There is suggestive evidence that some windows at Pueblo Bonito were used for calendrical purposes. Two second-story rooms on the Southeast end of the building have corner windows opening to the Southeast. Corner windows or doors are not common architectural features in ancient pueblo buildings; only seven can be found in the 650 rooms of Pueblo Bonito.
From the vicinity of Pueblo Bonito, in the months preceding the winter solstice the eastern horizon provides adequate features for an horizon calendar only until the end of October. It is about then that the rising Sun begins to shine through the window onto the West wall of the room. Earlier in the season sunlight does not shine directly through the window into the room, because of the significant thickness of the walls (see slide 5, figure 3, inset [A]). On the slide, the sunlight seen through the window illuminating the West wall of the room shines from above, where the ceiling used to be; this photograph was taken in mid-morning on 21 June 1997. In the following weeks the light beam widens as it moves across to the NW corner of the room, eventually casting a square patch of light on the North wall just against the NW corner.
From end of October to winter solstice, over seven weeks the beam moves a linear distance of 1.5 meters across the W and N walls, amounting to an average displacement of about three centimeters per day. This is in all likelihood sufficient to allow accurate anticipatory measurement of the winter solstice. The main uncertainty concerning the calendrical role of the two SW-facing corner windows is of an architectural nature. The rooms are not located on the outermost row of rooms in Pueblo Bonito, but one row within. If the outer row of rooms was also two stories high, then the view out of the corner windows to the winter solstice horizon would evidently have been obstructed (unless larger, properly positioned corner windows also existed in the second story outer rooms). Whether by direct examination of the remaining walls or of the field notes of the archeologists having carried out the first excavations at Pueblo Bonito, there is no strong evidence for either the presence or absence of a second story on the outer row of rooms.