# On which altitude does the heaven turns in black?

1. Jan 12, 2013

### nickek

Hi!
Just got a question from my 6 years old nephew who just travelled in an aeroplane for the first time. He wonders where (which altitude) the heaven turns in black? Someone who knows?

2. Jan 12, 2013

### mathman

I don't have any specific numbers, but the transition is gradual (not sudden) as you go higher.

3. Jan 12, 2013

### Chronos

Test pilots unanimously assert the sky appears black at 100,000 feet. Concorde passengers have claimed the sky appears black at 60,000 feet. The air pressure at 60,000 feet is about 1 lb/in^2, at 100,000 feet it is 0.162 lbs/in^2, at 250,000 feet it is 0.000 lbs/in^2. Obviously, the sky is indisputably black when atmospheric pressure reaches 0.000.

4. Jan 12, 2013

### nickek

5. Jan 12, 2013

### VantagePoint72

While there's no point at which the sky turns black, you could ask when has it turned black by (in the same sense that you can't say exactly when it gets dark at night, but you could say that it will happen sometime before, e.g., 9pm. At around 70,000ft it will be very dark, and it will be more or less black by 100,000ft.

Edit: Oops, looks like Chronos snuck in while I was typing. Though, Chronos, maybe I'm misremembering but I thought the Concorde passengers said it looked black if they looked up out of their windows. So, not necessarily ambient blackness.

6. Jan 13, 2013

### Straw_Cat

Nickek: That's a very bright question. You're nephew's one smart guy. :-)