What it was
Hold a fingernail out at arm's length and you can hide the Moon behind it. The young Moon sat much nearer Earth. It would have spilled beyond your fist instead.
The Moon tugs harder on the side of Earth facing it than on the far side. That uneven pull stretches our world and makes tides. Water rises and falls, but rock moves too. The solid ground lifts slowly enough that you cannot feel it beneath your shoes.
When and where
The Moon formed early in Earth's history, most likely after a giant impact. Since then it has crept outward. Tides pass motion between the worlds, pushing the Moon away and slowing Earth's spin. A longer day comes with a more distant Moon.
How big
Our Moon is about a quarter of Earth's width. The average gap between their centers is about 239,000 miles (384,400 kilometers). You could line up roughly 30 Earths across that distance. The small disk in the sky hangs a long way from your hand.
How we know
We bounce lasers off reflectors left on the Moon and time their return. Old coral growth bands and tidal stripes in rock also point to shorter days in the past. Moon rocks brought home to labs keep records of lava flows and impacts that changed its face.
Still a mystery
Scientists still argue about why the far side has a thicker shell and fewer dark lava plains. A smaller moon could have spread across it in a crash. Or heat from young Earth helped the sides cool differently. Gravity maps show the uneven shell, but cannot yet choose the cause.