Albert Barnes Commentary 2 Kings 20:9

"And Isaiah said, This shall be the sign unto thee from Jehovah, that Jehovah will do the thing that he hath spoken: shall the shadow go forward ten steps, or go back ten steps?" — 2 Kings 20:9 (ASV)

Ten degrees—literally, "ten steps." It is perhaps not entirely certain whether the "dial of Ahaz" (2 Kings 20:11) was a true dial with a gnomon in the center and degrees marked around it, or a construction for marking time by means of "steps." While sundials had been invented by the Babylonians before the time of Herodotus, the instrument mentioned here was likely a set of steps, or stairs, with an obelisk at the top. The shadow of this obelisk would descend or ascend the steps as the sun rose higher in the sky or began to set.

The question of how the shadow's return was produced has prompted many different opinions. Recently, some have argued that the true cause of the phenomenon was a solar eclipse, in which the moon obscured the entire upper portion of the sun. It has been clearly shown that if such an event occurred shortly before noon, it would produce the described effect. During the obscuring of the sun's upper part, shadows would noticeably lengthen, causing the obelisk's shadow to descend the stairs. As the eclipse passed, the reverse would happen: shadows would shorten, and the obelisk's shadow would once again move back up the steps. If this is the correct explanation, the miracle would lie in Isaiah’s supernatural foreknowledge of an event that the astronomy of the age was completely incapable of predicting. It would also lie in the providential guidance of Hezekiah’s will, leading him to choose the very "sign" that was about to occur naturally.