According to the Round Earth Theory...
The sun is roughly 92,960,000 miles from the Earth.
And about 870,000 miles in Diameter.
By Flat Earth
The sun is 3000 miles from the Earth
and is 32 miles in diamter
That means that the size/distance ratio is 30986.67/27187.5
Since the size of Mercury isn't disputed, I will just use the current observations and use the size/distance ratio to change it down to what the size SHOULD be under a Flat Earth model in order to keep the observations accurate.
Mercury is about 3031.92 miles in diameter
Since the Round Earth Theory requires all planets to orbit the sun, Mercury's distance from Earth varies greatly throughout the year from about 138 million to about 48 million. There are obvious problems with this when it comes to having a set value for a ratio, and so I will just use both values to create two different distances for Mercury according to Flat Earth.
Distance from Earth: 138,000,000 and 48,000,000.
Now to do some conversion.
3021.92/30986.67= 0.1 of a mile in diameter.
Distance should range from about 5075.86 miles to 1765.52 miles.
The calculations for Venus following the same method
0.24 of a mile in diameter (Roughly 1320 feet)
Distance ranges from 5896.6 miles to 959.91 miles
Remember, I'm no math wizard and so I am unsure if I did the right thing in order to convert it between models. But as you can see, the distance doesn't seem to be a problem. With those values, it could easily orbit the sun without crashing into anything. However, it is the size that is what confuses me. How is Venus even observable when it is 959.91 miles away at only 1320 feet long? Even worse, how is Mercury even orbiting anything or considered a planet being less then 600 feet in diameter?
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Figures put into a chart for convienince
Round Earth Theory Figures
Sun
Diameter - 870,000
Distance from Earth - 92,960,000 miles
Mercury
Diameter - 3031.92 miles
Min. distance from Earth -48,000,000 miles
Max distance from Earth - 138,000,000 miles
Venus
Diameter - 7,520.83 miles
Min distance from Earth - 26,097,590.1 miles
Max distance from Earth - 160,313,768 miles
Flat Earth Theory Figures (Figured out as shown above by coverting using size/distance ratio based on the only solid agreed point - The Sun)
Sun
Diameter - 32 miles
Distance from Earth - 3000 miles
Mercury
Diameter - 0.1 mile (528 feet)
Min. distance from Earth - 1765.52 miles
Max. distance from Earth - 5075.86 miles
Venus
Diameter - 0.24 mile (Roughly 1267.2 feet)
Min. distance from Earth - 959.91 miles
Max. distance from Earth - 5896.6 miles
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This idea is based off perspective and size. Two very small objects, in order to remain their correct observed proportions, must have a correct distance/size conversion, or else the ratio will be unbalanced and it will be much bigger or smaller then it really is.
Imagine putting a bunch of balls on a flat surface (assuming they wouldn't roll) and then you lay flat to observe it. You would then have some observable distances and sizes from your position. Replace those balls with bigger ones, however, and they will appear bigger then the first UNLESS you put them at a far enough distance that they actually appear to be the same size. This is, of course, not taking into account that farther away objects become blured. It is for the sake of consistency.
This assumes that Mercury and Venus are both not hoaxes and that they oribt the sun and not the Earth. If they orbited the Earth, there would be quite a number of inconsistencies. Mainly why they wouldn't be observable anymore since they dissapear behind the sun sometimes.