Kudos to The Mac Observer‘s John Martellaro for dusting off his geometry and calculating the diameter of the circle that would circumscribe the Pentagon’s five 921-foot sides. Answer: 1,566 feet.

But he went astray when he eyeballed the floor plans for Apple’s (AAPL) proposed new headquarters — made public last week by the Cupertino city council — to estimate the diameter the main office building. He came up with 1,615 feet “plus or minus a few feet.” That’s the diameter he used to draw the image below that shows the headquarters of the U.S. Department of Defense slipping neatly inside Steve Jobs’ “spaceship” with room to spare.

Martellaro’s diagram was widely reposted Tuesday despite the fact — pointed out by several of his readers — that the outer radius of the Apple’s proposed headquarters building (listed on page 2 of the floor plan), is actually 760 feet and 7 inches. That makes the diameter of Apple’s HQ a bit over 1,521 feet — or nearly 94 feet smaller than than the Pentagon’s circumscribing circle.

Martellaro’s comparison goes even further astray when you start comparing the area and office space of the two buildings. According to published reports, the Pentagon has double the office space and houses twice as many workers. See the tale of the tape above.