Why do aircraft appear lower?

[mikehoran]

When there is a line of aircraft on approach to an airport on a clear night they appear in an arc. The ones farthest away, although at a higher altitude, appear lower. Is this due to the curvature of the earth?

Thanks!

[brachiopod]

Not really, it's due to perspective.

If you had a line of skyscrapers of equal height, the tops of those further away would appear increasingly lower than those closer to you.

Even if the buildings were of increasing height the further away they got, the effect would be the same. Ie a house nearby looks taller than a skyscraper in the distance.

[qapmoc]

Well, yeah..., but the horizon is only about 3 miles away at eye level and some of the aircraft are a lot further away than 3 miles so there must be a large effect related to the earths curvature to factor in also.

[sddsddean]

'fraid not qapmoc, its all due to perspective as brachy says. At 3 miles, the curvature of the Earth only accounts for a 6 feet height difference. I seem to remember reading somewhere that the horizon at eye level is 16 miles away. Even at this distance the height difference is only 170 feet.

[mikehoran]

Thanks you all! Can I ask for some clarification though so I can understand. This is how it might look:

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* *

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The second and third "aircraft" are probably at least 10 and 20 miles away respectively but they appear higher. Why do these not appear lower? On a clear night where I live I guestimate that the farthest plane out I can see is about 30-50 miles. Is there no significant curvature of the earth over a distance like this?

Thanks again,

Mike

[mikehoran]

Sorry,

My diagram didn't work once i sent it. It's supposed to look like an arc.

Best,

Mike