Hastings' Dictionary of the Bible (1898)
DI'AL, an instrument employed to measure time, or to determine the apparent progress of the sun by the shadow which the gnomon, or point in the centre of a graduated arc, casts. The "dial of Ahaz" is the only one mentioned in the Bible. 2 Kgs 20:11; Isa 38:8. The sign of Hezekiah's recovery was that the shadow of the sun went ten degrees backward upon it. " It was therefore probably modelled after those in familiar use with the ally of Ahaz, Tiglath-pileser. The fact that ambassadors came from Babylon to inquire of the wonder proves that the fame thereof had reached that city.
It is a question of considerable importance whether this miracle was wrought upon the rays of the sun, by which they were deflected in an extraordinary manner, so as to produce this retrograde motion of the shadow, while the sun itself seemed to go on its way, or whether the motion of the earth or the position of the sun was so changed as to produce this result. It was this miracle to which reference is made in 2 Chr 32:31.