[experimenter]

Hello flat earthers.  Please all join me in an experiment if you can.

I plan to perform mine on March 25, 2018 (weather permitting) but other dates will be available.

Equipment:
* mobile phone (international calling enabled) x 2
* camera with telephoto lens x 2

Procedure:

1. First find a buddy who lives at your longitude plus 180 degrees (or thereabouts).  For example, I live in Plymouth, Devon, UK, and my buddy lives in Wellington, NZ.
  **** Important!! Make sure this buddy is someone who you really trust.  I chose my brother ****
2. Near the date of the equinox, choose a time when the half moon is setting at your location.  I choose March 25, 2018.
3. At that time, get on the phone to your buddy.
4. Line up the moon in your camera.  Make sure the magnification is high enough to see some features.
5. As the moon is setting, give a countdown to your buddy over the phone and both of you take a picture of the moon at exactly the same time.
6. Share your pictures with each other.

Now you need to explain how one of you has taken a picture of the moon with the top half illuminated, and one with the bottom half illuminated.  Also note that in both cases the illuminated half contains the same features, but like the moon they are flipped across the horizontal.

I've attached two files showing what I think the pictures will look like.

Looking forward to this experiment, hope you will join me.

[mtnman]

I like your idea, basically find someone on the opposite side of the (round) Earth and in the other hemisphere.

If I could make a suggestion, instead of each person needing to find their partner, perhaps people could show there interest in participating using the forum, then people could be matched up.

I would participate from Atlanta, GA, USA.

[experimenter]

Yeah we could do that.  For Atlanta, you'd need to find someone in Myanmar or Inner China to pair with, though.

[3DGeek]

You can really do this experiment on any day when the moon is visible - but instead of picking someone on the opposite longitude to you - just pick someone at a very different latitude.   If you live in Europe - almost anyone in Africa will do - if you live in the USA, then anyone in South America will do.

[experimenter]

I get your point, but it's better to have the observations separated in longitude.  Reason being that if separated only in latitude, the moon only has a left-right inversion (see attached Cape Town pic from approx the same time as my previous post).

If you are separated in longitude, one of you sees the moon lit from the top and the other sees it lit from the bottom.  It's very difficult to explain with a fixed light source and two observers with the same normal vector.

[HorstFue]

I also think, you could do the experiment with any two observers. Both should be able to see the moon and the distance between these two observers should be significant.
If both take an image of the moon, these two images can differ in size due to camera optics - should be scaled accordingly - and if turned in the right way, these two images should match exactly (for the part with the moon).
This can only be true, if the distance to the moon by far exceeds the distance between the observers. The distance to the moon must be something at least 10 times the distance between the observers.

And something else is in the second part, as you have to turn the images: It's not the moon which is tilted, the orientation of the observers/cameras is different. If this tilt is evaluated in the right way, you can only deduce, that observers must be on a sphere.

Example: I'm living here about 49°N. If I look east at a rising crescent moon, I see the "C" of the moon not standing upright but tilted - i did not measure it exactly, but 41° looks reasonable (That's 90°-49°). The horizon line I'm looking at in the east is not parallel to the astronomic plane, but tilted by these 41°. Later in the night, when the moon is in the south I see an upright "C", as the horizon line in the south is parallel to the astronomic plane.