[QUOTE=chalsall;289461]Some talk about the "dark side of the moon". But that only refers to what we cannot see from Earth, not what is actually dark.

[/QUOTE]Yeah, I have to keep reminding myself, "say FAR side, not DARK side".

[QUOTE=chalsall;289461]As an aside, I always find it amusing how few understand why a lunar eclipse can only occur during a full moon....[/QUOTE]

Indeed, I've seen both of those (the above and the solar-eclipse/new-moon) fubared by people who should know better, e.g. SciFi writers.

Slightly more subtly, IIRC the above 'phenomena' also help explain why the 2 types of eclipses ofetn come in pairs.

[QUOTE=ewmayer;289474]Slightly more subtly, IIRC the above 'phenomena' also help explain why the 2 types of eclipses ofetn come in pairs.[/QUOTE]

Yeah... Until you understand planes of orbits, you are lost.

Once you do, you instantly understand why eclipses are grouped.

[QUOTE=chalsall;289461]Some talk about the "dark side of the moon". But that only refers to what we cannot see from Earth, not what is actually dark.[/QUOTE]There's no dark side of the moon really. As a matter of fact it's all dark.

[QUOTE=Jeff Gilchrist;289460]I never realized that a day on Venus was longer than its year.

[URL="http://www.cbc.ca/news/technology/story/2012/02/15/science-venus-slow-rotation.html"]Venus slowdown puzzles planetary scientists[/URL][/QUOTE]What is far more interesting, IMO, is the size of the change in the length of the day.

To some people, this posting really belongs in the "WTF?" thread.

[QUOTE=xilman;289489]What is far more interesting, IMO, is the size of the change in the length of the day.

To some people, this posting really belongs in the "WTF?" thread.[/QUOTE]

What is being forgotten here is the results being compared and reported are between modern instrumentation and those of 16 years ago.

Which is more likely: the planet has suddenly slowed in its rotation rate, or the earlier measurements were wrong?

[QUOTE=xilman;289489]What is far more interesting, IMO, is the size of the change in the length of the day.[/QUOTE]

Yes that is the more interesting part which is why I linked to the article. I was just adding a note about how I never thought about days and years closely enough to notice that a planet like Venus had such a state.

[QUOTE=Jeff Gilchrist;289497]I was just adding a note about how I never thought about days and years closely enough to notice that a planet like Venus had such a state.[/QUOTE]

This is meant as a joke: Do you think the Earth is flat? :smile:

[QUOTE=chalsall;289495]What is being forgotten here is the results being compared and reported are between modern instrumentation and those of 16 years ago.

Which is more likely: the planet has suddenly slowed in its rotation rate, or the earlier measurements were wrong?[/QUOTE]

We are talking about a [URL="http://www.wolframalpha.com/input/?i=%28243.023-243.0185%29%2F243.0185*100"]0.00185%[/URL] shift in measurement. That, combined with old instrument error, massive atmospheric pressure --> greater drag, and the "decades long weather patterns" I think is enough to account for the change, at least to the point of "it's not a big deal". [I]Which[/I] of the above causes doesn't matter, my point is that there are more than enough variables that could cause it. Let the Venus folks figure exactly what, but it's not an earth-shattering (bad metaphor?) revelation all things considered.

[QUOTE=Dubslow;289506]We are talking about a [URL="http://www.wolframalpha.com/input/?i=%28243.023-243.0185%29%2F243.0185*100"]0.00185%[/URL] shift in measurement. That, combined with old instrument error, massive atmospheric pressure --> greater drag, and the "decades long weather patterns" I think is enough to account for the change, at least to the point of "it's not a big deal". [I]Which[/I] of the above causes doesn't matter, my point is that there are more than enough variables that could cause it.[/QUOTE]Still, that's a [i]bunch[/i] of angular momentum change. I'm interested in seeing the energy and atmospheric drag calculations.

Hmm.

[URL="http://www.wolframalpha.com/input/?i=2%2F5*%28mass+of+Venus%29*%28radius+of+Venus%29%5E2*2*pi*%281%2F%28243.023+days%29-1%2F%28243.0185+days%29%29"]delta-L ~ 4*10^26 Js[/URL]

But... again, it's total spin angular momentum is probably somewhere north of 10^30 Js.