Acts 1:9

“And when he had spoken these things, etc.] That the times and seasons were not to be known by them, but to be kept a secret by the Father: that they should tarry at Jerusalem, and in a few days be baptized with the Holy Ghost, and receive such power, abilities, strength, and courage thereby, as to bear a noble testimony for Christ, not only there, but in all the world; and when he had given them a fresh commission, and told them where they should go, what they should preach, and what miracles they should perform, and blessed them. While they be held; all the Oriental versions, add, “him”; that is Christ, while they looked wistly at him, being attentive to what he said to them, so that they…”

“while they beheld, he was taken up--See on Luk 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" (Kg2 2:10); "And Elisha saw it" (Act 1:12). (See on Luk 9:32.)”

“While they beheld - While they saw him. It was of importance to state that circumstance, and to state it distinctly. It is not affirmed in the New Testament that they “saw him rise” from the dead, because the evidence of that fact could be better established by their seeing him after he was risen. But the truth of his “ascension to heaven” could not be confirmed in that manner. Hence, it was so arranged that he should ascend in open day, and in the presence of his apostles; and that not when they were asleep, or were inattentive to what was occurring, but when they were engaged in a conversation that’ would fix the attention, and even when they were looking upon him. Had Jesus vanished…”

“when: Act 1:2; Psa 68:18; Mar 16:19; Luk 24:50, Luk 24:51; Joh 6:62; Eph 4:8-12 a cloud: Exo 19:9, Exo 34:5; Isa 19:1; Dan 7:13; Luk 21:27; Rev 1:7, Rev 11:12, Rev 14:4 Acts 1:10”