Buy one of these, and make your friend look at the moon through it. https://a.co/d/2N0cqI1

If that isn’t enough, then do this experiment:

You and a friend go to different locations very far apart, preferably on opposite sides of the Earth. You can both use a sundial to measure the angle of the sun's shadow at the exact same time of day. By comparing the angle of the shadow at each location, you can see that the angles are different, which can only be explained by the curvature of the Earth. If the Earth were hollow or flat, the angle of the sun's shadow would be the same at both locations.

For example, if you and your friend are in New York and Sydney, respectively, and you both use a sundial at 12:00 pm local time, you'll see that the angle of the sun's shadow on your sundial is different from the angle of the sun's shadow on your friend's sundial. This is because the sun is hitting the two locations at different angles, which can only be explained by the Earth being round.

For a more detailed experiment:

• Choose a location: You and your friend should choose two locations that are far apart from each other, preferably on opposite sides of the Earth.
• Set up your sundials: At the exact same time of day, both of you should set up your sundials in a clear and unobstructed spot, making sure that the sundial is accurately calibrated to the local time.
• Observe the shadow: Observe the shadow cast by the sundial's gnomon and make note of the angle of the shadow with respect to the dial. If you're using a vertical sundial, measure the angle between the gnomon and the dial. If you're using a horizontal sundial, measure the angle between the gnomon and the ground.
• Compare the angles: Using your video call, compare the angles of the shadows on your sundials. If the angles are different, this indicates that the sun is at different positions in the sky at the two locations. This can only be explained by the curvature of the Earth.
• Conclusion: The fact that the angles are different means that the sun is hitting the two locations at different angles, which would not be possible if the Earth were flat or hollow. The only explanation for the difference is that the Earth is round.

It's important to note that this method assumes that the sundials are accurate, the local time is the same at both locations, and the weather conditions are similar at both locations. However, if these assumptions are met, this method can be an effective way to prove the Earth's roundness using only sundials and a video call.