1. Mick West

    Mick West Administrator Staff Member

    A classic demonstration of the curvature of the Earth was performed by Alfred Russel Wallace in 1870:

    (Original source is Wallace's "My Life", Page 364-376)

    Put more simply, we have:

    A) A telescope (which we would replace with a digital camera with powerful zoom lens)
    B) A target 3 miles away
    C) A target 6 miles away, in line with A and B.

    A,B, and C are all the same height above the waterline. So you just look from A towards B and C, and if B is visually higher than C then the water surface is curved up in the middle.

    Rather surprisingly I can't find any well documented recreations of this experiment. It seems like it would be an ideal science project. There are significant difficulties though as you need a suitable stretch of water, the targets are 12 feet above the water, and the six mile distance makes it difficult to coordinate.

    But it's really a very straightforward demonstration. So what I'd like to do in this thread is discuss places where the experiment might best be carried out, and what procedures could be used to make the execution of the experiment as straightforward and foolproof as possible.
    Last edited: Jul 21, 2019
  2. Mick West

    Mick West Administrator Staff Member

    One idea to open up more locations would be to use a periscope at point A. You could make quite tall lightweight periscope with very simple cheap construction This gives you a lot more flexibility in terms on positioning the camera and targets.

    You could use some 1" PVC Schedule 40, which would also be good for the target supports. You could make a clip-together or telescoping arrangement of the PVC pipe for easy transport, even adjustable heights. It's $4 for a 10' section.
  3. Spectrar Ghost

    Spectrar Ghost Senior Member

    A salt flat might work. You need something very level, but it doesn't need to be water, neccessarily.
  4. Mick West

    Mick West Administrator Staff Member

    An ice lake might also work, like Balaton in the winter.
  5. Hama Neggs

    Hama Neggs Senior Member

    How do you prove a salt flat is level?
  6. Spectrar Ghost

    Spectrar Ghost Senior Member

    You have a point. Pointing at Topo Maps wouldn't work. Ice lake would be better. Of course, there's the effects of heaving there...
  7. Mick West

    Mick West Administrator Staff Member

    A series of three bridges such as in the original seems ideal, as you can use a simple plumb measure to get the height.

    The original Bedford Level is still there, and has been used to kind of recreate the useless "man in a boat" version of the experiment. But it's not clear if it's still suitable for the Wallace experiment. These seem to be the bridges:
    20160912-141904-gg5f7. (See attached Google Earth File)

    However the middle bridge is a railway bridge and does not seem to have access.

    Attached Files:

  8. Hama Neggs

    Hama Neggs Senior Member

    A flowing body of water will have a sloped surface. Else it wouldn't be flowing. There has to be some factor figured in for that.
  9. Mick West

    Mick West Administrator Staff Member

    That should not matter, so long as it's not curved.
  10. Clouds Givemethewillies

    Clouds Givemethewillies Active Member

    Cardinal marks make good targets if you can find one between two suitable bits of land. Runnel Stone.JPG
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  11. Mick West

    Mick West Administrator Staff Member

    A really long straight causeway could be a opportunity for multiple targets. Might be tricky though

    Seven Mile Bridge, Florida

    Pontchartrain causeway, Lousiana
  12. Bass In Your Face

    Bass In Your Face Active Member

    I live next to this lake. There are 4 possible places to do this, tell me what you think:





    The locations of the left hand side from the pictures, from top->down, are:

    -A dock/boat loading area
    -Beach/boat loading area
    -Beach/boat loading area
    -A small water inlet with a boat loading area

    The right side locations from the pictures, from top->down, are:

    -Accessible shore from road, or by boat
    -Accessible shore from road, or by boat
    -Accessible shore from road/park, or by boat
    -Accessible shore from road/park, or by boat

    Also, many people ice fish on this lake in the winter.. (and would probably ice fish in the summer if there was ice! Wisconsin!!!)
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  13. Mick West

    Mick West Administrator Staff Member

    It's hard to get a spot exactly in the middle. At this scale we can approximate the curve as a triangle, and for an offset midpoint at a distance m from the end (where m<d/2), then we can approximate the "bulge" as bulge*2m/d

    So for 5 miles, bulge is 4.17 feet, but if the middle marker were at 1.5 miles, then it 4.17*2*1.5/5 = ~2.5 feet.

    So a 5 mile span should still work, even if you can't get the middle exactly.
    Last edited: Sep 12, 2016
  14. Bruno D.

    Bruno D. Senior Member

  15. Mick West

    Mick West Administrator Staff Member

    Folsom Lake is plausible. There's a 4 mile stretch with a pretty centered spit of land (depending on the water level) 20160912-171849-9vwfr.

    The middle is only really accessible by boat though.
  16. Mick West

    Mick West Administrator Staff Member

    It's hard measuring from a water surface in a canal, etc. But if you are on a sloping shore, then you can make the targets of fixed pre-measured height with a nice solid base, and simply place them with the base on the waterline. Would need some adjustment for slope though
  17. JRBids

    JRBids Senior Member

    Robins Island is smack dab in the middle.

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  18. Mick West

    Mick West Administrator Staff Member

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  19. Mick West

    Mick West Administrator Staff Member

    Wallace's correspondence from the time is still relevant now.

    Six miles is a very long way. However I think a P900 or similar would be sufficient. Here is a zoom test of 5km (3.1 miles)

    Source: https://www.youtube.com/watch?v=VQPZuBlCvvg


    So that's the type of angular resolution and distortion you would get at the midpoint. Sufficient to resolve railings that are likely only an inch thick or less.

    The far point, at 6 miles, would be half the size, and with additional atmospheric distortion. Wallace used "a large sheet of white calico, six feet long and three feet deep, with a thick black band along the centre". I'd suggest the same, and based on the above a 3" thick black band. Specifically I suggest using two strips of Gorilla Tape, which besides being black is also very unlikely to fall off the board (and black duct tape could do, Gorilla Tape is more heavy duty). The board itself could be a piece of plywood painted white, or some poster board (might be too fragile), or even an actual dry-erase whiteboard.

    I also suggest giving the board a distinctive border with tape, and maybe some colored squares to make it easier to find, like:
    Last edited: Sep 13, 2016
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  20. Trailblazer

    Trailblazer Moderator Staff Member

    There are footpaths/bridleways along both sides of the Old Bedford River all the way along: http://streetmap.co.uk/map.srf?x=55...ty/Town/Village]&searchp=ids.srf&mapp=map.srf

    Street View suggests that overhanging trees might be a problem, though.


    There's also a dead straight 6 mile stretch in the other direction from the Welney bridge to the next bridge:

  21. Grand Traverse Bay in Michigan would be a good contender.

    There's a peninsula in the middle, and it's at about the halfway point.

    For some reason, my computer doesn't want to screenshot it.
    Anyway, it's about ten miles from the peninsula near sutton's bay to the mainland, and where the Old Mission Light is is roughly halfway. You could put the targets in water, maybe have them mounted on a 5-gallon bucket filled with concrete, put a 'water level' mark on them wade them out that far, and presto. Easy way to replicate the Wallace experiment, and if you made them ten or twelve feet high, your'd potentially minimize the effects of refaction near the surface.

    Now, if you had a boat, you could make floating targets (a tripod supported on three inner tubes) and use your GPS to space them however many miles apart you wanted for the experiment, and you could really do that nearly anywhere on the Great Lakes.
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  22. Z.W. Wolf

    Z.W. Wolf Senior Member

    There are quite a few dry lake beds in California and Nevada. How do you tell the surface is exactly level? When the winter rains come along, it's common for a lake bed to be covered with a thin layer of water only inches deep. You can still drive over the lake bed, but on a still day the surface of the water should be a very good gravity equipotential surface.

    As for the Bedford Level, I've previously asked people on another forum if there were any UK members interested. I got some casual interest. I'll ask again, more seriously.
  23. mowgli

    mowgli New Member

    Am keeping an eye on this.. I am based about 1hr drive from the original Canal. Not sure how involved I can be directly though as lots of family commitments :eek:(
  24. tinkertailor

    tinkertailor Senior Member

    Mayybe Lake Tahoe?
    tahoe1.PNG tahoe2.PNG
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  25. Mick West

    Mick West Administrator Staff Member

    I just got the same camera, a Nikon P900, mostly for videoing planes, but it's also ideal for earth curvature experiments with a 2000mm equivalent zoom. It's also much more practical that a DSLR with a long lens, as it's much lighter and smaller. Here's an "actual pixels" handheld shot of a gas station awning about 5km (3 miles) away. It's a hot day (around 85°F)


    Here's the same type of gas station

    So you can see you get reasonable contrast with the approx 1 foot blue stripe on a 2 foot white background. The red and blue Arco logo is also reasonably visible.

    Remember though that the Wallace experiment was twice as far as this, so this represents a bit past the midpoint.
    Last edited: Sep 28, 2016
  26. Trailblazer

    Trailblazer Moderator Staff Member

    That level of heat haze is reasonably unlikely in East Anglia.
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  27. Mick West

    Mick West Administrator Staff Member

    It was a bit hot for the time of year here. I'll try taking photos of the same scene and recording weather conditions, to see what the effect is. There's some varied weather coming.

  28. Trailspotter

    Trailspotter Senior Member

    You also can try taking short HD videos of the scene. Although there will be fewer pixels per frame, by going through the video frame-to-frame, it is possible to find a frame with a sharper image than in a single still photo. I found this trick helpful when trying to read the tail number of a distant aircraft.
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  29. Mick West

    Mick West Administrator Staff Member

    I took a short video of the freeway about 300 feet from the gas station. Like you say there are occasional moments of clarity from the camera movement and the heat haze (turbulent atmospheric refraction)

    Source: https://www.youtube.com/watch?v=ihn9kANb5Qo

    A tripod will help :) But I think it's clear that a midpoint target need only be around 1 foot in diameter, providing it is of sufficiently high contrast (like a red disc).
  30. Mick West

    Mick West Administrator Staff Member

    Holding up a large board might be problematic in terms of transporting it, and it waving around in the wind. Perhaps a more modern solution would be to use a single pole with LED lights on it of varying colors.

    Something like:
    https://www.amazon.com/Super-Bright-Waterproof-Flexible-Lights/dp/B005EQKOAG 20160928-061557-0d2kz.

    Cheap, and comes in different colors. You could just wrap it around the pole a few time to make a band, using three colors.
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  31. Mick West

    Mick West Administrator Staff Member

    Although, shot this early morning so I could see the gas station lights. Did not come out incredibly well handheld


    Still, the strip of blue lights are quite distinguishable.
    Last edited: Sep 28, 2016
  32. Trailspotter

    Trailspotter Senior Member

    Actually, I was thinking about the noise reduction by taking multiple images of the same scene (with help of a tripod mounted camera) and averaging them. Taking a video probably is the easiest way of doing it. There may be burst and continuous shooting options, but they require holding the shutter button that may cause the camera drift.
  33. Mick West

    Mick West Administrator Staff Member

    That's in interesting idea, I know you can increase effective resolution, as explained here:
    However there's some suggestion it might not work well with atmospheric distortion.
    I'll tripod the camera and give it a go.
  34. Mick West

    Mick West Administrator Staff Member

    So I'm trying to figure out best time of day. Here's a shot at first sunlight:
    Still rather a lot of atmospheric distortion. It was about 70°F/21°C
    Last edited: Sep 28, 2016
  35. Clouds Givemethewillies

    Clouds Givemethewillies Active Member

    Overcast days seem best. Pity about where you live..
    Last edited by a moderator: Sep 28, 2016
  36. Mick West

    Mick West Administrator Staff Member

    Yes it's terrible here. Warm and sunny all the time, and worst of all, no contrails for the last two months!!

    Overcast would be ideal as there would be basically no heating of the ground, greatly reducing convection turbulence. Maybe later in the week.
  37. Hama Neggs

    Hama Neggs Senior Member

    I saw a woman from California talking on Youtube about the missing "chemtrails" this time of year. Somehow she explained it as still part of the program and conspiracy.
    Last edited: Sep 28, 2016
  38. Mick West

    Mick West Administrator Staff Member

    Well, a tripod and more sunlight made an astonishing difference, 9:20, around 80°

    That's on a tripod with manual shutter release (i.e. I pressed the button) and image stabilization on

    Compare with yesterdays afternoon handheld

    Note the difference in the red and blue signs. Barely visible in the older shot.
    Last edited: Sep 28, 2016
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  39. Mick West

    Mick West Administrator Staff Member

    I suspect a large part of the reason why the above image is so good is the closeness of air temperature and ground temperature.

    Outside right now, measure the shaded air temp at 76 and the ground temp at 72 (using a Thermopen meat thermometer both in soil and an unheated spa). The sun has not yet heated the ground sufficiently to cause it to warm the air above it (and create convection).
  40. Trailblazer

    Trailblazer Moderator Staff Member

    I can't quite tell from the crop, are those wires at the gas station (or at least close by)? That's pretty incredible to be able to pick out individual utility cables from 5km away.