How far away can I see an aircraft?

Efftup

Senior Member.
I think the answer to this question might help people understand better what they are seeing in the sky.

I live in Bristol in the UK.
the road I live on is a few degrees off a straight North/South direction, so my lounge window is pointing slightly south West.
I was looking out of my window and saw a plane making a contrail slightly more south west of my position.
I then checked on flight radar 24 without actually measuring exact angles etc, but the sky was quite empty at that point, and there was only one plane that could possibly match up, and it was flying near Cardiff, approximately 26 miles from my location. It did look really tiny and I would definitely not have seen it without the trail behind it.

I then a few minutes later saw another one which APPEARED to be at the same angle and the only plane that matched up on Flight Radar 24 was over Barnstaple(at about 36,000ft) approximately 32 miles away and more to the South.

Considering just how hard it is to judge angle and distance if you aren't used to it, is this normal?
or is it possible I was seeing Flybe jets that have no ADS-B transponder and only show up on MLAT
data if you are lucky or military planes that won't show up anyway?

They did seem to correspond to the North/South air corridor that takes a lot of traffic down to Southern Europe.
 

scombrid

Senior Member.
I can see Miami-Dallas flights leaving trails over the Gulf or Mexico from Cocoa Beach when it is good and clear in the lower atmosphere but airplane exhaust is condensing at cruise altitude.
 

Efftup

Senior Member.
If I am in my flat on the first floor, do I need to work out how far up I am to get a good angle? or is that negated by the fact it is 6 miles up and my height is neglible?
 

Efftup

Senior Member.
I also don't have a smart phone so will have to use an old fashioned protractor to get my angle.
 

Hevach

Senior Member.
If I am in my flat on the first floor, do I need to work out how far up I am to get a good angle? or is that negated by the fact it is 6 miles up and my height is neglible?
If you're one floor off the ground, that's around 9-12 feet depending on the building. Compared to somebody on the ground outside, that's at most a 0.04% difference in your relative distances to a plane flying at 30,000 feet, when it's overhead and the relative difference is at its greatest (if it's a few miles down range, that 12 feet ends up having even less .

Unless for some reason you want to determine the plane's altitude to an accuracy finer than your height, you can basically ignore it. Chances are using a protractor your margin of error will be much larger than the 12 feet between you and the ground anyway.
 

Efftup

Senior Member.
Yeah, I kind of guessed that. any error in the angle will be greatly magnified over a distance of 20 miles or more.
 

Mick West

Administrator
Staff member
I recommend getting a theodolite app for your mobile device:


The angle from the horizon is generally a lot less than you think it is.
 

Trailblazer

Moderator
Staff member
I think the answer to this question might help people understand better what they are seeing in the sky.

I live in Bristol in the UK.
the road I live on is a few degrees off a straight North/South direction, so my lounge window is pointing slightly south West.
I was looking out of my window and saw a plane making a contrail slightly more south west of my position.
I then checked on flight radar 24 without actually measuring exact angles etc, but the sky was quite empty at that point, and there was only one plane that could possibly match up, and it was flying near Cardiff, approximately 26 miles from my location. It did look really tiny and I would definitely not have seen it without the trail behind it.

I then a few minutes later saw another one which APPEARED to be at the same angle and the only plane that matched up on Flight Radar 24 was over Barnstaple(at about 36,000ft) approximately 32 miles away and more to the South.

Yes, you can see planes (or at least, the trails they are making!) 50 miles or more away under good conditions.

I took this picture a couple of weeks ago around sunset:
turn1.jpg

That distinctive turn made it easy to find on FlightRadar24:
turn2.jpg

Measuring on Google Maps, the turning point was almost exactly 40 miles from my house. So at a guess, the point where the trail disappeared into the cloud was more like 60 miles away.
 

Mick West

Administrator
Staff member
Try the attached file in Google Earth.

You will need to position the target over your location
https://www.metabunk.org/sk/Google_Earth_2014-07-30_12-25-40_2014-07-30_12-25-44.jpg

then


Then when you get down to ground level, switch to 3D building mode to get out of street view.



(Drag the slider in the middle here)


(The address in the images above is just random, where I happened to land when I flew down from orbit in GE)
 

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Efftup

Senior Member.
I recommend getting a theodolite app for your mobile device:


The angle from the horizon is generally a lot less than you think it is.
as I already mentioned, I don't have a mobile device (i.e Tab or smart phone) If you would like to buy me one with all that money you get for being a paid shill (lol) then feel free. To be honest, now I have seen what THOSE planes look like at those distances, if I see other planes, experience will help me get a rough idea. If I ever do get a better phone, I think an app like this would be great.
 

Jason

Senior Member
Ah, then instead try a piece of paper, some string, and a rock:
http://www.mathsisfun.com/activity/how-high.html

Thats a great method Mick, but you could also use a ruler or a stick. Simply hold the ruler perpendicular to the surface of earth (straight up), and start the protractor "up-side down" at the 90 degree mark. It won't be 100% exact but it will give you a rough estimate. When you tilt the protractor up to line up with the observed object in the sky, you want to make sure the center of the protractor always remaines in the same spot. And you will be able to read it the same was as above. In case you don't have string and a rock.
 

Mick West

Administrator
Staff member
Thats a great method Mick, but you could also use a ruler or a stick. Simply hold the ruler perpendicular to the surface of earth (straight up), and start the protractor "up-side down" at the 90 degree mark. It won't be 100% exact but it will give you a rough estimate. When you tilt the protractor up to line up with the observed object in the sky, you want to make sure the center of the protractor always remaines in the same spot. And you will be able to read it the same was as above. In case you don't have string and a rock.

Problem with that is you are eyeballing "straight up", and you are likely to be around >5° off. The rock and string gives you a guaranteed vertical, accurate to a degree.
 

WeedWhacker

Senior Member
Simply hold the ruler perpendicular to the surface of earth (straight up)

As noted, it's hard to get it accurately vertical. You could attach a small bubble level to the ruler, as a suggestion to achieve plumb. Making certain the level is attached at exactly 90° to the ruler.
 

Jason

Senior Member
As noted, it's hard to get it accurately vertical. You could attach a small bubble level to the ruler, as a suggestion to achieve plumb. Making certain the level is attached at exactly 90° to the ruler.
I agree with you and @Mick West 100% if you're wish is to be extremely accurate, and as I stated above, using a stick or ruler isn't accurate but if you're the type that likes to "eye" things up, then it could work for you.

And the method Mick discussed above has been around since the time of the pyramid construction.
 

Mick West

Administrator
Staff member
Always up for some backyard science, I made a theodolite.


Aimed it at the top of a utility pole:


Used my thumb to "capture" the data. 50 degrees, so 40 degrees above the horizon.


Captured the same view with the theodolite app. 42 degrees, pretty close
 

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Efftup

Senior Member.
that's a really cool simple method. Now I have to see if I actually DO have a protractor buried somewhere.
 

Mick West

Administrator
Staff member
I'm lost. so if I determine a 5 degree angle (contrail to horizon) what is that telling me?

Contrails generally form at about 6 to 8 miles up. (Roughly 31,000 to 42,000 feet, although they can form higher or lower). Six miles is a good rule of thumb. So if you assume the contrails is 6 miles up, and you know the angle, then you can calculate how roughly far away it is.

If it's 5 degrees, and 6 miles up, then it's 68 miles away.

http://contrailscience.com/how-far-away-is-that-contrail/
 

Mick West

Administrator
Staff member
I've created a circular version of the grid, which is more useful for visualizing distances from the viewer. The circles are at 5 mile intervals. The altitude of the circle is 11,000m (36,000 feet)

Open this in Google Earth
https://www.metabunk.org/attachments/concentrc-5-mile-kml.8250/





Notice the 100 to 200 mile range (between yellow and orange, basically the region under the bridge. That is all over Belgium.


The picture at the horizon changes if you gain a little altitude:
 

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