I don't really understand how it's drawn on sketches of the celestial sphere. At first I thought it goes through east and west, but it does not. How do you know where to draw it?
Example from Roy and Clarke, Astronomy: Principles and Practice:
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$\begingroup$ When you say "celestial sphere sketch" do you mean a sketch of an actual sphere with stars drawn on the outside of the sphere? Or do you mean a sketch of the flattened map where the x axis goes from -180 to +180 and the y axis from -90 to +90? If you are sketching an actual sphere, then imagine cutting through it with a plane tilted at 23 degrees from the equator. If you are sketching the flattened map, then draw a "sine-wave"-like line that goes up and down by +/- 23 degrees. $\endgroup$– uhohJul 2, 2019 at 7:57
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$\begingroup$ An actual sphere sketch. A plane 23 degrees through the equator, but where does it intersect the horizon? At East and West? $\endgroup$– T jeyJul 2, 2019 at 11:22
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1$\begingroup$ You've lost me. The plane is not through the equator, it is tilted with respect to the equator by 23 degrees, so it only intersects the sphere and and the equator at two opposite points. I don't know what "...where does it intersect the horizon?" means. The celestial sphere is just a sphere, it has nothing to do with a specific point on the Earth or a horizon. $\endgroup$– uhohJul 2, 2019 at 11:24
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$\begingroup$ Oops, I didn't double-check before adding the comment. $\endgroup$– T jeyJul 2, 2019 at 22:39
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$\begingroup$ What I meant was, where does it intersect the celestial horizon? The plane of the celestial equator (which would be ring-like in the sketch) goes through the east and west points. $\endgroup$– T jeyJul 2, 2019 at 22:42
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2 Answers
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The ecliptic is the plane of the Earth's orbit around the Sun. From the Earth's point of view, the Sun appears to migrate along the ecliptic, eastward one cycle per year. The ecliptic and equator intersect at the equinoxes. The Sun appears to cross the equator northward at the vernal equinox (♈) around March 20 and southward at the autumnal equinox (♎) around September 22.
Equatorial coordinates are linked to the equinoxes; ♈ is at right ascension 0h, and ♎ is at RA 12h. Where these points appear in the sky depends on the local sidereal time:
- ♈ rises in the east at 18h, crosses the meridian at 0h, and sets in the west at 6h.
- ♎ rises in the east at 6h, crosses the meridian at 12h, and sets in the west at 18h.
- At sidereal times between the above, apply a polar rotation of 15° per hour to interpolate.
Once you've determined where on the celestial equator to mark ♈ and ♎, then you can draw the ecliptic through them as another great circle 23.4° oblique to the equator. The ecliptic runs north of the equator east of ♈, and south of the equator east of ♎, in the sense that eastward is counterclockwise as seen from the north celestial pole.
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The ecliptic is the mean apparent path of the Sun throughout the year. Thus it is the plane of the Earth's orbit. The ecliptic crosses the celestial equator from south to north at the vernal equinox.
answered Jul 1, 2019 at 16:26