One of the many amazing facts about the three pyramids of the Giza Plateau in Egypt, built almost 5000 years ago, is that they are aligned remarkably accurately to the cardinal points. How do we know this? Mostly because, um, a few people measured them about a century or more ago, and we just keep repeating what they said.
But no longer do we have to rely on old, possibly inaccurate measurements! Archaeologists Clive Ruggles and Erin Nell undertook a week-long survey of the famous pyramid complex, aiming to clarify the data concerning the main three pyramids' orientation, and also determine the orientations "of as many as possible of the associated structures" surrounding them. To do so, the pair departed from the usual method of using the corners of the buildings, and instead identified a series of points along the best preserved structural segments of each side.
The result? Nell and Ruggles found that the pyramids truly were aligned very accurately to the cardinal points, that there was "only a very slight difference in orientation" between the north-south axes of the two larger pyramids at Giza, those of Khufu and Khafre (approximately 0.5 arc minutes), and that the sides of Khafre's pyramid are actually more perfectly perpendicular than those of the 'Great Pyramid' of Khufu.
But perhaps the most interesting discovery was that the east-west axis of both pyramids was even closer to "true cardinality" than the north-south axis. For many years there has been some debate as to whether the alignment of the pyramids was executed by sighting the circumpolar stars of the northern sky, or via the Sun (using noon shadows or rising point on the equinox). This debate has also sometimes been associated with a debate over Egyptian culture of that time being centered around a 'solar cult' or a 'stellar cult', with a possible change from stellar to solar between Khufu and his son Khafre (note the 're' on the end of the latter's name, denoting the Sun).
Nevertheless, Nell and Ruggles concluded that the main pyramids were probably aligned using the circumpolar stars. However, they also noted from their data that the "broader context of associated structures suggests that the east-west orientation in relation to sunrise or (in one case) sunset may have been a, or even the, key factor in many cases."
Read: "The orientations of the Giza pyramids and associated structures", published in the journal Archaeoastronomy.