Some minor spoilers to Riven as well...

Investigations into the Book of Atrus

So often in my work I'm reading an ancient story or account for things the author wasn't trying to say--the anachronisms that point more to her own time than the tale she's telling, the underlying assumptions, the omissions of ignorance or antipathy, the little details that shed a flicker of candlelight on the long-vanished world with which the author assumed his readers were quite familiar.

No wonder then that I 'read' Riven even as I played it, noting the overlapping of a wood-based, nature-oriented religion (the Jungle Temple, the Moiety Stones) with a more severe, obedience-and-punishment religion where the symbols were fashioned of bone, ceramic, and metal (the whark gallows, the whark throne, all the grills and cages). The children's game told me volumes. Little details like that made Riven the easiest of the three games for me to figure out.

Now I'm turning my archaeologist eye to the Book of Atrus, to see what's hidden there. Can we tell where the cleft really was? Things about D'ni technology? Anna's knowledge?

Where are they living? The answer may lie in the stars.

The Book of Ti'ana is needed to answer this question fully, but here's some scraps I've found.
Analysis:
Africa has an ocean "hundreds of miles to the east". The Middle East does not: it has Asia.

I don't believe the constellations were picked at random; the planetarium in MYST shows that star charts and accurate astronomy are dear to someone's heart over in Miller-land.

Praesepe (M44) is a large star cluster--like the Pleiades-- covering an area roughly the size of the moon. Its stars are much fainter; in a clear sky it looks like a sort of fuzzy smudge. It is located in Cancer the Crab.
The "marker star" is some major point of reference. If it's Polaris, the North Star (the most likely), then they can't be south of the Equator. (doesn't help us much since we guessed that already). There are a few other very bright stars it might be like Capella or Sirius, but most of those don't mark anything.
The hunter is most certainly Orion: it has a distinct belt and bow or sword, recognized as a hunter or hero by cultures from Greece to Egypt to many native American peoples. It isn't visible from late May through early August at all.
First quarter moons rise during the daytime and set before midnight; the evening star (Venus) also appears in the west. So this is the west.
I can't find any particularly obvious line of stars between Orion and the western horizon that he could "look past," but there are scattered dim constellations over there. Might be Pisces.
Cancer is to the left of Orion (when he's standing up) by a couple of moon-widths. Nice bit of accuracy: the ecliptic, the plane of the solar system through which moon and planets pass, cuts right through Praesepe (because of course that's the line of the zodiac, "Venus in the house of Scorpio and all that).
To get Cancer over on the western side of the sky, you need it to be mid April through June. But Orion is below the horizon and not visible, it can't be past around April 15 because after that Orion sets before sundown, and is "lost". So this has to be perhaps the second week of April.
This particular description occurred on Atrus' 7th birthday! So he's an Aries. That fits very well; so am I.
I was doing all this to try and find some constellation not visible south of a certain latitude. These are all on the southern horizon though. If I'm reading my charts right, Orion's knees are cut off to the if you're farther north than 30 degrees latitude. 30 degrees skirts neatly along the top of Africa; Cairo is exactly at 30 degrees. Unfortunately the ocean to the east is actually southeast, here; too far north and you've got Saudi Arabia and Asia to the east.
If all my suppostions above are correct, they have to be between 30 degrees north and the equator, probably closer to the equator than not. Yep, that's the Sahara all right, which goes about 30 degrees to 15 degrees north. Countries from west to east at that latitude you find Senegal (too far west, I'm sure), Mauretania, Mali, Algeria, Niger, Libya, Chad, the Sudan. Algeria through Chad seem most likely to me as a place that would get weather from the Indian Ocean as opposed to the Atlantic.