Acts 1:10

And while they were looking stedfastly into heaven as he went, behold two men stood by them in white apparel.

Of course, "heaven" as used here merely means that they were looking upward, not that they actually saw Jesus entering into the heaven of heavens which is the place of God's throne. And, as Bruce observed:

We need not be alarmed by suggestions that the ascension story is bound up with a pre-Copernican conception of the universe, and that the former is therefore as obsolete as the latter. Anyone appearing to leave the earth's surface must appear to spectators to be ascending.[16]

Two men ... in white ... These were angels, so identified from their dazzling apparel, as frequently spoken of in Scripture (Matthew 28:3; John 20:12).