Saturday, October 2, 2010

The Night Sky

I'm pretty sure that I dealt with this subject before, but I'm going to do it again, anyway, because it is so interesting.

When you look up at the stars at night, you think, "Oh! Look at the stars up there!"

But that is a mistake.

Strictly speaking, those stars are not now there.

What you are looking at is where each star was, in the past, at a large variety of different times in the past. And sometimes even that is not true, due to any effect called "gravitational lensing."

The 3 nearest stars, Proxima Centauri, Alpha Centauri and Alpha Centauri B, all visible from the southern hemisphere, are all a little over 4 light years away. That means that the light from them took a little over 4 years to reach the Earth. So, when you look up at them from Brazil, you see them where they WERE, a little over 4 years ago, NOT where they are.

Barnard's Star, the next closest star, about 6 light years away, is seen where it was 6 years ago. And more than any other star, it is not even close to its apparent position. It moves very fast through the sky.

Wolf 359 is almost 8 light years away. When you look at it, you are seeing it where it was 8 years ago.

Do you see the problem?

The problem gets larger and larger with naked-eye views of the sky, until we get to the Andromeda Galaxy.

Andromeda Galaxy is 2 million light years away.

As the two million year old light from Andromeda tries to pass the Milky Way Galaxy, where we are, the gravity of our galaxy actually bends the beam of starlight from Andromeda, so that when the light falls upon the Earth, what appears up above the horizon may actually be below the horizon -- not anywhere near where we see it, not just because it has moved substantially since the light from Andromeda which we are seeing on Earth today left Andromeda 2 million years ago, but also because that light has had a lot of time to be bent in our direction by gravity.

You are seeing where it was 2 million years ago, not where it is today.


  1. You keep looking at the dark part of space 3 degrees above where andromeda galaxy is, and say, ah, the REAL andromeda.

    Me? I'll keep looking at the light ;-)

    The way you describe it sort of takes the fun out of astronomy.

    Let me take it one step further.
    All our appreciation of astronomy is, is a little electron light show going on in the back of our eye ball.
    Light wave hits the back of the eyeball
    electrons jump energy platforms and go back, jump again(as we continue to view). and the special nerves pick up on this activity and send it to the brain.

    Thats all astronomy is, entertaining ourselves with the pattern of sensory perception going on in the back of the eyeball.

    Its all 98% immagination.

    Not to pooh pooh it totally. I like to get out there too with scope or binocs and check things out also.

    Imagine, God might have been displeased by various occupants of the Andromeda galaxy and, just yesterday, wiped it out. We won't know that for two million years when suddenly, it just goes dark.

  2. Actually, if some moron there built a $20,000,000,000 Large Hadron Collider which accidentally generated a mini black hole with Hawking Radiation infinitely delayed by relativistic time dilation, the Andromeda Galaxy might end up looking like this in 2,000,000 years...