That period seems very close to the combined times of Io being in Jupiter's shadow and behind the planet's disk itself, in a specific orientation (Jovian system orientation with respect to earth). You might see if it matches up on the night Romer made the timings. On Dec 30, 2007 12:27 PM, Kurt Fisher <fisherka@csolutions.net> wrote:
I am trying to figure out what orbit event of Io is referred to in the 1677 paper reporting Romer's experiment measuring the speed of light. Romer's estimate was confirmed by the Royal Academey measuring "the emersion of the first satellite observed at Paris the 9th of November [1676] last at 5 [hours], 35 [minutes], 45 [seconds] at night, 10 minutes later . . . that had been observed in the month of August." quoted at Bartusiak, Archives of the Universe at 120. Checking my 2008 RASC Galiean satellite table, I cannot find any combination of Io orbit events that correspond to a period of nearly 5:35:45.
Anyone have any idea what this "emersion" refers to?