Naked Science Forum

General Science => General Science => Topic started by: latebind on 02/03/2010 10:24:32

Title: Why do planes look like they fly slow in the sky?
Post by: latebind on 02/03/2010 10:24:32

LOL

When I look up at the airlines flying way up in the atmosphere, I know that they are moving fast but it seems as though they are in slow motion because of my relatively large distance away from the plane.

Why does it 'seem' like they move slow?

Title: Why do planes look like they fly slow in the sky?
Post by: stereologist on 13/04/2010 05:29:20

In general we know that things close to us move faster than objects far away. We use this as a visual clue to determine distance. This is known as motion parallax. Imagine sitting in a car and looking at distant hills. A  line of fence posts near the road has the fence posts quickly snapping by. A line of fence posts far from the road do not snap by so quickly. The farther from the road the posts are the slower they appear to go by even though they go by at the same rate.

With the plane, the plane is in motion and you are stationary.

Title: Why do planes look like they fly slow in the sky?
Post by: latebind on 13/04/2010 15:26:47

I recently saw a plane fly directly over my house, and it was very close and very big in the sky (no more than 100 meters away) - this is because I have moved to a new place close to heathrow airport.

I can honestly say that the planes still look like they fly slow even when they are close by, albeit they have just taken off, but that requires at leat 400kph or something, and the plane looks like its doing a lazy 80kph at most...

Title: Why do planes look like they fly slow in the sky?
Post by: krytie75 on 14/04/2010 17:09:45

I think it might also have something to do with lack of reference points when you're looking at the sky.  In the same way that the moon and sun look bigger when they're low in the sky because you also have the horizon and anything between you and it in view, when you look at an airplane in the sky, there's usually no reference points close to the plane so this can further create the illusion that the plane is moving slowly.  I hope that makes sense!

Title: Why do planes look like they fly slow in the sky?
Post by: latebind on 15/04/2010 09:47:32

Yes that makes a lot of sense!

Title: Why do planes look like they fly slow in the sky?
Post by: LeeE on 16/04/2010 16:56:21

Quote from: latebind on 13/04/2010 15:26:47

I recently saw a plane fly directly over my house, and it was very close and very big in the sky (no more than 100 meters away) - this is because I have moved to a new place close to heathrow airport.

I can honestly say that the planes still look like they fly slow even when they are close by, albeit they have just taken off, but that requires at leat 400kph or something, and the plane looks like its doing a lazy 80kph at most...

A clearance of just 100 metres would indicate that you're about 1.91 km from the landing touchdown point on the runway (assuming a relatively low glideslope angle of ~3 degrees) and there do indeed appear to be residential buildings within this distance at Heathrow.  However, the angle at which airliners climb out is much steeper than 3 degrees, so I suspect that the ones that you're seeing overhead at ~100 metres are coming in to land and haven't just taken off (at 100 metres during take off, it would considerably louder than during landing too).  During take off, I'd expect most airliners to be well above 100 metres before they've passed the end of the runway, let alone passed over your new home.

Take offs are performed at a steeper angle for a couple of reasons: the more altitude that can be gained before a possible engine failure during take off means that the pilot has a greater degree of choice about what he's going to do about it (speed may be 'life', but altitude is life-insurance) but it's also used as a means of keeping the speed of the aircraft down while the undercarriage is retracted (excessive speed with the undercarriage still down may risk damaging the undercarriage and/or the undercarriage doors).

Typical take off speeds for airliners are around ~140 to ~155 kts (250kph - 290 kph), with typical landing speeds tending to be lower by ~5-10 kts.

I believe that the reason that large close things can appear to be moving slowly is similar to the reason why very distant small things appear to move slowly: you're trying to use an angular displacement to estimate a linear displacement.  When something large is close to you it occupies a large angular displacement, filling a large percentage of your field of vision, so even though it may actually be moving quite fast it'll stay in your field of vision for a relatively long time, which what you'd normally associate with slow moving things.