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
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