Jamieson, Fausset and Brown on Acts 1:9-11

9-11. while they beheld, he was taken up--See on Lu 24:50-53. Lest it should be thought He had disappeared when they were looking in some other direction, and so was only concluded to have gone up to heaven, it is here expressly said that "while they were looking He was taken up, and a cloud received Him out of their sight." So Elijah, "If thou see me when I am taken from thee" (2Ki 2:10); "And Elisha saw it" (Ac 1:12). (See on Lu 9:32.)