Many attempts have been made to date the monuments by means of astronomy. All these start from the assumption that it was erected in connection with the worship of the sun, or at least in order to take certain observations with regard to the sun. Sir Norman Lockyer noticed that the avenue at Stonehenge pointed approximately to the spot where the sun rises at the midsummer solstice, and therefore thought that Stonehenge was erected to observe this midsummer rising. If he could find the exact direction of the avenue he would know where the sun rose at midsummer in the year when the circle was built. From this he could easily fix the date, for, owing to the precession of the equinoxes, the point of the midsummer rising is continually altering, and the position for any year being known the date of that year can be found astronomically. But how was the precise direction of this very irregular avenue to be fixed? The line from the altar stone to the Friar's Heel, which is popularly supposed to point to the midsummer rising, has certainly never done so in the last ten thousand years, and therefore could not be used as the direction of the avenue. Eventually Sir Norman decided to use a line from the centre of the circle to a modern benchmark on Sidbury Hill, eight miles north-east of Stonehenge. On this line the sun rose in 1680 B.C. with a possible error of two hundred years each way: this Sir Norman takes to be the date of Stonehenge. Sir Norman's reasoning has been severely handled by his fellow-astronomer Mr. Hinks, who points out that the direction chosen for the avenue is purely arbitrary, since Sidbury Hill has no connection with Stonehenge at all. Moreover, Sir Norman determines sunrise for Stonehenge as being the instant when the edge of the sun's disk first appears, while in his attempts to date the Egyptian temple of Karnak he defined it as the moment when the sun's centre reached the horizon. We cannot say which alternative the builders would have chosen, and therefore we cannot determine the date of building. Sir Norman Lockyer has since modified his views. He now argues that the trilithons and outer circle are later additions to an earlier temple to which the blue-stones belong. This earlier temple was made to observe "primarily but not exclusively the May year," while the later temple "represented a change of cult, and was dedicated primarily to the solstitial year." This view seems to be disproved by the excavations of 1901, which made it clear that the trilithons were erected before and not after the blue-stones.