LRW, on 05 January 2013 - 02:23 PM, said:
Of course the people who depicted such a system of a star and planets knew the earth was round.
There should not be any doubts about it.

If that was actually the solar system, then I'd tend to agree.
However, what's shown there is an important star - likely Venus (Ishtar) surrounded by less important stars (probably some constellation that Venus is passing through.)
Seals like that one often include date stamps, which is what that depiction actually is. It indicates the time of year of the transaction recorded by the cylinder seal. Or, possibly, the time of year that the vignette of the two standing figures and the seated figure (a god, you can tell by his "flounced" clothing and the style of his hat) supposedly took place.
While the star in the seal admittedly has only six points, it still matches the iconography for Venus quite well. The iconography itself was often used for any star/planet that was being emphasized in the pic, not just Venus, and the depictions of stars usually had eight points, but there are many examples of those with six points and even seven points. That's the convention they used - there simply no question about it, and if you want to claim otherwise, you'll need to back it up (somehow) with (at least) textual evidence to the contrary.
Here's how they depicted Venus:
That's Venus at left, with the Moon in the center and the Sun at right. Please note - since the seal you posted is only a few inches long, finer details such as the lines often included in the "rays" coming from the star that you can see above in the Venus icon would be impractical to replicate on such a small thing (and unnecessary in any event.)
Like I stated, other stars/planets were depicted in a similar way to that of Venus:

The above seal, however, is not Sumerian. IIRC, it's Assyrian. You can tell by the shape of the winged disk at left. That is, if I'm remembering the iconography correctly, and I'm likely to be wrong here, not being an expert. The winged disk icon evolved over the several centuries it was used in Mesopotamia.
At any rate, you can see eight-pointed stars at left and round, plain stars at right in this seal. The round stars represent the Pleiades in this case.
Note in my first pic that the Sun is depicted as a disk (of course.) See the wavy lines the Sun is emitting? Sumerians depict the Sun in that way in every single depiction they made (that we've found.)
Here's another one of the Sun:
And sketched here for more clarity:
Obviously, the star in the center of the depiction is not the Sumerian representation of the Sun.
The seal itself is the record of a transaction between a mason/bricklayer and some person named Dubsiga. We know this from the writing on the seal.
Harte
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