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? I am working up a star party cheat sheet on Romer's 1676 experiment measuring the speed of light using Jupiter's Moon Io. Romer used the 42.5 hour period of Io's eclipse entry into Jupiter's shadow as his remote timing indicator at Jupiter. 42 hours is an inconvienent measuring period to use for star party purposes, but is easier to measure since the change in light travel time from Jupiter varies by about 15 minutes throughout the year for that 42.5 hour interval. Shorter event periods could be used, such as those mentioned in the Paris confirmation observations above. For star party purposes, I will probably instead try the transit of Io across the face of Jupiter to replicate Romer's experiment. Although these are approximately 3 hour events and the time difference of Io's transit near conjunction and opposition is only about 1 minute, modern videorecording with time signals easily would have that precision - a level of precision beyond Romer in 1677. But if there is an Io orbital event with a 5 hour period, that might be more useable. Again, any idea what the term "emersion" refers to in the above quote from the 1677 report of Romer's observations? - Kurt _______________________________________________ Sent via CSolutions - http://www.csolutions.net