How big is the Earth?

Space scientist Carolin Crawford answered this question...

Carolin - Tiny. Just about insignificant. That's the short answer. It depends what you mean by big. If you meant in terms of size, a dime to the Earth, well it's twelve thousand, seven hundred and forty kilometres across, which means that you could pack about a thousand Earths inside Jupiter; maybe a million Earths inside the Sun. We are truly insignificant. And in terms of mass, it's not much better. The mass of the Earth, it's sort of one three-hundredth of the mass of Jupiter; one three-hundred-thousandth of the mass of the Sun. So the Earth is tiny, whether you think in size or mass, and I'm afraid we are truly insignificant!

Chris - Talking of things that are not insignificant though: the Moon is a very big presence in our sky. And I watched a really interesting documentary and it taught me something the other day - I hadn't realised this - which is that the side of the moon that faces the Earth is completely different from the side of the Moon that faces out into space, in terms of its surface appearance...

Carolin - That's right. The side of the Moon towards the Earth has a lot of those dark - we call them seas, or maria - but they’re actually just flat volcanic…

Chris - ...splodges. What we call man-in-the-moon-type pictures?

Carolin - Yeah, or they describe the bunny rabbit in the moon, if you see that. But the far side of the moon has only got one of those large mares, or seas, and most of it is the much lighter-weight sort of cratered terrain. So it is a big difference between the two sides.

Chris - So it really is a dark side of the moon, in terms of its mysteriousness!

Carolin - Well, actually, it's a lighter side of the moon because it has fewer of the seas.

Chris - And that’s true, and it gets more sunshine, doesn’t it?

Carolin - Yes, it does. Yeah.

Colm - It's odd because you'd expect... I would always imagined that the side of the moon that faces the Earth would have been more protected from collisions.

Carolin - Well that's certainly true....you get moons around Saturn, for example, I think it's Dione, which one side is far more cratered than the other side. An explanation for that is they think it's gone through a 180 degree twist in its history. And so one side was a lot more pockmarked by meteors, and then it got turned round, and it's got this weird dichotomy of its surfaces. I don't think that's the case for the moon.