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This is very difficult on a forum and probably only possible face-to-face. You can give a try and read more on Special Relativity by Einstein which is what he Twin Paradox is basically about.

This has some formulae and all but worth a go
http://galileoandeinstein.physics.vi.../srelwhat.html

Yes I agree time dilation can be a hard thing to fathom. I think you should start looking into the mathematical basis of the special theory of relativity to get a better perspective of time dilation. Consulting an elementary physics book might help. i recommend this one.
http://www.flipkart.com/relativity-s...e-b73833fdd939

Since a couple of people were talking about the ISS. Here is an amazing picture taken by astrophotographer Thierry Legault. This was taken 4 years back. The size of ISS is roughly equivalent to an average Walmart store.

Click for High Resolution pic:
Source: Thierry Legault [High-Res]

When I gaze at the night sky, I used to get confused about which planet and constellation I am seeing. Then I stumbled upon about Stellarium, the free and open source planetarium software. It is an amazing piece of software for astronomy enthusiasts. I use it all the time I do star gazing.

As I said earlier, I have been watching a bunch of documentaries on Universe and I have a question. (In reference to Stephen Hawkings observation about the Mad scientist paradox)

http://www.youtube.com/watch?v=sUqxzH0652w

In order to realize time travel, he mentioned about being very close to an object with Huge mass and such a mass is present at the center of our galaxy which is a black hole. However some other doc mentioned it is dark matter ( a place holder for which we don't have any definition)

So what is the difference between a Black hole and Dark matter? and is it defined currently as to what is at the center of our galaxy?

From what I understand, dark matter is matter which is supposed to exist but has not yet been "detected" or seen. Estimates for this type of matter are high along with something called dark energy (which is the reason the universe is expanding and not contracting or keeping the status quo), while our usual matter which makes us and all the observable stars/galaxies does not seem to fulfill the total mass of the universe.

As for a black hole, it is a location with a very very high mass and hence extraordinary gravitational pull such that even light waves cannot escape - making the object "black" or invisible in any electro-magnetic wavelength..It has only been detected by indirect methods - basically radiation it causes when interacting with other visible matter (stars/gas clouds etc)

But it stil is our "normal" matter, formed by the collapse of a very heavy star at the end of its life after a supernova explosion has occurred.

I hope most of you know about Coursera.
Happened to see this today - https://www.coursera.org/course/astrotech

Something good for us is on the way:D



Cheers!

Vinu

Folks,

This morning suddenly an idea struck to me and I just did some quick computations. Thought of sharing the same with you.

The idea is to scale down the observable Universe with radius of 46.6 billion light years to the size of earth whose radius is a meager 6370 kms. When we do that it works out that every millimetre corresponds to roughly 45 trillion kms or a light year is about 0.13 milimeters. Further the breadth of our Milkyway galaxy works out 1.37 metres. This is perhaps one of the easiest of ways to comprehend the vastness of the universe. Any thoughts?

Cheers,
Naren

^Quite a nice way to put it, especially to show the vastness of the universe compared to the already gigantic size of our own galaxy.
This helps in making more "sense" of the larger distances, but then our day to day distances will go well down to the scale of nm or much smaller than that ( takes it to the sub-atomic realm -which is equally difficult to get a perspective on just like the vast galactic distances)

Just finished watching the pilot episode of Cosmos and it does do justice to the original Carl sagan's Cosmos. The major improvement is, the visuals are stunning and it also touches on sensitive topic between religion and astronomy.

The only thing it lacks is Carl Sagan's punch. When he narrates/describes it reaches straight to the heart. Not this one though.

OT: Obama, came to introduce the Cosmos and asked everyone to keep an open mind. I think this is one of his message to urge kids to be involved in Science and Math.

We did it guys, we are orbiting mars several million miles away and that too on a staggeringly cheap budget. NASA's MAVEN orbiter cost ten times as much :) . How many of you believed we could do it on the first attempt? I believe 74 million bucks is pittance to what we can gain by the bump in reputation we got by achieving this milestone. India can now be a destination for low cost satellite launches and we can finally say we have entered the space age clap:

Little off-topic, but maybe of interest to people who have interest in astronomy. There is this concept of 'star people' or 'starseeds'. These starseeds are people who feel a strong bondage with out-of-the-world stuff -- the stars, the galaxies, another world -- in short, with anything that lies beyond the earth. The starseeds often have the feel that though they are born in earth, but they don't belong here, and their real home lies in some other world, and they have been sent to earth for a purpose. Though this is still a nascent concept in India (or maybe a lot of people will just laugh it off), but a lot of people in the Western and Northern Europe and North America, Australia, etc. identify with this concept. In fact, there are some prominent associations of so-called starseeds as well, based in these countries. They can easily be Google-ed as well. Just put up this info, as it may be of interest to some BHPians!

Do you'll mean something like this ?

http://www.youtube.com/watch?v=17jymDn0W6U

-Bhargav

Sure, we did it and we should be proud about it. But let's put things in a little perspective atleast...MAVEN's hardware is going to be probably 10 times as complicated as the Indian MOM, so just saying MOM is cheaper by a factor of 10 is very misleading when they aren't really at the same level of complexity. :)