intellegent-
That’s a good point – if there was no atmosphere, the brochos on lightning and thunder (no weather) shiluach hakan (no flying) would also be affected, among many others.
I was specifically asking about Shabbos and Yom Tov.
The answer is – there would be no bain hashmoshos. That means that Shabbos and Yom Tov would be shortened slightly for someone living on the equator, and by a larger amount the further one moved from the equator.
Currently, since the equator is moving fastest, its twilight period is shortest.
Atmospheric refraction is what causes the sky to remain somewhat light after the sun dips below the horizon (and lighten before sunrise), but without it daybreak and nightfall would be instant.
Beams of light traveling thru the air are only visible because of water vapor, smoke or dust in the air that they illuminate, which is what I think is what you meant (the moon, with no atmosphere, has a night and day).