THE sun's sroTs. 73 



give separate rotations of 24d. 28m. and 26d. 46m. Our 

 knowledge of the actual period of the rotation of the Sun can 

 therefore only be regarded as the mea?i of a large number of 

 observations of those maculae, which, from their permanence 

 of form, and invariability of position in reference to other co- 

 existent spots, may form the basis of reliable observations. 



Although solar macula) may be more frequently seen by the 

 naked eye than is generally supposed, if the Sun's disk be at- 

 tentively observed, there yet occur not more than two or three 

 notices of this phenomenon between the beginning of the ninth 

 and of the seventeenth centuries, on the accuracy of which we 

 can rely. Among these I would reckon the supposed reten- 

 tion of Mercury within the Sun's disk for eight days, in the 

 year 807, as recorded in the annals of the Frankish kings, 

 first ascribed to an astronomer of the Benedictine order, and 

 subsequently to Eginhard ; the 91-days transit of Venus over 

 the Sun, under the Calif Al-Motassem, in the year 840 ; and 

 the Signa in Sole of the year 1096, as noticed in the Stain- 

 delii Chronicon. I have, during several years, made the 

 epochs of the mysterious obscurations of the Sun which have 

 been recorded in history — or, to use a more correct expression, 

 the periods of the more or less prolonged diminution of bright 

 daylight — the subject of special investigation, both in a mete- 

 orological and a cosmical point of view.^ Since large num- 



* Although it can not be doubted that individual Greeks and Romans 

 may have seen large Sun-spots with the naked eye, it is at all events 

 certain that such observations have never been referred to in any of the 

 works of Greek and Roman authors that have come down to us. The 

 passages of Theophrastus, DeSignis, iv., 1, p. 797 ; of Aratus, Diosem., 

 v., 90-92 ; and of Proclus, Parapkr., 11, 14, in which the younger ldeler 

 (Melcorol. Veterum, p. 201, and in the Commentary to Aristotle, Meteor., 

 torn, i., p. 374) thought he could discover references to the Sun's spots, 

 merely imply that the Sun's disk, which indicates fine weather, exhib- 

 its no difference on its surface, nothing remarkable (/z?/oe rt afjpa <pepoi), 

 but, on the contrary, perfect uniformity. The of//ua % the dappled sur- 

 face, is expressly ascribed to light clouds, the atmosphere (the scholia.-: 

 of Aratus says, to the thickness of the air); hence we always hear of 

 the morning and evening Sun, because their disk, independently of all 

 Sun-spots, are supposed, even in the present day, according to an old 

 belief, not wholly unworthy of regard, to give notice to the farmer and 

 the mariner, as diaphanomelera, of coming changes of weather. The 

 Sun's disk, on the horizon, gives an indication of the condition of the 

 lower atmospheric strata which are nearer the Earth. The first of the 

 Sun-spots noticed in the text as visible to the naked eye, and falsely re- 

 garded in the years 807 and 840 as transits of Mercury and Venus, is 

 recorded in the great historical collection of Justus Reuberus, Vetere& 

 Scriptores (1726), in the section Annates Regum Francorum Pipini, 

 Karoli Magni et Ludovici, a quodam ejus cctatis Aslronomo. Ludovici re- 



Vol. IV.— D 



