APOD: Arp 94 (2013 Oct 09)

[Chris Peterson]

Personally, as a rank amateur with a six-inch telescope, I have never made an observation which has required me to infer the existence of dark matter, nor dark energy. I am, however, quite comfortable with gravity, and I find it has a great many uses in my day to day life as well. Call me simple if you must.

So you've never looked at a galaxy through your telescope? Because the actual structure of galaxies is as dependent upon the existence of dark matter as on gravity. Without dark matter, galaxies would look very different (if they existed at all). So your lack of a need to infer the existence of dark matter is merely a consequence of your degree of knowledge, not of what you were actually observing. (Please don't take that as an insult... it certainly isn't meant that way. It is only an observation that our knowledge fundamentally colors how we interpret our observations.)

[Nitpicker]

So you've never looked at a galaxy through your telescope? Because the actual structure of galaxies is as dependent upon the existence of dark matter as on gravity. Without dark matter, galaxies would look very different (if they existed at all). So your lack of a need to infer the existence of dark matter is merely a consequence of your degree of knowledge, not of what you were actually observing. (Please don't take that as an insult... it certainly isn't meant that way. It is only an observation that our knowledge fundamentally colors how we interpret our observations.)

Not insulted at all Chris. If I hadn't read about the existence of dark matter, I would have no way to tell that the few barely visible galaxies I can see in my modest scope, do not have enough (conventional) matter to explain their structure and (unobservable) motion. Not sure I'd even know how to tell with a professional scope.

My point was only that if a bona-fide layperson wants to get a better understanding of the universe, they might start with the Big Bang and gravity. If they are still paying attention, they might then consider dark matter. And if they really want to ponder the ultimate fate of the universe, they might then consider dark energy.

But one can still ponder the interaction between two distant galaxies, as in this APOD, with only a 19th century understanding. It still makes sense qualitatively.

[Anthony Barreiro]

... Over 50 million light-years distant toward the constellation Leo, the pair's appearance has earned them the designation Arp 94 in the classic catalog of peculiar galaxies. But such galactic collisions and mergers are now thought to represent a normal course in the evolution of galaxies, including our own Milky Way. ...

It's interesting to me that as we learn more about the history of the universe, fewer and fewer things appear static, and more and more things are revealed as processes. So an Arp galaxy is only peculiar because we happen to see it at a moment of intense interaction and change.

By the way, this is a lovely picture, and I would name this the "Sea Turtle Galaxy Pair."

[Chris Peterson]

It's interesting to me that as we learn more about the history of the universe, fewer and fewer things appear static, and more and more things are revealed as processes. So an Arp galaxy is only peculiar because we happen to see it at a moment of intense interaction and change.

I would suggest this has little to do with our increased understanding of the Universe, but rather represents a shift in our approach to scientific understanding in general. It is a viewpoint that extends to all areas of science, not just astronomy.

[stephen63]

But one can still ponder the interaction between two distant galaxies, as in this APOD, with only a 19th century understanding. It still makes sense qualitatively.

To a non professional astronomer, observations, for the most part, are qualitative!

[Anthony Barreiro]

It's interesting to me that as we learn more about the history of the universe, fewer and fewer things appear static, and more and more things are revealed as processes. So an Arp galaxy is only peculiar because we happen to see it at a moment of intense interaction and change.

I would suggest this has little to do with our increased understanding of the Universe, but rather represents a shift in our approach to scientific understanding in general. It is a viewpoint that extends to all areas of science, not just astronomy.

I thought that's what I said. The last time I checked, everything was part of the universe. Thanks for clarifying.

[Chris Peterson]

I would suggest this has little to do with our increased understanding of the Universe, but rather represents a shift in our approach to scientific understanding in general. It is a viewpoint that extends to all areas of science, not just astronomy.

I thought that's what I said. The last time I checked, everything was part of the universe. Thanks for clarifying. :lol2:

I wondered. It isn't always clear what people mean when they talk about the Universe. As a physicist, I tend to view everything as a subset of physics: astronomy, cosmology, biology, chemistry. But some non-physicists don't see it that way.

[Nitpicker]

But one can still ponder the interaction between two distant galaxies, as in this APOD, with only a 19th century understanding. It still makes sense qualitatively.

To a non professional astronomer, observations, for the most part, are qualitative!

For galaxies and lots of other deep sky thingies, I think you are right. But there seem to be an increasing number of amateurs doing useful, quantitative things like, for example: astrometric measurements of binaries, accurate timings of lunar occultations, mapping meteor showers, supernova hunting, and even measuring the brightness and spectrum of variable stars. And some others, including me, enjoy doing less useful things, like measuring (as best I can from my rudimentary photos) the motion of already well-known Solar System objects. I find it calming to know that I haven't yet observed any discrepancies in the motion of the Moon and planets predicted by the professionals. It also compensates for the fact that I'm unlikely to ever develop the talent for the beautiful deep sky photography so often seen on this site. We all have our quirks.