Charles Ellicott Commentary Isaiah 38:7

"And this shall be the sign unto thee from Jehovah, that Jehovah will do this thing that he hath spoken:" — Isaiah 38:7 (ASV)

This shall be a sign to you... —The offer reminds us of that made to Ahaz, but it was received in a far different spirit. In 2 Kings 20:8–11, the story is more fully told. Hezekiah asks for a sign and is offered his choice: Shall the shadow go forward or backward? With a certain childlike simplicity, he chooses the latter, as the more difficult of the two.

The sun-dial of Ahaz, probably copied from Syrian or Assyrian art like his altar (2 Kings 16:10)—Herodotus (ii. 109) attributes the sun-clock to the Chaldeans—seems to have been an obelisk standing on steps (the literal meaning of the Hebrew word for dial), casting its shadow to indicate the time, with each step representing an hour or half-hour.

The nature of the phenomenon seems as curiously limited as that of the darkness of the crucifixion. There was no prolongation of the day in the rest of Palestine or Jerusalem, for the backward movement was limited to the step-dial. At Babylon, no such phenomenon had been observed, and one ostensible purpose of Merodach-baladan’s embassy was to investigate its nature (2 Chronicles 32:31). An inquiry into the causation of a miracle is almost a contradiction in terms, but the most probable explanation of the recorded fact is that it was the effect of a supernatural, but exceedingly circumscribed, refraction. A prolonged afterglow following the sunset, reviving for a time the brightness of the day, might produce an effect such as that described to one who gazed at the step-dial.