Io disappeared into Jupiter's shadow and returned with enough regularity to serve as a distant clock. Yet its observed schedule accumulated a delay while Earth receded from Jupiter and recovered it while Earth approached. Cassini had considered a light-travel explanation; Rømer followed the pattern through and made a successful eclipse prediction from it.

The clock was not faltering. Its signals had different distances to cross. In the diagram, a longer line becomes a later arrival: the first quantitative evidence that light does not reach us instantaneously.

Rømer's report gave about 22 minutes for light to cross the diameter of Earth's orbit; the modern value is about 16 minutes 40 seconds. That was a travel-time estimate, not yet a published value in kilometres per second. Converting it into a speed also required a trustworthy scale for Earth's orbit.

The correction does not diminish the discovery. It sharpens it. Sunlight reaching Earth is already about eight minutes old; starlight may have travelled for years or millennia. 'Looking back in time' is the right astronomical consequence, not a claim that nearby touch, sound, and sight all share one perceptual delay.

Rømer's diagram turns astronomy into correspondence. Io enters shadow; light carries that event outward; Earth receives the news later. The farther the source, the older the news.

Calling this 'seeing the past' is not merely poetic. It describes the mechanism of observation across space. But the diagram does not prove a single cosmic present hidden behind the signals, nor that human knowledge is sealed inside an absolute bubble. Its quieter gift is enough: every image has a journey, and the journey has a duration.