Like the shadow on the moon?
Yes, the shadow of Earth on the moon during an eclipse is one piece of evidence that Earth is round.
whats goin on?
This has already been dealt with previously on this site, or something very close to it.
The person is being quite dishonest.
It isn't an "empty horizon". It is just that at that zoom, you do not have enough resolution to resolve the boats.
As you zoom in, you get enough resolution to start resolving the boats.
Notice the boat before the horizon? It is clearly before the horizon, and was not obscured by the horizon at all.
Once you could resolve it, you could resolve it. It didn't appear from the bottom up.
For the boats actually past the horizon, you have a mirage, which makes the bottom obscured.
As for the analysis, it is somewhat problematic as it starts off already past the horizon.
It would be much better if they started earlier so we could see it before it goes over the horizon to compare.
But then they just start out lying. It isn't ~ a 6th of the height. It starts off much closer to a 7th. If you go to the "mirror line" it is 1 in 6.8. If instead you go to the horizon it is 1 in 7.4 as for the full length of the mast they used, that is 1 in 2.9 and 1 in 3.2 using the mirror line and horizon respectively.
Later in the video, at around 6:45, they dishonestly put the top orange line LOWER than the top of the mast.
Unlike what they claim, the relative size has increased significantly.
The top of the mast is now 1 in 4.7 compared to the mirror line and 1 in 6.3 compared to the horizon.
This is a significant reduction and shows that a significant portion of the boat is now missing.
Likewise, for the height of the mast used, it has now dropped to 1 in 1.8 or 1 in 2.4, relative to the mirror line or horizon. Again showing a significant part of the boat is missing.