Why do we always see the same side of the Moon from Earth?

The Moon's rotation is tidally locked relative to the Earth so that it always presents the same face towards us. This is due to the Earth's greater gravity pulling against the rotation of the Moon on its own axis.

The Moon does rotate on its own axis still, it makes one complete rotation for every orbit around the Earth, i.e. one day on the Moon is the same length of time it takes to make one orbit around the Earth (about one month). You can demonstrate this by drawing a dot on a ball and rotating it about an object so that the dot always faces the object. You will notice that the dot must make one rotation about the axis of the ball for this to occur.

This does mean of course that there is no 'dark side' of the Moon either. All parts of the Moon (apart from the bottoms of a few craters near the poles) see sunlight at some point during a lunar day. e.g. when the Moon is New (directly between us and the Sun) the side that we cannot see is fully sunlit, the side facing us is in darkness.

Because the Moon's orbit is not perfectly circular we can actually see 'around the corners' as the Moon orbits the Earth. These libration effects mean that the Moon presents a slightly different face to us at any given time. We are therefore able to see 59% of the Moon's total surface from the Earth.

The moon is tidally locked to the Earth.  The forces between the Earth and the moon have gradually decelerated the moon until its rotation period is equal to its revolution period.

How is the age of the universe determined to be approximately 13.8 billion years?

When we talk about the age of the universe, we usually mean the age of theobservable universe.  All of our observations occur from Earth or its near vicinity.

In the above picture, imagine that Earth is the circle in the center.  We look outwards and the farthest we can see is a radius (d) that equals the age of the universe (a) times the speed of light (v).  So, if we see something 13.8 billion light years away, the universe must be at least 13.8 billion years old.

What complicates this is that we believe the universe is not a static object.  It is expanding.  The big bang theory tells us that the universe started as an extremely dense and compact object and has been expanding outwards, ever since the big bang.  It is also cooling.

Early attempts to determine the age of the universe focused on redshift.

Redshift

The Doppler effect tells us that if an object is moving towards us, the waves it emits are compressed and if it is moving away from us, the waves are elongated.  You may notice this when a police car or ambulance passes you, that the siren sounds different as the vehicle is approaching you than when it is moving away.  The same thing happens with color.

The visible light spectrum looks like this:

So, if an object is approaching, the light it is emitting will be shifted slightly in the direction of blue.  If the object is moving away, it will be shifted slightly in the direction of red.

If we start with the premise that the big bang happened - that the universe, at the beginning, was infinitesimally small and has been expanding ever since, we can imagine that the galaxies are all moving away from each other.  The word choice here can get a little finicky.  It isn't necessarily so much that the galaxies are being propelled away, but space is expanding, between those galaxies, so they are becoming farther apart.

That means if we look at a very far away galaxy, it should appear more red than we would expect it to be.  We can analyze the components of a galaxy and based on its size, mass, temperature and activity determine what color it should naturally be.  The difference between the color it should be, and the color it appears to us is called the redshift.  If we correctly understand how fast space expands, we can derive the distance between the two objects.  That rate of expansion is called the Hubble Constant (H0).  H0 = v/d where v is the radial velocity of the galaxy we're observing and d is the distance from Earth.

If we determine the distance to that far away galaxy (d) and we know that the speed of light (c) is constant, then we know how long it took that light to reach us.  The universe cannot be younger than that amount of time.  So, based on our current understanding of the Hubble constant and the farthest away galaxies we've been able to observe, we know that the universe has to be at least 13.8 billion years old for us to be looking at those galaxies.

Cosmic Microwave Background Radiation

More recently, observations have focused on the cosmic microwave background radiation.  Cosmologists modeling the big bang believe that for the first ~370,000 years after the big bang, the universe was too dense for photons to be emitted.  But at around 370,000 years, it had expanded enough that photons were emitted that described the state of the universe at that time via a very specific ripple pattern, from the combination of electrons and protons combining to produce the first hydrogen.  This radiation has also been redshifted.

While mapping this cosmic background radiation, observers have detected variations called anisotropies.  These variations are expected because of the expansion of the universe - in fact, study of the anisotropies can reveal how much the universe has expanded and thus tell us how old the observable universe is.

Over the last decade, data from WMAP and now Planck (two observatories out in space) have given us a very detailed map that tells us, if we understand the physics properly, that the universe is about 13.8 billion years old.

Can an astronaut standing on the moon observe earth's rotation with the naked eye?

Yes, as long as one was patient.  If one was on the Moon and stared up at the Earth and Washington DC was in the center of their view, in about thirty minutes, Lexington, Kentucky will have taken its place and in less than an hour Chicago, Illinois will have taken its place.  Add another thirty minutes and he or she would be looking at Topeka, Kansas.

What are the coolest facts about black holes?

Blackholes - Blind love of astrophysicists.

There are many interesting facts, many are observed phenomena and many other are purely theoretical as we cannot go and experiment near a blackhole.

1. Nothing can escape
The very famous thing about blackholes which everyone knows is the fact that,nothing can escape the gravity of blackholes, not even light. Thoughphotons are massless, but it is the bend in space time curve (General Relativity) caused by blackhole, which forces the light to travel through the bent space inside blackhole.

2. Singularity
Singularity is a point which doesn't contain any spatial dimension, means that point doesn't contain any height length or breadth. It is just asingular point with no shape and size, and infinite density. Singularity point was also the starting point of Big Bang.
My other answer on singularity is here.

3. Mystery beyond Event Horizon 
Event horizon is the boundary line of blackhole. Anything which crosses event horizon cannot return back, hence it is also called point of no return. It is called as event horizon because whatever events occurs beyond that boundary line, it cannot affect an outside observer. Rotating blackholes can havedistorted and non spherical event horizons.

4. Really, are blackholes really black? Hawking Radiation.
Blackhole sucks up everything, even light, then how it can emit anything? Thanks to Sir Hawking for finding out the answer to this mystery.
Due to quantum fluctuations, pair of virtual particle appear in space and then they disappear almost immediately.

What is Hawking Radiation?
Imagine a pair of virtual particles, say A and B gets created at the event horizon, and before they disappear, particle A enters the event horizon while particle B goes away in opposite direction. Hawking has considered these quantum fluctuations and quantum corrections at the event horizon for  black hole, and found that the blackholes emits these non cancelled particles (like particle B), known as Hawking Radiation.

5. Nothing is forever, not even blackholes.
Who can destroy the most massive and powerful object in the universe?Blackholes evaporates over time, and thus one day the black hole willcompletely disappear. We know that blackholes emits Hawking radiation, and that Hawking radiation carries away some mass and energy along with it. Thus blackholes gets evaporated slowly over time in the form of Hawking Radiation.

6. Death of a massive star - Birth of a Blackhole.
Blackholes are created as a result of most violent events in the universe -Supernova. When the fuel of star gets over, the  star tries to fuse heavier atoms in it's core. Based on the size and mass of star, either it will die a slow death and become a neutron star, or the star will explode with a very bigexplosion called as supernova. Once the star has exploded into supernova, it will leave behind a blackhole, which is so massive that it curves spacetime in itself. 

Source: Home - remnant of supernova 1E 0102.2-7219, about 190,000 light-years.

7. Whiteholes - Opposite of blackholes.
It was said that at the core of blackhole  lies singularity. Recent studies suggests that a black hole might vomit up all the matter and energy at the other end of it, resulting in white hole. Black hole curves space time and thus emits massive energy to somewhere else, as a whitehole. One such whitehole we know BIG BANG, who knows may be the big bang is a white hole of a giant black hole.

8. Blackhole also gives rise to most brightest object in the universe - Quasars.
Blackholes possess mass billions of times the mass of the Sun. Due to it's immense gravity, the matter around around the blackholes also gets attracted by it. When the matter gets too close, it forms an accretion disk around the black hole. Due to immense gravity and collision of matter particles, thematerial heats up to millions of degrees, blasting out an enormous amount of radiation. The magnetic environment around the black hole forms twin jets of material which could be many light-years long.

9. Blackholes - The Architect of Universe.
It was discovered that almost every galaxy has a massive blackhole at it's center. It is this center galactic black hole, around which the entire galaxy rotates. Our own milky way is also having a massive blackhole, which isbillions of times more massive than sun. It is blackholes which shapedthe galaxies, and shaped the universe the way we observe it today. Infact quasars are generally formed by center galactic black holes.

10. Gravitational waves are caused by blackholes.
When two massive object moves around space in some kind of harmony, then it may cause gravitational wave. Imagine two blackholes dancing salsa. There are two black holes which are rotating around each other. Their gravity will bend the fabric of space-time, and thus the continuous rotating motion will create a wave in ripple form.
My detailed answer on Gravitation Wave is here.

11. Black holes freezes time
Blackholes freezes time beyond event horizon. As per general relativity the clock will tick slower at higher gravity than at lower gravity. This phenomena is also known as gravitational time dilation.

Please read my detailed answer on this Vivek Keshore's answer to Does time depend on gravity?

11. Black holes and Theory of Everything (TOE)
Theory of Everything is like holygrail for physicists. String theory is one of the solid contender which can lead to the ultimate theory - Theory of Everything. String theory is not proved till now experimentally. But we know blackholes are out there in universe, and string theory predicts supersymmetric black holes. Thus making string theory a more strong theory which is like one step closer towards TOE.

Please read more about string theory, black holes and TOE in my other answer here.

12. Blackholes, holograms and Information paradox
Taking a snippet of my other anwser.
What is information paradox?
Imagine this, you are preparing a milk shake in a blender. While blending a piece of paper from your pocked falls inside blender and the paper gets   ripped off and gets mixed with the shake. As per quantum mechanics the   information written on your paper is still inside the blender, though you cannot recognize it in normal way, but it is there. Information is not lost, it is in some other form inside blender. Quantum mechanics says the information cannot be lost.

The information stored in a black hole is proportional to its surface area (in two dimensions) rather than its volume (in three dimensions). This could be explained by quantum gravity, where the three dimensions of space could be reconstructed from a two-dimensional world without gravity – much like a hologram. String theory was shown to be holographic in this way.

Using holography we can describe the evaporation of the black hole in the two-dimensional world without gravity. So holography tells us thatinformation is not lost in black holes.

Also it leads to a very fascinating question, are we all just a hologram on the surface of this universe?

13. Blackholes are the future of the universe.
We know that stars die after they burn all their fuel. So, after few billion years when all the stars would have burned their fuel, only black holes will remain in the universe. The black universe, where there will be no light. Soon, blackholes will also evaporate over time, and there will be nothing in this universe. Not a single ray of light. As the universe started with a big bang, it will one day end with either big crunch or big rip.

IIT Jam 2015

Hello Friends ......... Today I'm uploading IIT JAM 2015 Physics question paper.
Go Download it ......... Enjoy ... :)

IIT Jam 2015 Physics Question Paper

Where exactly is the edge of universe and whts beyond it?

The edge of the universe might be simply space that is expanding at a speed greater than that of light. It can be called an edge because it is well defined and cannot be crossed. Beyond this edge is more space.

On the other hand, the edge of the universe might be the beginning of a higher dimension. What's beyond this edge is space of the higher dimension, in which other universes (called 3-branes) are present. At least that is what string theory says. 

It's all theoretical and not yet verified. Cheers :)

Is there another planet like Earth?

NASA's Kepler Space Telescope, astronomers have discovered the first Earth-size planet orbiting a star in the "habitable zone" -- the range of distance from a star where liquid water might pool on the surface of an orbiting planet. The discovery of Kepler-186f confirms that planets the size of Earth exist in the habitable zone of stars other than our sun.
While planets have previously been found in the habitable zone, they are all at least 40 percent larger in size than Earth and understanding their makeup is challenging. Kepler-186f is more reminiscent of Earth.

The diagram compares the planets of our inner solar system to Kepler-186, a five-planet star system about 500 light-years from Earth in the constellation Cygnus. The five planets of Kepler-186 orbit an M dwarf, a star that is is half the size and mass of the sun.

"The discovery of Kepler-186f is a significant step toward finding worlds like our planet Earth," said Paul Hertz, NASA's Astrophysics Division director at the agency's headquarters in Washington. "Future NASA missions, like the Transiting Exoplanet Survey Satellite and the James Webb Space Telescope, will discover the nearest rocky exoplanets and determine their composition and atmospheric conditions, continuing humankind's quest to find truly Earth-like worlds."
Although the size of Kepler-186f is known, its mass and composition are not. Previous research, however, suggests that a planet the size of Kepler-186f is likely to be rocky.

What are some unsolved problems in astronomy?

Everybody's questions are very interesting. Here are some more:

Will a spacecraft that we send out ever touch E.T life?

How do dark matter and dark energy come together and expand the universe ever so slowly?

How will settle on Mars? (Is their a full-thought plan ready to put in effect..?)

How did the Great Red Spot on Jupiter form, and why it is starting to show signs of dying? 

Is it possible to land on Europa? If it is, what can we learn from Jupiter's moon?

Will there be a huge solar flare from the sun (X-Class) that could stop everything on earth?

If we compare all these earth-like planets (found by the Kepler Telescope), will we get a total understanding of our planet?

**Why is the outer layer of the sun MUCH MUCH warmer than the photosphere (the part that we can see from the earth, which is around 6000K)**

How do we classify something in space as a star or a brown dwarf? Does this rule apply for all stars and brown dwarfs?

Including the earth, what percent of the entire milky way contains planets and planetesimals that have some form of water?

Where does our solar system actually end? And where does Interstellar space begin. (The Voyager 1 Controversy)

How do black holes form? Are they at the center of every galaxy?

What causes the different shapes of galaxies?

One way to generate spiral structure is through star formation triggered by a spiral density wave. A spiral density wave is traveling through the galaxy disk, causing the collapse of gas clouds which form new stars. In this case, the spiral structure appears prominently (especially in  blue optical images) because new stars emit most strongly in the blue. If you look at many spiral galaxies in redder colors (or even the near-infrared), the spiral pattern become less prominent, because you are now looking at the older stars which are more uniformly distributed.

Another way to generate spiral structure is through gravitational interactions between galaxies (collisions or mergers). Tidal forces can create strong spiral structure that is seen in both young and old stars because it is a physical arrangement of stars. An example of that is Arp 240 (shown below [1]). While the galaxies may have had spiral arms before their collision (from the previously described density waves), the spiral structure in the image below has undoubtedly been amplified.

How fast is the Universe expanding?

The Universe is expanding with a Hubble parameter of
H0≈70km/s/Mpc.
These are the units most typically used to state this quantity by astronomers.
To figure out how fast the apparent recession velocity of an object is due to the expansion of the universe, use "Hubble's law":
v=Hd,
where d is just the distance to that object.

Note that this "law" only works for objects which are far enough away that expansion is important compared to proper motion (say, outside of the local group, a few Mpc) but close enough that 1) the linear approximation to Hubble's law is valid and 2) it doesn't matter which distance measure you used (they all agree in the small distance limit).

As an example, consider the Coma cluster, which is roughly 100Mpc away. The apparent recession velocity due to expansion will be
v≈70km/s/Mpc∗100Mpc≈7000km/s.

[1] 1Mpc≈3∗1022m