The Moon probably was born in a “big whack” – a massive collision between the young Earth and another planet. Scientists are still working out the details of how that worked, but they mostly agree on the overall picture.

But it took a long time to reach that conclusion. For much of the 20th century, in fact, scientists debated three other possible scenarios for the Moon’s birth.

One said that Earth and Moon were born together – they both coalesced from the disk of gas and dust that encircled the newborn Sun. But the motions of the Earth-Moon system seemed too high for such a formation. And there are big differences in how Earth and the Moon are put together – Earth has a massive iron core, while the Moon has a small core.

Another scenario said that Earth captured the Moon as it wandered close by. But that doesn’t work well, either. It’s hard for one body to snag another, and especially hard when the two objects have such a similar mass.

The most popular idea said the Moon was a spin-off – the young, molten Earth was spinning so fast that it flung off a big blob of itself. But the physics of such a formation are pretty daunting as well.

It wasn’t until the 1980s that scientists began to settle on the current model: the Moon was born in a big whack.Look for the crescent Moon quite low in the southwest as twilight fades this evening. The brilliant planet Venus – the evening star” – stands close by.

Script by Damond Benningfield