General Question

Ltryptophan's avatar

Is the sky transparent at night?

Asked by Ltryptophan (10710points) May 21st, 2016 from iPhone

How much light does it take before the sky becomes visible?

How high up do you need to be before you see stars.

Observing members: 0 Composing members: 0

16 Answers

Ltryptophan's avatar

i’ve been in a lightning storm at night that seemed to illuminate the sky as if it were day

XOIIO's avatar

…The sky is always transparent, what you are seeing is refracted light from the water in the air.

kritiper's avatar

I heard a story of a man who was digging a deep shaft in the earth. In broad daylight, when at the bottom of the shaft, he could see stars.

Rarebear's avatar

The sky is not completely transparent. That’s why stars twinkle, because air is moving around. But it’s mostly transparent. How high up do you need to see the stars? On the ground.

@kritiper That story is false.

kritiper's avatar

@Rarebear If stars twinkle because of what you say, why don’t planets?

Rarebear's avatar

Great question. Do you want the longer, more scientific answer, or the short one liner answer?

kritiper's avatar

@Rarebear Neither. It was rhetorical. OH, and BTW, I can see the moon during the day!

Rarebear's avatar

Yes. You can see the moon and sometimes even a planet. But not stars

XOIIO's avatar

@Rarebear Technically, you can see a star during the day, quite easily.

Rarebear's avatar

@XOIIO Well, yes. One star, obviously. Unless it’s cloudy out.

dxs's avatar

How are we defining sky? I thought the sky never went away, it’s just whatever we see when we’re outside and look up.

RocketGuy's avatar

@kritiper – planets would twinkle due to atmospheric disturbance just like stars, but they appear much brighter and wider than stars, so we don’t see any twinkling. See a more detailed explanation here:

RocketGuy's avatar

Look here for a graph of atmospheric opacity (opposite of transparency) vs wavelength:

As you can see, only certain wavelengths can get through. This assumes no clouds or other junk in the air. The higher you are, the less the available junk in the air. This is why the best ground-based observatories are at high elevation locations. To do better than that, there are airborne observatories: To do better than that, there are space observatories:

Brian1946's avatar


“planets would twinkle due to atmospheric disturbance just like stars, but they appear much brighter and wider than stars, so we don’t see any twinkling.”

Although it’s a satellite and not a planet, I guess the moon’s dearth of twinkle is an easily observable example of that.

Rarebear's avatar

@Brian1946 Basically apparent magnitude and angular size is what is important. The bigger and brighter something is, the less likely the object will twinkle.

Twinkling is caused by disturbance in the air in the atmosphere.
Wikipedia has a bit of a technical article about it.

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