Search found 1565 matches

by JohnD
Sun Feb 25, 2007 12:24 pm
Forum: The Bridge: Discuss an Astronomy Picture of the Day
Topic: battered moon (APOD 25 Feb 2007)
Replies: 20
Views: 6070

Because the Moon was not always tidally locked to the Earth. When it was formed, it would have had its own rotation. See: http://en.wikipedia.org/wiki/Tidal_locking The original cratering was probably even all over the Moon. The later maria formed where the crust was thinner, smoothed out the crater...
by JohnD
Tue Feb 20, 2007 10:27 pm
Forum: The Observation Deck: Latest Sky Photography
Topic: New impact on Mars
Replies: 3
Views: 3630

Hey, CC, keep up.

Although no one took up the challenge here, there was a PREVIOUS illuminating but not explanatory exchange of views on BAUT: http://www.bautforum.com/showthread.php?t=52156

John
by JohnD
Sat Feb 10, 2007 10:32 pm
Forum: The Asterisk Café: Discuss Anything Astronomy Related
Topic: Craters are an optical illusion?
Replies: 16
Views: 6969

Seeing things in 'reverse' on such pictures is common. Much of our 'seeing' is processing in the eye and brain, that relies on familiar cues and light direction. When those are absent, we cannot 'see' things as they are. A famous illusion relies on this - as in the staircase on this page: http://wil...
by JohnD
Fri Jan 26, 2007 6:14 pm
Forum: The Observation Deck: Latest Sky Photography
Topic: hubble's photos of regular matter and dark matter
Replies: 3
Views: 3779

kovil, The article is almost as obscure as dark matter itself! But it does say,"The dark matter map was constructed by measuring the shapes of half a million faraway galaxies. To reach Hubble, their light has had to travel through intervening dark matter, and the path of the light is slightly ...
by JohnD
Fri Jan 26, 2007 6:14 pm
Forum: The Observation Deck: Latest Sky Photography
Topic: hubble's photos of regular matter and dark matter
Replies: 3
Views: 3779

kovil, The aratcile is almost as obscure as dark matter itself! But it does say,"The dark matter map was constructed by measuring the shapes of half a million faraway galaxies. To reach Hubble, their light has had to travel through intervening dark matter, and the path of the light is slightly ...
by JohnD
Tue Jan 16, 2007 12:50 pm
Forum: The Bridge: Discuss an Astronomy Picture of the Day
Topic: APOD: Kepler's Supernova Remnant in X-rays (2007 Jan 16)
Replies: 4
Views: 2846

Can't answer the above, but I followed the same link and found the Wkipedia saying, "For more massive stars, electron degeneracy pressure will not keep the core from collapsing to very great density, leading to formation of a neutron star, black hole, or, speculatively, a quark star." A &q...
by JohnD
Fri Jan 12, 2007 5:06 pm
Forum: The Observation Deck: Latest Sky Photography
Topic: Circumhorizontal Arc [Fire Rainbow]
Replies: 15
Views: 16065

Rather more useful links are found aplenty by Googling for 'circumhorizontal', or just going to the Wikipedia entry:
http://en.wikipedia.org/wiki/Circumhorizontal_arc

How are you going to link this in with interplanetary war, CC?

John
by JohnD
Mon Jan 01, 2007 11:13 am
Forum: The Bridge: Discuss an Astronomy Picture of the Day
Topic: Stereo effect (APOD 12 Dec 2006)
Replies: 2
Views: 1667

Well done, flieg! And that ?sink hole is almost right in line with the streak, so is likely to be associated. "Flieg's sink hole"? I admire your ability to cross your eyes. Even with my nose on the screen, I couldn't fuse the images, so here's another way, not as good, but may be applicabl...
by JohnD
Sun Dec 31, 2006 11:57 am
Forum: The Observation Deck: Latest Sky Photography
Topic: New impact on Mars
Replies: 3
Views: 3630

New impact on Mars

http://www.nasa.gov/mission_pages/mars/ ... 09023.html

Love to read discussion of the assymtry of this impact. Rays to one side, secondary cratering to thhe other.

John
by JohnD
Fri Dec 15, 2006 5:38 pm
Forum: The Bridge: Discuss an Astronomy Picture of the Day
Topic: mountains of titan (APOD 14 Dec 2006)
Replies: 4
Views: 2047

Later: BINGO! Cassini-Huygens website: http://www.esa.int/SPECIALS/Cassini-Huygens/SEMAJIZTIVE_0.html "The mountains probably formed when material welled up from below to fill the gaps opened when tectonic plates pull apart, similar to the way mid-ocean ridges are formed on Earth. " Maybe ...
by JohnD
Fri Dec 15, 2006 1:50 pm
Forum: The Bridge: Discuss an Astronomy Picture of the Day
Topic: mountains of titan (APOD 14 Dec 2006)
Replies: 4
Views: 2047

The black shadow - clouds of ice particles? They reflect radar. And a straight line - are we into crater chains again? A linear strike, that opens a crack down to liquid, that then wells up, into a linear mountain chain? Like the Mid-ocean rift on Earth, but with less gravity and no tectonics? John
by JohnD
Fri Dec 15, 2006 12:06 am
Forum: The Bridge: Discuss an Astronomy Picture of the Day
Topic: Phobos: Irregular Shape? (APOD 3 Dec 2006)
Replies: 43
Views: 15218

ERROR ALERT!

My apologies. I wrote above that: If the ball enters a smaller gravity field with a steeper gradient, like that of a planet, then objects on the far side of the CCoM from the planet are going too slow for their new orbit; they experience a force pulling them away from the planet into a higher slower...
by JohnD
Tue Dec 12, 2006 9:07 am
Forum: The Bridge: Discuss an Astronomy Picture of the Day
Topic: Moon Panorama: shadows, dust, stars? (APOD 10 Dec 2006)
Replies: 26
Views: 9935

Thanks, Bruce. Strangely, while it doesn't work as a link, it does work with the backslash "\" when entered into my browser (IE). And that is the way the address appeared when I searched for Pacholka's name, went to the Orange County site and found that pic in the album. I just copied the ...
by JohnD
Tue Dec 12, 2006 1:15 am
Forum: The Bridge: Discuss an Astronomy Picture of the Day
Topic: Moon Panorama: shadows, dust, stars? (APOD 10 Dec 2006)
Replies: 26
Views: 9935

If you are a good enough photographer, you can use flash and see stars in the result! See the work of Wally Pacholka. Some of his work looks as if it is staged, such as this one, where the mesas appear to cast a flash shadow on the starry background! Obviously a stage set! http://www.ocastronomers.o...
by JohnD
Tue Dec 12, 2006 1:04 am
Forum: The Bridge: Discuss an Astronomy Picture of the Day
Topic: Moon Panorama: shadows, dust, stars? (APOD 10 Dec 2006)
Replies: 26
Views: 9935

Bob, I feel your pain! But maybe we're just too geocentric, deposition would be quite unlike Earthly windblown detritus. And look at two places in that pic: 1/ the lumpy rock, aboput four o'clock from the astronaut. There's dust in the depressions and 'lapping' up the sdie nearest the camera that's ...
by JohnD
Sat Dec 09, 2006 11:23 pm
Forum: The Asterisk Café: Discuss Anything Astronomy Related
Topic: "Report this post" button needed?
Replies: 6
Views: 3450

nereid,
Thank you for your considered answer!

John
by JohnD
Fri Dec 08, 2006 10:45 pm
Forum: The Bridge: Discuss an Astronomy Picture of the Day
Topic: APOD: Antikythera Mechanism (2006 Dec 05)
Replies: 30
Views: 11920

13, your example of the shuttle is appropriate. I read, and it seems more than likely, that unless the promise of funding and opportunity for NASA to go to the Moon again soon, like in the next ten years, most of those with the experience of the Apollos will be dead. An enormous storehouse of expert...
by JohnD
Fri Dec 08, 2006 11:12 am
Forum: The Bridge: Discuss an Astronomy Picture of the Day
Topic: APOD: Antikythera Mechanism (2006 Dec 05)
Replies: 30
Views: 11920

All, In contrition for having hijacked the thread, may I offer Archimedes' Screw for discussion? This is a much simpler mechanism than the Antikythyra, indeed it is difficult to see how this could have been lead up to by earlier, simpler devices. Can anyone suggest an earlier, simpler device than th...
by JohnD
Fri Dec 08, 2006 10:59 am
Forum: The Bridge: Discuss an Astronomy Picture of the Day
Topic: APOD: Antikythera Mechanism (2006 Dec 05)
Replies: 30
Views: 11920

GG, All, Irony is not entirely absent from the Bible, there are some instances. I cannot tell if they were provied by a god or not. Job suffered mightily from the cruelty of Jehova, and lamented mightily too. He was upbraded by his unsympathetic neighbours, who quoted the prophets and the holy law t...
by JohnD
Fri Dec 08, 2006 10:29 am
Forum: The Bridge: Discuss an Astronomy Picture of the Day
Topic: Phobos: Irregular Shape? (APOD 3 Dec 2006)
Replies: 43
Views: 15218

Ken, Thank you! What an interesting and informative paper! It raises a Q for me - Assuming that grooves are just crater chains below the resolution of the photos, the authors suggest that ejecta from Mars impacts caused these markings. Does (do?) 'ejecta' leave the impact site as a 'line of gravel',...
by JohnD
Thu Dec 07, 2006 11:03 pm
Forum: The Bridge: Discuss an Astronomy Picture of the Day
Topic: APOD: Antikythera Mechanism (2006 Dec 05)
Replies: 30
Views: 11920

You're right, Andy, irony is a dangerous form of humour, but one much loved by gods, I fear And when you think about it, a god must have a sense of humour. Sitting about for centuries, while all those little people make such a complete mess of their lives, don't even thank you and when they do it's ...
by JohnD
Thu Dec 07, 2006 10:43 pm
Forum: The Bridge: Discuss an Astronomy Picture of the Day
Topic: Phobos: Irregular Shape? (APOD 3 Dec 2006)
Replies: 43
Views: 15218

13, Since you mention it, may I mention that it is the effect of which you write which causes tidal disruption? That ball of gravel would orbit in a very large gravity field, like the Sun's, with the individual particles orbiting around their common centre of mass (CCoM), or stuck together loosely w...
by JohnD
Thu Dec 07, 2006 3:48 pm
Forum: The Bridge: Discuss an Astronomy Picture of the Day
Topic: Phobos: Irregular Shape? (APOD 3 Dec 2006)
Replies: 43
Views: 15218

Al, Don't forget the resolution of this picture - about 7 meters (an astounding achievement!). This may allow a chain of craters that are 250 meters wide to be seen as craters, but if they are c.25 meters, then the chain will appear as a groove. I fear that even 'dragging' won't work! Gravel thrown ...
by JohnD
Thu Dec 07, 2006 11:23 am
Forum: The Bridge: Discuss an Astronomy Picture of the Day
Topic: Phobos: Irregular Shape? (APOD 3 Dec 2006)
Replies: 43
Views: 15218

FI, I'd feel proud if you weren't such an easy target. But it wouldn't look good on a T-shirt: "I got FI to bluster, rather than offer a reasoned argument for the theory of a hollow Phobos" And in my physics, acceleration="rate of change of velocity". A rate of change can be posi...
by JohnD
Wed Dec 06, 2006 11:33 pm
Forum: The Bridge: Discuss an Astronomy Picture of the Day
Topic: Phobos: Irregular Shape? (APOD 3 Dec 2006)
Replies: 43
Views: 15218

What's wrong with that, FI?
Acceleration can be positive or negative. Drag is a force in opposition to motion, casuing negative acceleration.


Oh, and please answer my question about penetration of the hollow Phobos?

John