I stated (_Dawn of Astronomy_, p. 319) that Spica was the star the heliacal rising of which heralded the sun on May-day 3200 B.C. in the temple of Menu at Thebes. Sirius was associated with the summer solstice at about the same time. Mr. Penrose found this May-day worship continued at Athens on foundations built in 1495 B.C. and 2020 B.C., on which the Hecatompedon and older Erechtheum respectively were subsequently built, the warning star being now no longer Spica, but the cluster of the Pleiades rising, or Antares setting, in the dawn. It is generally known that Stonehenge is associated with the solstitial year, and I have suggested that it was originally connected with the May year; but the probable date of its re-dedication, 1680 B.C., was determined by Mr. Penrose and myself by the change of obliquity. Now if Stonehenge or any other British stone circle could be proved to have used observations of warning stars, the determination of the date when such observations were made would be enormously facilitated. Mr. Penrose and myself were content to think that our date might be within 200 years of the truth, whereas if we could use the rapid movement of stars in declination brought about by the precession of the equinoxes, instead of the slow change of the sun’s declination brought about by the change of the value of the obliquity, a possible error of 200 years would be reduced to one of 10 years. In spite of this enormous advantage, no one so far as I know has yet made any inquiry to connect star observations with any of the British circles. I have recently obtained clear evidence that some circles in different parts of Britain were used for night work and also in relation to the May year, which we know was general over the whole of Europe in early times, and which still determines the quarter-days in Scotland. If the Egyptian and Greek practice were continued here, we should expect then to find some indications of the star observations utilised at the temple of Min and at the Hecatompedon for the beginning, or the other chief months, of the May year. I have found them, and I will now show the method employed. To begin with, if we assume that the astronomer-priests here did attempt such observations, what is the most likely way in which they would have gone to work? The easiest way for the astronomer-priests to conduct such observations in a stone circle would be to erect a stone or barrow indicating the direction of the place on the horizon at which the star would rise as seen from the centre of the circle. If the dawn the star was to herald occurred in the summer, the stone or barrow itself might be visible if not too far away, but there was a reason why they should not be too close; in a solemn ceremonial the less seen of the machinery the better.