The full harvest supermoon will shine brightly in the night sky Tuesday evening. A partial lunar eclipse, with a “bite” taken from the moon, will also be visible.

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,

Mainers can expect a vivid scene in the Tuesday night sky when a supermoon coincides with a partial lunar eclipse. NASA predicts that the full moon will be partially covered by Earth’s shadow for several hours Tuesday night.