We are just a tiny dot in the universe, How could we see the whole universe picture? Does our telescope got such a wide angle?
This could be answered many ways. We can't, would be one way. We can only see what we call the "visible Universe", or we see much of the Universe through powerful telescopes one small piece at a time or, yes, we see the CMB (Cosmic Microwave Background) by measuring what is all around us.
The finite age of the universe means that we can only see as far as the distance that light could travel since the big bang.* There's no way we could see beyond that. --- * There is a slight complication here in that space has expanded since the big bang, so we can actually see further than 13.7 billion light years. I believe the limit is about 90 billion light years at the moment.
not to mention the whole universe, but the milky way galaxy itself. the earth is relatively very small compared to the whole galaxy, and it is in the galaxy, how can we see the whole galaxy from the earth? it is like you are in a mountain, how can you see the whole picture of the mountain? unless you stay far from it and view it from a distance.
We can't see it from the outside although that has nothing to do with how small Earth is. The problem would be the same even if Earth was much bigger.
Perhaps you're asking how we know the milky way is a spiral galaxy. The answer is that we have mapped out lots of stars that we can see in our galaxy. That's not easy, because as you say we're stuck out in one of the spiral arms and viewing it from the inside. Lots of gas and dust blocks our view of the galactic centre, for instance. However, we have examined things at wavelengths other than the visible as well. If I recall correctly, it was only quite recently that astronomers worked out that the Milky Way is most likely a barred spiral galaxy. We can also infer quite a lot about our galaxy by comparing what we can observe to what we see in the billions of other galaxies that we can see from a distance. We also know that there's a supermassive black hole at the centre of our galaxy. To sum up, we can't see everything in our galaxy from Earth, but we can see a lot of it - certainly enough to deduce its large-scale structure.
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