"as they watched." The apostles "were looking" at Jesus because Jesus rose into the sky. Alternate translation: "as they were looking up at the sky" (See: [[rc://en/ta/man/translate/figs-explicit]])
This can be stated in active form. Alternate translation: "he rose up into the sky" or "God raised him up into the sky" (See: [[rc://en/ta/man/translate/figs-activepassive]])