MarkBour wrote:This is pretty neat, and certainly a beautiful composition. Is there a name for this, as an astronomical diagram?
An analemma? We only see the tip.
Thank you! That word may be the one to use. The most basic meaning of analemma seems to be a tracing of the Sun's position in the sky from a given point on the Earth at a given time of day over the period of a year. (After which it "repeats", although there is probably subtle movement if you watch long enough?) Wikipedia suggests the term may be expanded; they have, in the article https://en.wikipedia.org/wiki/Analemma
, "Although the term 'analemma' usually refers to the Earth's solar analemma, it can be applied to other celestial bodies as well." I cannot tell from just this short sentence in what sense they mean "other celestial bodies", though. I found online computed images of the analemma of Sol from other planets
, such as one from a point on Mars that was a prior APOD: http://apod.nasa.gov/apod/ap061230.html
I'm looking for a term for a diagram of the points in the sky (above or below the horizon) for any celestial object as seen from a given point on Earth at a given time of day over the course of time. Perhaps analemma can be used for this for any object. The APOD: http://apod.nasa.gov/apod/ap050713.html
seems to be using analemma in just this sense, when it considers "an analemma of the Moon". Although apparently the image in that particular APOD was constructed somewhat differently.
So, much like the current APOD suggests, if we traced the points for an object such as Mars or Mercury from a point on the Earth such as Stonehenge, I assume we would get a far more involved shape than we do for the Sun's analemma. I haven't seen any such observationally-constructed diagram for a planet anywhere, yet, though this APOD is closely related. I think such a diagram would have been the foundation for discussion of such matters as the heliocentric model in ancient times. It would surely show those retrograde motions that were so vexing to natural philosophers.