I haven't posted for a while, but felt I might be able to contribute to this question. I love my 1916 edition of Funk and Wagnalls Dictionary. It retains some old word usages. It gives a common and an astronomical meaning for "emersion", the latter being "The reappearance of a heavenly body after being eclipsed or occulted". So I would agree with Chuck that it means when Io exits from the shadow. I think the word order in the sentence might be confusing, and I think the word "that" was a typo for "than". I think he was referring to a point in time, rather than a length of time. From reading the following sentence "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", I would rewrite it to say "the emersion of the first satellite observed at Paris last November at 5:35:45 GMT, was 10 minutes later than that observed in the month of August". Bob Grant