Can anyone tell me the meaning of these two lines in the lyrics?
I look up to the heavens
But night has clouded over
No spark of constellation
No Vela no Orion
A constellation is a pattern of stars in the sky. "No spark of constellation" means no stars can be seen. Vela and Orion are names of constellations, so Enya is emphasizing that the sky is pitch black. It may be important that Orion is visible in the northern hemisphere and Vela in the southern hemisphere, so the whole sky is empty.
As FumbleFingers pointed out, "spark" here is a bright point of light, like a star. And it might also be important that Orion is a hunter and Vela is the sails of a ship.
Further edit: After reflecting on FumbleFingers comment about Enya being Irish, I looked at the lyrics, and a few sites have it as Vega not Vela (it sounds like Vela to me; I haven't heard Vega pronounced with a long E). This would make some sense, since Vega is a very bright star in the constellation of Lyra (the Lyre) and Orion is also a very bright constellation. The Wikipedia article verifies that Vega is also a very important star culturally.
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