The night sky on Tuesday will treat us to a partial lunar eclipse during a supermoon. It’s an astronomical wonder that almost anyone in the Western Hemisphere can experience, and it’s the latest reminder that our home planet is part of a bigger cosmic system.

If you happened to notice the supermoon looking especially large as it was rising, you may have fallen victim to a “well-known, poorly studied optical illusion,” said NASA’s Noah Petro. Here’s the scientific explanation of what you were seeing,