Showing posts with label Astronomy Picture of the Day. Show all posts
Showing posts with label Astronomy Picture of the Day. Show all posts
Monday, October 26, 2009
Wednesday, October 21, 2009
I can stop posting about APOD any time I want. Really.
Here's a cheesy analogy: In astronomy, studying an object gives you a chance not just to look through space, but through time, even to near the beginnings of the known universe. Searching through the Astronomy Picture of the Day archives likewise gives a glimpse back in time, if not to the very beginning then to an earlier, more primitive web.
I said it was cheesy.
Nonetheless, looking at the first few months of the archive gives some idea of the changes both in astronomy and in the web over the past 14 years or so:
I said it was cheesy.
Nonetheless, looking at the first few months of the archive gives some idea of the changes both in astronomy and in the web over the past 14 years or so:
- The early pictures are all GIFs, generally with either conspicuous dithering or a small color palette (like the very first picture). Broadband? What's that?
- Many of the early pictures were old even when they were posted. Early NASA is well represented, including the Voyager probes and even Skylab.
- The sources are generally well-known institutions. There is little if any contribution from individuals.
- The prose is plainer and there are many fewer links. Some recent APOD entries seem almost to have more links than plain text.
- The next/previous links, now standard in just about any slide show site, didn't come along until later [November 11, 1996 to be precise. JPEGs start to show up a bit before that].
- There's a reference to something called a "WWW page" and one to a "node" along with the now-standard "web page" and "web site" [There are also references to images available "over the WWW"]
- Besides being presented in higher resolution, recent images are much more detailed to begin with. In 1990, this was a "premier view" of the center of our galaxy. Three years later, observations began that eventually traced the orbits of individual stars there.
Monday, October 5, 2009
Doing math (or not) with Alpha
One of the pleasant surprises of the Baker's Dozen exercise was Wolfram Alpha. It didn't always come up with a full answer, but when it did, what an answer! So when I was browsing the Astronomy Picture of the Day and wanted to do a quick calculation, Alpha was the natural choice.
The question in question: In a 16-year tour de force, astronomers from the European Southern Observatory tracked the orbit of several stars around the center of the Milky Way [unfortunately, the video link appears broken]. From this, they confirmed the existence of a supermassive black hole there and measured its distance, based not on some chain of inferences involving standard candles and such (which also works and gives a consistent answer), but by pointing a telescope and watching things move. For sixteen years.
My question was, how good a telescope do you need to do that? The stars in question were on the order of light-days from the core, and the core is about 27,000 light-years away. Doing back-of-the-envelope calculations in my head and picking 2 light-days for reasons I don't remember, I made that to be around .02 arcseconds (the calculations aren't that hard, since for that small an angle you can easily ignore trig). But do double-check, I thought I'd ask Alpha.
For comparison, here's the same exercise with Google:
Now I just need to look up what kind of telescope you need to resolve hundredths of arcseconds (actually, you need considerably better to be able to plot the position of the stars in orbit and figure out the orbital elements, but at least it's a start). [Actually, you don't. 2 light-days is the closest point of a fairly eccentric ellipse. The full orbit is considerably bigger.][Alpha now has no problem with arctan((2 days) / (27,000 years)) in arcseconds, though it does think that 27 000 without the comma means 27 times 000 --D.H. May 2015]
The question in question: In a 16-year tour de force, astronomers from the European Southern Observatory tracked the orbit of several stars around the center of the Milky Way [unfortunately, the video link appears broken]. From this, they confirmed the existence of a supermassive black hole there and measured its distance, based not on some chain of inferences involving standard candles and such (which also works and gives a consistent answer), but by pointing a telescope and watching things move. For sixteen years.
My question was, how good a telescope do you need to do that? The stars in question were on the order of light-days from the core, and the core is about 27,000 light-years away. Doing back-of-the-envelope calculations in my head and picking 2 light-days for reasons I don't remember, I made that to be around .02 arcseconds (the calculations aren't that hard, since for that small an angle you can easily ignore trig). But do double-check, I thought I'd ask Alpha.
- I say (almost correctly): arc tan (27,000 years/2 days). Alpha thinks I mean tan-1(27, (0 years/2 days)).
- All right, take out the comma. Alpha says "Result: tan^(-1)(4927500)". Hovering over this I see the pointy hand indicating a link. So I click on it. Up comes a box that lets me cut and paste the text. Not quite what I was expecting. OK, so put that back into the entry box at the top
- Ah ... now I get a lot of results. A huge long decimal expansion, a conversion to degrees saying 90 degrees -- oops, I meant 2 days/27,000 years, not the other way around -- and then a bunch of alternate representations, including a continued fraction, integrals with gamma functions and other such. Well, Alpha does have its roots in Mathematica ...
- Fix the fraction, and try again, including the extra cut-n-paste. I get a similar display with a conversion to degrees: 1.163x10^-5deg. But I wanted arc seconds, so ...
- tan^(-1)(1/4927500) in arc seconds. It offers me "convert tan^(-1)(1/4927500) to arc seconds" to paste in, so ...
- For some reason I'm now getting back the same thing to paste in again. Before, I believe I got an answer in numbers.
For comparison, here's the same exercise with Google:
- arc tan (27,000 years/2 days) gives me a link to Did you mean: arctan (27,000 years/2 days)? Click the link:
- arctan((27 000 years) / (2 days)) = 1.57079612 Now that's more like it. Fix the fraction and ask for the units:
- arctan (2 days/27,000 years) in arcseconds gives arctan((2 days) / (27 000 years)) = 0.0418321722 arcseconds.
Now I just need to look up what kind of telescope you need to resolve hundredths of arcseconds (actually, you need considerably better to be able to plot the position of the stars in orbit and figure out the orbital elements, but at least it's a start). [Actually, you don't. 2 light-days is the closest point of a fairly eccentric ellipse. The full orbit is considerably bigger.][Alpha now has no problem with arctan((2 days) / (27,000 years)) in arcseconds, though it does think that 27 000 without the comma means 27 times 000 --D.H. May 2015]
Thursday, September 24, 2009
Lost in a web of stars
The day job is less busy now. In the inevitable letdown period, my attention has wandered skyward, to the Astronomy Picture of the Day. Along with the Galaxy Zoo and other random sites, the APOD largely satisfies my desire to learn a little astronomy without, um, actually going out and looking at the sky. Besides, I learn more this way, or at least I learn things that just looking up at the sky gives little hint of. That's why they have all those telescopes and high-tech instruments, after all.
For example, while you'll often see pretty posters of the Orion nebulae or the Trifid nebula, it's another thing entirely to see them in context and realize that, were our eyes sufficiently sensitive (and our surroundings sufficiently dark) even a clear, dark sky would be cloudy. And of course, Van Gogh's Sterrennacht springs to mind.
In the night sky most places we see very little beyond local stars. In major cities it can be hard to see even that much. This limited view reveals very little about the universe at large. Recent theories hold that the universe was born out of some sort of quantum foam and still reflects that structure. What are they talking about? If you zoom out far enough, it starts to make sense.
How can astronomers develop theories of how stars and galaxies form when the timescales involved are much, much longer than anyone's lifetime? They look at lots and lots and lots of stars and galaxies. On a clear, dark night the unaided eye can pick out a few thousand stars. Galaxies have stars by the billion, and there are plenty of galaxies. The Hubble Deep Field, for example, covers about two millionths of the night sky and comprises about 3,000 galaxies. Even the Galaxy Zoo's original million are only a small sample of what's out there.
From common experience, stars (except our sun, of course) are little pinpricks of light. Science tells us that's because they're mind-bogglingly far away. Only objects in the solar system are close enough to appear as anything more than points [well ... you have the Sun, the Moon, the Andromeda galaxy, the Magellanic clouds and the occasional comet ... but let's just agree that you'll see a lot more with a telescope, especially a big one or one in space --D.H. Dec 2015]. But with a good telescope, you can not only tell stars from points of light, you can not only see that stars are round, you can see one that isn't and pick out individual stars in a galaxy far, far away (well, actually a pretty close one by galactic standards).
With special equipment astronomers can see colors the eye can't, as in this lovely image of the Andromeda galaxy in ultraviolet (make sure your cursor isn't over the picture), or pick out otherwise hidden features and reveal the complexity of the processes at work in a nebula, or even show us what's right in front of our faces.
This is a really small sample of the APOD archive. Wander through it yourself and you'll find all kinds of wonders and not a few oddities. But beyond the pretty pictures, the real value lies in the descriptions, written by professional astronomers. It's one thing to read in a science article about this or that theory or process, quite another to see a principle illustrated by a real live picture from a real live observatory accompanied by a clear, concise paragraph rich in links to further pictures and other resources.
This is the kind of thing the web was made for. Certainly it's long been possible to subscribe to an astronomy magazine or go to the local library and get information of a similar quality, but the web enhances the experience considerably.
For example, while you'll often see pretty posters of the Orion nebulae or the Trifid nebula, it's another thing entirely to see them in context and realize that, were our eyes sufficiently sensitive (and our surroundings sufficiently dark) even a clear, dark sky would be cloudy. And of course, Van Gogh's Sterrennacht springs to mind.
In the night sky most places we see very little beyond local stars. In major cities it can be hard to see even that much. This limited view reveals very little about the universe at large. Recent theories hold that the universe was born out of some sort of quantum foam and still reflects that structure. What are they talking about? If you zoom out far enough, it starts to make sense.
How can astronomers develop theories of how stars and galaxies form when the timescales involved are much, much longer than anyone's lifetime? They look at lots and lots and lots of stars and galaxies. On a clear, dark night the unaided eye can pick out a few thousand stars. Galaxies have stars by the billion, and there are plenty of galaxies. The Hubble Deep Field, for example, covers about two millionths of the night sky and comprises about 3,000 galaxies. Even the Galaxy Zoo's original million are only a small sample of what's out there.
From common experience, stars (except our sun, of course) are little pinpricks of light. Science tells us that's because they're mind-bogglingly far away. Only objects in the solar system are close enough to appear as anything more than points [well ... you have the Sun, the Moon, the Andromeda galaxy, the Magellanic clouds and the occasional comet ... but let's just agree that you'll see a lot more with a telescope, especially a big one or one in space --D.H. Dec 2015]. But with a good telescope, you can not only tell stars from points of light, you can not only see that stars are round, you can see one that isn't and pick out individual stars in a galaxy far, far away (well, actually a pretty close one by galactic standards).
With special equipment astronomers can see colors the eye can't, as in this lovely image of the Andromeda galaxy in ultraviolet (make sure your cursor isn't over the picture), or pick out otherwise hidden features and reveal the complexity of the processes at work in a nebula, or even show us what's right in front of our faces.
This is a really small sample of the APOD archive. Wander through it yourself and you'll find all kinds of wonders and not a few oddities. But beyond the pretty pictures, the real value lies in the descriptions, written by professional astronomers. It's one thing to read in a science article about this or that theory or process, quite another to see a principle illustrated by a real live picture from a real live observatory accompanied by a clear, concise paragraph rich in links to further pictures and other resources.
This is the kind of thing the web was made for. Certainly it's long been possible to subscribe to an astronomy magazine or go to the local library and get information of a similar quality, but the web enhances the experience considerably.
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