No less than three accounts are provided by biblical authors regarding the unusual sign that God gave Hezekiah to assure him that his life would be extended. Isaiah had first announced to the king that his life was near its end, but when Hezekiah pleaded for a reprieve, Isaiah returned and told him that God had heard his prayer and would add another 15 years to his life. As a sign confirming this prediction, Yahweh invited Hezekiah to decide whether the shadow on the time-marker would move forward or backward, and Hezekiah chose backward. The details of the story are recounted both by Isaiah and the compiler of the Kings record (Isa. 38:1-8; 2 Kgs. 20:1-11), while the account in the Chroniclers’ History is very abbreviated (2 Chr. 32:24). That the accounts in Isaiah and 2 Kings are for the most part verbatim in the Hebrew text has convinced most scholars that one account is dependent on the other (Isaiah is likely the earlier one).
There are, however, some lingering questions about the incident. First, there is the nature of the time-instrument itself. The Masoretic Text of both Isaiah and 2 Kings describes it as a marker with “steps”, which suggests that the instrument was some sort of device that cast shadows on a wall as the sun moved through its daily course. The great Isaiah scroll recovered from the first cave at Qumran, however (1QIsaa), describes it as “the upper dial of Ahaz”. This description presumes that the instrument was inherited by Hezekiah and might very well have been of Assyrian origin due to Ahaz’ fascination with Assyrian devices (cf. 2 Kg. 16:10-18). Either way, Hezekiah’s remark that he wanted the shadow to move backward, since it was too simple to have the shadow move forward, seems a bit strange. From a modern point of view, whether the shadow moved forward or backward seems sufficiently miraculous by all definitions!
So how did the shadow move? We can immediately dismiss the legendary story that scientists at the Goodard Space Flight Center in Greenbelt, MD, using a sophisticated computer program, discovered a missing 40 minutes in the astronomical record. The scientists at Goddard have repeatedly disclaimed this story outright, but nonetheless, it has been floating around for decades in one form or another. A version of it was first published as far back as 1936 by Harry Rimmer, who in turn cited an otherwise unknown 1890s source. It was picked up by a gullible Harold Hill and repeated in a new and improved version in his 1974 book, How to Live Like a King’s Kid. Now, with the advent of the internet, it is still making the rounds, albeit in an updated fashion (see the debunking at www.snopes.com/religion/lostday.asp). A much more plausible explanation is that the moving shadow was related to a total solar eclipse (a total solar eclipse is when the moon completely blocks the sun, creating a short period of darkness). We know, for instance, that just such an eclipse occurred in 702 BC, the 16th year before Hezekiah would eventually die (you can find it online at NASA’s Five Millennium Catalog of Solar Eclipses), and unlike the story by Harold Hill, this one is not bogus.
Solar eclipses can create strange visual phenomena. If a “steps” device in Hezekiah’s court was shrouded in darkness for a short period and then restored to daylight, the shadow might very well have seemed to go backward. A solar eclipse seems a more likely explanation than some disruption of the planet’s daily revolution. Such total solar eclipses are rare (the last one in America extending coast-to-coast was in 1918, but Hawaii had one in 1991 and some of the western states saw one in 1979). Indeed, should you be interested, you’ll be able to see another on August 21, 2017, and it will be from coast to coast, its path of totality extending from Lincoln City, Oregon to Charleston, South Carolina, passing through Idaho, Wyoming, Nebraska, Kansas, Missouri, Illinois, Kentucky, Tennessee and Georgia. Darkness will last longest at Carbondale, Illinois (a few seconds beyond 2.5 minutes).
At the end of the day (to use a rather loaded metaphor in the present context), I suppose the real question concerns the legitimacy of biblical prophecy more than celestial disruption. The moving shadow was a predicted sign to Hezekiah, and Isaiah was to be regarded as a true prophet. If you can believe in a God who knows the future, then it was not hard for him to inform Hezekiah that the steps of the shadow would move backward. Was this the result of a total solar eclipse? There is no way to know for certain, of course, but in my mind this is a satisfactory explanation.