A Year of the Moon in 2.5 Minutes - it wobbles more than you might think [video] | Hacker News Reader

A Year of the Moon in 2.5 Minutes - it wobbles more than you might think [video]

youtu.be

147 pointsColinWright32 comments

This animation was produced at the Scientific Visualization Studio at NASA Goddard Space Flight Center. I work in the Studio and the visualizer who created it is in the next office over. All of our work is on the web at http://svs.gsfc.nasa.gov. This animation can be found at http://svs.gsfc.nasa.gov/vis/a000000/a003800/a003810/index.h...

I agree it would be interesting to see an animation that eliminated the rotation and scaling and depicted only the wobbling. Producing that might be harder than you think, the coordinate systems and transformations involved in making an animation like this get complicated pretty quickly.

Fun trick:

Find a high res picture of the moon. Say this one: http://universe-beauty.com/albums/userpics/2011y/04/19/1/18/... Go to http://www.tineye.com/ (Reverse Image Search) and search for that image. Click one of the results (make sure it isn't just the same exact image) and then click the "switch" button repeatedly to wobble the moon. Because Tin Eye matches the rotation and scale of the images, it creates a neat effect.

It is quite amazing that in spite of the moon's rotation about its own axis and our (the Earth's) revolution around the sun where the moon comes along for the ride, we still only ever see just a bit over 50% of the surface of the moon. That takes some amount of dexterity on the part of the moon.

This is very common among natural satellites. Here's the phenomenon: http://en.wikipedia.org/wiki/Tidal_locking (Look at the list of tidally locked bodies)

I did not know that, thank you.

I find it more amazing that the apparent size of the moon is very very close to the apparent size of the sun.

That won't be true forever. The Moon gets about 38 mm further away every year, so at some point there will be no more total solar eclipses, only annular eclipses.

Shame for future humans or trans-humans, but great for us to live at such a time.

>trans-humans

If they care, they'll fix it.

Libration: http://en.wikipedia.org/wiki/Libration

The sensation of movement seems to be mostly caused by rotation and scale changes, not wobbling. I'd like to see a video that compensated for all effects except wobbling.

Clear explanation of the different forms of libration:

http://www.nmm.ac.uk/gcse-astronomy/sun-and-moon/moon-moveme...

Link was 404

http://www.nmm.ac.uk/gcse-astronomy/sun-and-moon/moon-moveme...

How bizarre - I copy/pasted it from the URL bar of my browser.

Thanks for the corrected link.

Slightly OFF TOPIC - I've been trying to find references to any articles/stories about how early astronomers first estimated the distance from the Earth to the Sun (using only observational astronomy). It appears the distance was first estimated in multiples of the distance from the Earth to the Moon. If you happen to know of any quality articles about how these estimates were made (and how a modern amateur astronomer might duplicate them), I'd appreciate a link. Thus far, Google searches have only been mildly useful.

It hinges on observations made during the transit of Venus across the Sun: http://en.wikipedia.org/wiki/Transit_of_Venus

Eli Maor wrote a great book "Venus in Transit" that accounts this multi-century quest to accurately measure this event, which occurs very rarely and requires accurate timekeeping across large distances to compute the triangulation. It's a fascinating story.

Thank you for the link.

BTW, the next transit is in June 2012, and it's likely the last opportunity any of us living today will get to see it: http://www.transitofvenus.org/june2012/where-to-be

I don't have time to find more relevant things for you now, but some of the techniques appear in this paper:

http://www.solipsys.co.uk/HowHighTheMoon.pdf

In addition to that, early measurements were made by trying to compute the angle of the Sun to the horizon when the Moon was exactly at quarter. The angles involved are very small (or very close to 90 degrees) making the results very sensitive to errors. The wikipedia article gives a good overview of the ideas: http://en.wikipedia.org/wiki/Astronomical_unit#History

This article also covers the basics: http://galileoandeinstein.physics.virginia.edu/lectures/gkas...

But you probably knew all that.

I knew some, but not all of that. Thanks for the links!

Not exactly what you're looking for, but Newton explained the irregularities of the moon's orbit in the third book of his Principia. Unhappily I struggled through Cajori's revision of Mott's translation before a modern English translation became available in 1999.

The title claims that it is a year of the moon, but the description says that the images are from 2011 - which isn't over yet. So is this a year of the moon, or six months?

It's an animation.

Only in the sense that it's moving. These are, apparently, actual images of the Moon. I suspect it's a typo, and the images are from 2010, but I don't know.

"The animation archived on this page shows the geocentric phase, libration, position angle of the axis, and apparent diameter of the Moon throughout the year 2011, at hourly intervals. The Current Moon image is the frame from this animation for the current hour.

This marks the first time that accurate shadows at this level of detail are possible in such a computer simulation."

It's a sim.

Hah - interesting. I stand corrected, and I thank you.

NP. It was non-obvious from the YouTube description.

I had to dig around for confirmation it was a sim. I was scratching my head after your mild assertion.

When I saw moon libration for the first time I though this should have made people understand that Earth is spherical. But nobody noticed it for a long time...

But nobody noticed it for a long time...

When was the first animation of Libration produced? For many people, there's a big difference between knowing abstractly and seeing.

The Earth has been known by learned men to be spherical for well over 2,000 years, and it's circumference was first measured through experiment by Eratosthenes over 22 centuries ago.

Coincidentally, there is a lunar eclipse tonight. If you're in the UK, take a look at 9:15PM this evening - I'm afraid I don't know how visible it will be elsewhere.

Wobbles with respect to what?

With respect to an inertial frame of reference I guess. (A frame rotating relative to an inertial frame is not an inertial frame anymore.)

Edit: No, I was wrong. It wobbles with respect to someone viewing it from the the earth looking towards the moon. (Which is not an inertial frame of reference.)

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