This is really the icon of the Ascension, the earliest icon (though in a manuscript), that also established the iconography of the feast.
An icon does not depict a historical event, but a liturgical event: it is the liturgical meaning that the iconographer is trying to capture, not an historical scene.
The liturgical meaning of the Ascension is expressed in one of the verses for the kontakion (a verse sermon) of the feast, composed by Romanos the Melodist (c.500–after 555), just a few decades before the making of the sixth-century Rabbula Gospels:
When you had fulfilled your dispensation for us, and united things on earth with things in heaven, you were taken up in glory, Christ our God; in no way parted, but remaining inseparable, you cried to those who loved you: I am with you, and there is no one against you. (Lash 1994: 195)
The meaning of the Ascension for Christians is that everything Christ came to do has been fulfilled, that he is received into glory, without being separated from those who loved him, who have the assurance of his enduring presence with them. Christ departs, but his presence, his blessing (cf. Luke 24:51), remains.
In the icon, there is a clear separation between Christ ascending and the apostles who remain on earth, and yet that separation is not division—angels receive Christ; there are angels, too, with the Mother of God and the apostles. In this icon, Mary, the Mother of God, is always depicted at the centre of the apostolic band (though Acts is not so explicit), and depicted in prayer, in the orans position, for it is prayer that bridges the distance between heaven and earth. Nor is the apostolic band ‘historical’, because the apostle Paul is clearly depicted, opposite Peter, for this is the apostolic church in nuce, led by the leaders of the apostles, Peter and Paul.
Lash, Ephrem (Trans.). 1994. ‘On the Ascension, prelude 1,’ in Romanos the Melodist, On the Life of Christ: Kontakia (New York: HarperCollins)
19 So then the Lord Jesus, after he had spoken to them, was taken up into heaven, and sat down at the right hand of God. 20And they went forth and preached everywhere, while the Lord worked with them and confirmed the message by the signs that attended it. Amen.
50 Then he led them out as far as Bethany, and lifting up his hands he blessed them. 51While he blessed them, he parted from them, and was carried up into heaven. 52And they returned to Jerusalem with great joy, 53and were continually in the temple blessing God.
6 So when they had come together, they asked him, “Lord, will you at this time restore the kingdom to Israel?” 7He said to them, “It is not for you to know times or seasons which the Father has fixed by his own authority. 8But you shall receive power when the Holy Spirit has come upon you; and you shall be my witnesses in Jerusalem and in all Judea and Samarʹia and to the end of the earth.” 9And when he had said this, as they were looking on, he was lifted up, and a cloud took him out of their sight. 10And while they were gazing into heaven as he went, behold, two men stood by them in white robes, 11and said, “Men of Galilee, why do you stand looking into heaven? This Jesus, who was taken up from you into heaven, will come in the same way as you saw him go into heaven.”