the letter D on a standard QWERTY keyboard.
I'm south of the common, yet found by the middle.
Found almost directly below the commonest letter E, and in the middle row of the keyboard.
I'm due east of one article, and in the middle of another.
Due east of the letter A, which is an article in English. It's kind of on the way between A and N in the article "an" too.
The earth is north, west, and east of me,
The letters E, A, R, T, H are either above D or to the left and right of it.
but outer space is more southwest.
The space bar is found below it and to the right.
Mine is from the north to the east;
Yours is from the west to the northeast.
The letters M, I, N, E are found in the north and east, while the letters Y, O, U, R, S are found in the west and northeast.
Hers is 5342; his is 562.
This describes the fingers used to type each letter (similar to the position of the letter keys in each row): H, E, R, S use respectively the 5th, 3rd, 4th, 2nd fingers, and H, I, S the 5th, 6th, 2nd.
Riddle and Puzzling are 463373 and 85117654
Same deal again here. Note that we're using the standard (?) positioning of fingers for touch typing as seen here:
This was a very good riddle. The clues seem abstruse at first, suggesting many different possible directions of solution, but once you see the right answer everything falls neatly into place to confirm it. That's exactly how a riddle should be. The second line immediately made me think about "articles" in the linguistic sense (a/an/the), so I was thinking of letters (maybe H in the middle of THE?), but how to define directions? Then the giveaway was the fourth line, which made me think of the right meaning of "space".