As soon as sailing caught on, people noticed that when ships departed over the horizon, their hulls disappeared before their sails. This gave them the idea that the surface of the ocean was curved and that Earth was spherical, just as the Sun and Moon appeared to be.
Ancient Greeks found direct evidence of this by noticing that Earth cast a rounded shadow on the Moon during a lunar eclipse. Aristarchos of Samos observed that the Earth’s round shadow is a few times larger than the Moon. He also realised that this shadow is slightly smaller than Earth itself, because Earth is smaller than the Sun. He correctly concluded that the Moon is about 3.7 times smaller than Earth. Erathostenes had already figured out the size of Earth, so to find the size of the Moon he simply divided the size of the Earth by 3.7! Tegmark (2014) believes this to be the moment when our human imagination finally got off the ground and started conquering space.
Over an over again, people have realised that our physical reality is vastly larger than what we’d previously imagined it to be, and that everything we knew of was part of an even grander structure: a planet, a solar system, a galaxy, a galaxy supercluster, etc.
It is remarkable what discoveries have been made by cosmologists in recent years. No doubt, there are many more to be made.
It all started from those early observations of the Moon.
Source: Tegmark, Max, 2014.”Our mathematical Universe. My quest for the ultimate Nature of Reality”. Penguin Random House, UK.