The idea of a spherical Sun

[Hans]

-instead of a flaming disk.

When did this concept first surface?

[Hornblower]

-instead of a flaming disk.

When did this concept first surface?

My best guess would be the classical Greek era, starting a few centuries BC.

[Jens]

Apparently Chinese astronomers recorded sunspots in about 800 BC. If they did so, and notice that they moved, I can imagine that somebody would speculate that the sun might be a spinning sphere. Actually it occurs to me that not that many people might have even thought about it. I don't know if I would have thought about it unless I saw a model of the solar system at school or in some museum.

[George]

There may be several lines of evidence to serve people as early as the early Greeks, or earlier folks:

The one example case, the Moon, exhibits a spherical shape.
The planets don't go dark as would be expected when giving us an edge-on view of them.

[Hornblower]

It took a while for the Greek thinkers at the dawn of the Golden Age to come up with ideas about the Sun, Moon and stars that we take for granted now. The earliest ones, about 600 BC, envisioned them as openings in a celestial sphere which revealed fire behind them. It was another couple of centuries before they were envisioning the Moon as a nonluminous body illuminated by light from the Sun. They were developing sound geometry at this time, from which inference of a spherical shape was forthcoming. With their concluding that both the Earth and the Moon were spheres, it seems reasonable to guess that they would start seeing the Sun as likewise, although its appearance alone at the time would not preclude a flat disk that always faced the Earth.

[DaveC426913]

I recall that it was the observation of sunspots that did the trick.

The foreshortening near the edges will have made it quite obvious they were rotating on the surface of a sphere:

[Hornblower]

I recall that it was the observation of sunspots that did the trick.

The foreshortening near the edges will have made it quite obvious they were rotating on the surface of a sphere:

It is one thing for us to recognize that in hindsight with magnified images, but how much detail could ancient observers have seen with naked eye observations through murky air, let alone measured with any accuracy?

[DaveC426913]

It is one thing for us to recognize that in hindsight with magnified images, but how much detail could ancient observers have seen with naked eye observations through murky air, let alone measured with any accuracy?

My image is simply adding a visual to what I had read was the case.

All I can do is recall what I'd read: that large sunspots could be seen to start near one edge, cross the face over days, and disappear at the other edge.

[George]

My image is simply adding a visual to what I had read was the case.

All I can do is recall what I'd read: that large sunspots could be seen to start near one edge, cross the face over days, and disappear at the other edge.

Yes, this may have been why Kepler concluded that they were surface objects and not transits of planets, which was his proper use of objective evidence to counter his naked-eye earlier claim (1607) of a Mercury transit explanation for sunspots.

When telescopes (albeit small ones) were first put to use, there were disagreements about sunspots, especially the bitter ones between Scheiner and Galileo, who both claimed to be the first to discover them (telescopically, I assume). They were unaware of Fabricius' 22 page publication (Feb. 1611) on sunspots, which noted the western disappearance of sunspots and some that returned, which indicated surface rotation. I think, ironically, that it was Scheiner's excellent work that led Kepler to favor surface features. It should be noted that it was about 150 years later before observations were resolute enough to notice the perspective changes of sunspots across the disk (ie Wilson Effect).

[Hans]

Thanks for all the information - much appreciated