January 7, 1909] 



NA rURE 



28- 



SIRV EYING FOR ARCH.-EOLOCISTS.' 

 VI. — The Finding ok Dates. 

 (i) By Solstilial /Ui^uiiien/x. 

 T N the astronomical study of ancient monuments, the 

 A archasoiogist's measures of azimuth and altitude 

 enable him to dcterjiiine the declination of the celestial 

 bodies the rising and setting places of which are 

 indicated by the direction of avenues or of outstanding 

 stones seen from the centre of a circle. 



But this, after all, is but the means to an end; it is 

 only a first step. 



The second step is to find, if possible, from the 

 declinations, the time at which the sun or a star 

 occupied these declinations. This tells us when the 

 " ancient " stone monument was set out, and because 

 the monument is an ancient one it is certain that the 

 declination of the sun at a solstice and that of the 

 stars were dilTerenl from what they are now. I will 

 deal with the sun first. 



In consequence of causes which need not be gone 



ibliqui'y Years 



into here, the angle between the plane of the earth's 

 equator and of the ecliptic — called the obliquity of the 

 ecliptic — is getting smaller. The result is that the sun's 

 declination at a solstice, which defines the value of the 

 obliquity, is less now than it was in times past. 



This rate of change is very slow, as will be gathered 

 from the diagram — Fig. 25 — a little more than half a 

 degree in 4000 years. The present value is 23° 27' ; in 

 1680 B.C., the date of the erection of the sarsens at 

 Stonehengc, according to the measures made by Mr. 

 Penrose and myself, it was 23° 55'. 



Now in these latitudes this change of half a degree 

 in declination produces a greater change in the 

 azimuth. In a previous diagram I have given not only 

 the solstitial azimuth at the present day, in lat. 50° N., 

 but also that of 1680 B.C., showing that there is a 

 difference of nearlv one degree; still, this is not certain 

 of detection considering monument conditions. 



Hence, in attempting to deduce a definite date from 

 a solstitial alignment, favourable conditions of the 

 monument, such as the avenue at Stonehenge, and 



1 Continued from vol. Ixxviii., p. 574. 

 NO. 2045, VOL. 79] 



exceedingly careful observations are absolutely 

 essential. Any others are practically valueless, 

 because, as will be gathered from the curve, Fig. 25, 

 an error of only 10' in the derived declination pro- 

 duces an error of some 1300 years in the date. 



It is only the solstitial alignment that can help us, 

 in consequence of the sun then arriving at the extreme 

 declination. .An equinoctial alignment is of no use, 

 because_ with any value of the obliquity the sun's 

 declination at the equinox is always 0°. 



From May-November alignments it is impossible to 

 derive any date, owing to the rapidity With which the 

 sun's declination changes at those seasons of the 

 year — more than a quarter of a degree each day. 



The only serious attempt so far to derive a date by 

 an alignment to the solstice, using the change in the 

 obliquity of the ecliptic, was made by Mr. Penrose 

 and myself at Stonehenge, but there is little doubt 

 that as our knowledge of the monuments increases 

 other alignments as definite as the avenue at Stone- 

 henge will be found. 



The conditions of observation at 

 Stonehenge will be gathered from 

 F"ig. 26, in which the line drawn 

 through the centres of the naos, circle 

 and vallum, and passing to the north 

 of the Friar's Heel, represents the com- 

 mon direction of the avenue and «f the 

 axis of the temple. 



(2) By Stellar Aliginnciits. 

 In previous notes I showed how with 

 certain data, including a measured azi- 

 muth and altitude, the declination of 

 the star which rose on the alignment 

 indicated by the monument could be 

 found. Having this declination, the 

 next step is to inquire Vv'hich star occu- 

 pied that position in times past, and 



In dealing with stars, the problem 

 of finding a date is much more within 

 the possibility of observation than in 

 the case of the sun. The stars change 

 their declination 47° in 25,800 years, 

 that is, 1° in 550 years on the average, 

 and some stars at some times change 

 it much more rapidly. 



This relatively very great change in 

 the declination of stars from centurv 

 ^ '' to century is brought about by the 



action of the sun and moon. 

 The action referred to does not depend upon the 

 actual attractions of the sun and moon upon the earth 

 as a whole, which arc in the proportion of 120 to i, 

 but upon the difference of the attraction of each upon 

 the earth's bulge at the equator, arising from the fact 

 that the equatorial diameter is the larger. As the 

 sun's distance is so great compared with the diameter 

 of the earth, the differential effect of the sun's action 

 is small ; but, as the moon is so near, it is so con- 

 siderable that her precessional action is three times 

 that of the sun. 



An important result of the action on the protuber- 

 ance has now to be considered. The change in the 

 position of the equator caused by the attraction is 

 brought about by a rolling motion, which is neces- 

 sarily accompanied by a change in the earth's axis. 



In Fig. 27, ab represents the plane of the ecliptic, 

 CQ a line perpendicular to it, hfe the position of the 

 equator at any time at which it intersects the plane 

 of the ecliptic in c. The position of the earth's axis 

 is in the direction Cp. When, bv virtue of the pre- 

 cessional movement, the equator has taken up the 



