In Thucidydas History of the Peloponnesian War, Thucidydas says in the first page of chapter 7, introducing the subject of the plague of Athens, that, “it first began, it is said, in the parts of Ethiopia above Egypt, and thence descended into Egypt and Libya and into most of the king's country.” Here he states that the the plague descended from Ethiopia to Egypt (which is to the north, not the south, of Ethiopia) so I am wondering as to how the ancient Greeks in the 5th century viewed their geographical position and if they had seen Africa as to the north, not the south, of Greece. And also is that king he is mentioning the king of Persia, or rather a local king (knowing that in that period Egypt was a Persian colony). Thank you.
The description of descent here has nothing to do with north and south, but the flow of the Nile river. (Indeed, it's not really clear to what extent ancient authors thought about the world with a northward orientation, but I digress.) Ethiopia is above Egypt because following the Nile it is upriver of Egypt. This is also why, even today, upper Egypt is south of lower Egypt. (We find the same logic elsewhere, for example Upper Canada is to the south of Lower Canada, since the St Lawrence has an at least partially north-ward route.)
This is made entirely clear, for example, in Strabo's Geography. The geography of Egypt and Ethiopia, in book 17, is centred around the geography of the Nile:
Since, in my description of Arabia, I have also included the gulfs which pinch it and make it a peninsula, I mean the Persian and Arabian Gulfs, and at the same time have gone the rounds of certain parts both of Aegypt and of Aethiopia, I mean the countries of the Troglodytes and the peoples situated in order thereafter as far as the Cinnamon-bearing country, I must now set forth the remaining parts that are continuous with these tribes, that is, the parts in the neighbourhood of the Nile... (17.1.1)
And, of course, conceptually the geographical description follows the direction of the river, which of course flows from south to north:
The Nile flows from the Aethiopian boundaries towards the north in a straight line to the district called "Delta"... (17.1.4)
This grounds the logic of up and down in these descriptions, hence for example when describing the Island of Meroë, Strabo notes explicitly that above refers to the south:
It is bounded on the Libyan side by large sand-dunes, and on the Arabian side by continuous precipices, and above, on the south, by the confluence of the three rivers — the Astaboras, and the Astapus and the Astasobas and on the north by the next course of the Nile, which extends to Aegypt along the aforesaid windings of the river. (17.2.2)
That this logic of ascending refers to the Nile, not the cardinal points, is made totally explicit in an earlier reference to Menelaus travelling up the nile:
Everybody who tells the story of his own travels is a braggart; to this class belonged Menelaus, who had ascended the Nile as far as Ethiopia (1.2.23)
This is also clear in Herodotus's description of Egypt:
For even though a man has not before been told it he can at once see, if he have sense, that that Egypt to which the Greeks sail is land acquired by the Egyptians, given them by the river — not only the lower country but even all the land to three days' voyage above the aforesaid lake [i.e. Moeris], which is of the same nature as the other, though the priests added not this to what he said. (2.5)