Starship Asterisk*

Good day ! Thank you , very lovely and interesting composition. But why is the Polaris startrail a semi-circle ? instead of a dot or an almost complete circle ... Because Polaris doesn't outshine the Sun ! From the picture's legend : "Images taken at night compose the top half of the picture, w...

In fact, the light from this powerful laser would combine with light from our Sun to together appear only as bright as a faint and distant star. After atmosperic absorption and scattering, interactions with interstellar material, coherency loss and angular dispersion, and even assuming the absence ...

I've seen many crescents of the moon and do not remember any crescents being on the left side. Maybe all the ones i have seen has been when the moon is waxing and not waneing. Not the Great Quotationist either ;=) but in his absence, the Moon looks like a "C" open to the right when it's p...

Can we say the 100,000 yrs figure is not the result of a deterministic trajectory calculation, rather being a statistical mean (mathematical expectation) considering probable perturbations plus inaccuracies in the measured astrometric elements ?

A fantastic shot !

Anyone can estimate the or mass of the grain of matter that produced such a blazing ray ?

The robot,in the process of creating a new thread per picture/day, could add a pair of navigational links at the bottom of the page (to previous/next day's picture related thread). Link to to-morrow's thread would have to be to a permanent page warning the pic isn't online yet. At the same time it w...

Fantastic, almost erotic (!) picture, but once again the APOD caption is missing basic information :
what scale is it ? are the colours genuine or false ?

[Black Mamba:]"When looking at this holographic image, I can on occasion see three tea pots..." Are you sure it was only tea you poured into that pot ? :=) [Star Hopper:] "http://en.wikipedia.org/wiki/Autostereogram" I'm still not clear about why the colored dots after reading th...

Wow! Thank you, Star Hopper ! At last I've seen that teapot emerge from the background and floating in space ! Where could we find a simple explanation of the principles of such single image 3-D illusions ? Are the seamingly random colored dots essential ? Would the technique be applicable to 3-D ph...

... a fact pending discovery Now that's one scientific notion ;=) For the record, I am ... For the record, ... an idea must have some substance that can be examined to command my attention. And not egocentric in the least. We love APOD - we do - and as such are grateful to you inasmuch as you're co...

quote : "Sweep away what idea? " ISTM you're absolutely denying there /might/ be something to even wonder about. It's hard to understand why. At the same time "NASA scientists" (as journalists would put it) believe in the existence of many inhabited planets in the vast Universe a...

I stand corrected. :oops: (At least I would stand corrected if I understood. But that's my problem, not yours. :? Time for me to hit the books, or, nowadays, the web.) Rob Think of it thus : the tidal force is but a differential in attraction 'felt' by two neighbouring bits or particules of matter....

Neufer : Just curious, does your estimation of the drift take nonlinear [O(t²) and over] terms in account ? Is the recession speed increasing or decreasing by the way ? In any case the variation is very slow at the scale of the age of mankind, maybe even at the scale of the age, and probable lifetim...

It's just a coincidence. It's also a temporary coincidence. The Moon is gradually moving farther from the Earth; millions of years ago it covered more of the corona during eclipses, and millions of years from now every total eclipse will be annular- too bright for the corona to be seen at all. Just...

Sorry, I can't quite figure out what you're talking about. There's certainly nothing to suggest an Ice Age in the next few centuries. A lot is known about glacial and interglacials, much of it tied to the orbital dynamics of the Earth. There are clear patterns to the cycles, and in the absence of p...

Nobody knows when the next glacial period will arrive, but these come and go over much longer periods than a century. A very surprising statement. Are you telling me (on an astronomy forum) the triumphs of celestial mechanics and calculus of perturbations, from say Lagrange to Poincaré, joined to t...

- Since it is a heavier than air gas, gravity will tend to concentrate it a low atmospheric layer and be absorbed by vegetation on land, and dissolved in ocean waters. = No, this doesn't happen. There is no stratification of gases in the atmosphere below about 100 km, because the mixing rate is ver...

I'm a layman here, but I too am a bit skeptical of the predictive power of physical/mathematical/computer models of the climate. One question among a dozen, of which I'd like to read unprejudiced answers : how well understood is the global cycle of the (excess) CO² released in the atmosphere ? Since...

@ Kovil : How do you propose that the electromagnetic field, how ever strong, influenced the kinematics of globally neutral matter ?

<http://antwrp.gsfc.nasa.gov/apod/ap090218.html> This is not too reassuring is it ? I'd like to know if this collision was predicted correctly, how much in advance, and whether the predictions (if any) were given for certain or carried probability estimations ? Did the actual event confirm the advan...

Just estimating, but I'd say there's little difference. The beam has lost coherency by the time it gets to the Moon, and the light that reflects back isn't a tight beam at all. It illuminates a large area on the Earth- much larger than the distance the Earth moves during the time-of-flight. Ah, I s...

Re: lunar ranging. Can someone provide a calculated or estimated value for how much higher the intensity of the light received would be (per unit surface) if the detector were near the center of the returning beam ? Assuming it would be some order of magnitudes higher than the meager count of photon...

Can someone please give an indication about the size covered by today, April 27 2008's picture both in terms of angular measurement and, while we're at it, distance and estimated real size ? Generally I'd appreciate if the angular size of the area shown was alwas given with APOD ! I think I emailed ...

No data still off the height of those sand dunes (if they are sand dunes) ?
Is ~100 metres a correct approximation ? (ap080404)

A question regarding the spectacular picture in Neufer's link : is the vertical scale exaggerated ? Put differently, how deep are the canyons seen there ?

Similar question about the waves in to-day's aesthetical APOD.