Telescope Reviews: Observing Jura Mountains.

Welcome to the "Lunie Bin", Peter!

Were you observing Sinus Iridum just as the Moon was about to set? At such times the strong atmospheric refraction (which varies with wavelength) can stretch bright patches of light on the Moon -- or, in other phases, the entire bright crescent of the Moon -- into little rainbows of spectral colors, just as you can see if you look at Venus or a very bright star when they are very close to the horizon. But since this effect arises in the Earth's atmosphere, the quality of the telescope really has very little to do with it -- the rising/setting Moon looks no more or less colorful to a modern observer than it would have to Riccioli.

I don't know if Riccioli actually observed the Moon much himself or not. As I understand it, he mostly added labels to a map made by his friend Grimaldi. But yes, all the telescopes of that day (1651) were refractors with small singlet objectives, so that is what Grimaldi would have used. One characteristic of such telescopes is that the observer sees a halo of purple light around bright features, but the visual effect is much less striking than most people imagine, and, unless the telescope is badly out of alignment (or the object on the horizon), it doesn't look much like a rainbow. For an example of how a digital camera sees the color at the lunar terminator imaged with the modern equivalent of a very old telescope look here (the image on the right is the Sun photographed with a mylar filter over the objective); and for additional color photos of bright stars and planets here (notice that the camera sees lots of color, but the fringing is not usually obvious -- and the human eye sees even less).

I suspect your original assumption is correct: Riccioli was probably thinking more about the overall arc shape of the mountains that border his "bay", than of any color he may have observed there at sunrise. He probably also liked the play of words of putting his Rainbow on the shore of his Mare Imbrium (= Sea of Rain or Showers).

Regarding a time-lapse movie sequence showing sunrise over the Montes Jura, I don't know if there is one or not. I did recently start to a assemble a series of internet-posted still photos showing the changing appearance of Sinus Iridum as the sun rises over Montes Jura. They are displayed in order of increasing sun angle. But that's not quite the same as a movie.

Keep in mind that over the 7 or 8 hours that this phase is visible from a given location on a given night the sun angle over Sinus Iridum changes by only about 2°; so a single photographer can capture only a small part of the sequence. Capturing the "complete" sunrise sequence for a particular lunation would take a coordinated effort by astrophotographers on different continents.

-- Jim