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The Nebra Sky Disc has side-arcs which are interpreted as marking the extent of movement of the sunrise and sunset, during the course of a year. I think they can also be interpreted as marking the extent of movement of the ecliptic where it crosses the horizon, during the course of a single night (especially the winter solstice).

That's what it looks like when I simulate this in Stellarium, but I am looking for an authoritative statement of this (if correct), and I haven't been able to find one.

Am I correct? Anyone have a citation?

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  • $\begingroup$ An alternate explanation to the answer below: Since the Sun is on the ecliptic, the point where the Sun crosses the horizon is also a point where the ecliptic intersects the horizon. But the easier way is to use planetarium software, and you can visualize where the full ecliptic is at any point in time. $\endgroup$
    – Greg Miller
    Commented May 28 at 15:00

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Yes, the extent of movement of the sunrise and sunset during the course of a year is also the extent of movement of the ecliptic where it crosses the horizon during the course of a single day and night cycle.

The ecliptic is the annual path of the Sun on the celestial sphere, so the sunrise & sunset points on the horizon correspond to the points where the ecliptic crosses the horizon.

Here's an interactive 3D animated diagram which you may find helpful. The blue globe grid is the celestial sphere, with the polar axis labelled with N & S. The greenish-grey disc is the horizon plane, with the north-south and east-west axes in green. The red circle is the ecliptic. The letters around the ecliptic circle mark the Sun's location at the equinoxes and solstices. M is the March equinox, J is the June solstice, S is the September equinox, and D is the December solstice.

You can set the latitude, the number of animation steps, and the opacity of the horizon plane. Here's a typical frame.

Ecliptic on the celestial sphere

A high step number gives a smoother animation, but it takes longer to compute all the frames.

Here are the 3D interface controls.

  • Orbit - right mouse, or left mouse + ctrl/meta/shiftKey
    • touch: one-finger rotate
  • Zoom - middle mouse, or mousewheel
    • touch: two-finger spread or squish
  • Pan - left mouse, or arrow keys
    • touch: two-finger move
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  • $\begingroup$ The animation is great. It would be absolutely spectacular if I could replace the green-gray disk with a photo of the Nebra Sky Disk. $\endgroup$
    – DavidWalley
    Commented May 29 at 0:20
  • $\begingroup$ A little of the back-story: there is a debate about the meaning of the big circle on the disc. Some say it is the Sun, some say a full Moon. The evidence usually given is that the side-arcs mark the sunrise and sunset at the solstices, therefore, it must be the Sun. If the Sun, then that must be a "solar barge" at the bottom, so the disc is evidence of religious beliefs,... I think it is the full Moon, because, with your answer, everything on the disc can be related to the night sky (and the barge is the path of Vega kissing the horizon at True North seen from 51.3N on winter solstice). $\endgroup$
    – DavidWalley
    Commented May 29 at 0:43
  • $\begingroup$ @DavidWalley That would be tricky. The Sage 3D plotting facilities are designed for mathematics, not art, so it's rather tedious to import images, and the results generally don't look great. OTOH, Sage uses three.js as its 3D engine, which can do all sorts of interesting things, but Sage only exposes a tiny fraction of the capabilities of three.js. $\endgroup$
    – PM 2Ring
    Commented May 29 at 1:28

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