Tuesday, October 4, 2011

More on Terra Nova

I had a few comments on my last post (and a surprisingly large number of hits--who knew writing about something so topical would be so popular?). Although I addressed the questions in the comments section there, I'm not convinced anyone will see them, so I thought I'd repost and expand my responses here.

Stars, not moon

The first comment pointed out that geeky girl in Terra Nova blamed the expansion of the universe for the stars in the sky being in different places, not the moon. My mistake. She's still wrong, though. The stars we see in the sky are all part of the Milky Way and hence too close to be expanding away from us (or more accurately, because they are gravitationally bound to the galaxy). They would still look different 85 million years ago, however. This is because all these stars, including our sun, are orbiting the centre of the galaxy. It takes the sun about 250 million years to orbit the centre of the galaxy. Even in 85 million years it, and all the other stars, would have all orbited part way around and be in different positions. The sun would actually have moved about a third of the way around it's orbit, but different stars closer or further in have different circumferences and would appear to have gone further or less far around.

The second commenter also raised a good point about the characters remembering the constellations, though. If the parents were young when they last were able to see the moon, it seems unlikely that they would ever have seen the constellations. Perhaps geeky girl studied them in class? (The parents don't actually mention the stars, just that the moon looks different and that they were young last time they saw it.)

Runaway moon?

The commenter two asked how, if the moon is moving away from us by 2 cm a year, has it not moved too far away by now?

Laser Ranging Retro Reflector, used to measure the exact distance
between Earth and moon, and how much this distance is changing.
Deployed by Apollo 14 astronauts.
Credit: NASA Johnson Space Center (NASA-JSC)
First, let me say that from bouncing lasers off reflectors left on the moon by Apollo astronauts, we know that right now the moon is moving 3.8 cm away per year. However, various theories suggest that the moon was moving away at different rates in the past. This makes sense since the Earth's gravitational influence becomes weaker as the moon moves further away, and because things like ice-ages would influence how much water was available for tidally sloshing around and influencing the moon.

In Terra Nova, the girl actually says the moon is moving away half a centimetre a year. I chose 2 cm when explaining it by accident, but it turns out this is roughly what palaeontological estimates predict the average may have been over that time (link and citations within).

A recession of 2 cm a year over 85 million years gives us a moon less than 2000 km closer 85 million years ago. Given that the moon varies more than that as it moves through closer (perigee) and further away (apogee) parts of its orbit now (at perigee it appears about 15% larger than at apogee), the real question is would the moon really be that noticeably bigger? If you're keen, you can use what I wrote in this blog post to work it out.

Of course, it might be. We only have theories and models for how the moon's rate of recession changes. But even using the present-day value of 3.8 cm/year, it would still only be 3200 km close 85 million years ago, which is less than a percent closer on average.

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