Albert Barnes Commentary Luke 1:78

"Because of the tender mercy of our God, Whereby the dayspring from on high shall visit us," — Luke 1:78 (ASV)

Whereby the dayspring, and so forth. The word dayspring means the morning light, the aurora, the rising of the sun. It is called the dayspring from on high because the light of the gospel shines forth from heaven. God is its author, and through his mercy it shines on men. There is undoubtedly here a reference to Isaiah 40:1-2; indeed, almost the very words of that passage are quoted. Compare also Revelation 22:16.