of these drawings is to lay your face on the book where the earth is marked and look around: you don't see concentric circles; you see planets lined up in a single path surrounding you (the ecliptic). In the analemma case you must also visualize that your observations are on a plane tilted at the same angle as the earth's rotation axis. Once you are convinced of this, the vertical motion of the sun in the analemma (which is the horizontal axis in the EOT) becomes more apparent. An analemma is an EOT folded in half. Astronomy educators agree one of the best ways to introduce orbital relationships is to have a large scale model in which the participant is one of the celestial bodies (sun, moon, earth). Bel Murru wrote: > The second is that the plane of the earth's equator is inclined tot the > >plane of the earth's orbit. > >Please can anyone explain me the second cause so that I can conceive it. I > >am not a astronomer! > > > > If you have a globe that's tilted 23.45 degrees from vertical in its stand, > and you spin it, that's its rotational plane, the plane of its equator. > > If you move the globe around the table on its stand, that's the orbital > plane. > > It spins on one plane, and revolves around the sun on another. > > A way to see it - Put a light bulb in the center of the room. Take your > globe, holding the base level (parallel) to the floor, and the globe roughly > in line with the bulb. Spin the globe and walk around the bulb. This is the > interaction of the two planes. > > Now put a dot on your location on the globe. Point the edge of the frame > (the degree circle holding the globe) to the east. Keep it pointed east as > you walk around the bulb. You'll notice that from your location on the > globe, the bulb/sun would appear sometimes low, sometimes middle, sometimes > high in the sky depending on where you are in your orbit. > > If you were to make 365 steps around the bulb, and pinpointed the line of > sight to the bulb from your position on the globe at your putative noon > (perpendicularity to the day/night dividing line on the globe) you would > trace a line of a certain length parallel to the earth's axis on the side of > the globe. The ends of the line are the southern and northern limits of the > sun's declination, the solstices. The middle of the line is the equinox. > > If you could vary your speed accurately as you walked around the bulb in an > ellipse and pinpointed your noon - quicker towards the minor axis and slower > towards the major, you would find a figure eight instead of a line. This is > the analemma. Many globes have it traced already at the right declinations, > at noon on the International date line. > > Hope this helps. > > Ross Caldwell > ______________________________________________________ > Get Your Private, Free Email at http://www.hotmail.com -- =-=-=-=-=-=-=-=-=-=-=-=-= [EMAIL PROTECTED] Jeff Adkins Location: 38.00 N, 121.81 W CA, USA, Earth, Sol III
