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## Light year, explanation required?

One thing you might want to compare the lightyear to is the kilowatt-hour (kWh), the unit of energy the electric company bills you by. A kilowatt is a unit of power. It is equal to 1000 joules of energy per second. A 1 kilowatt motor, for example, is a motor that uses 1000 joules of energy every second. A kilowatt hour is that rate of energy use multiplied by 1 hour or 3600 seconds. So 1 kilowatt hour is 1,000 Joules per second * 3600 seconds = 3,600,000 Joules of energy.

A lightyear is the same idea. Light travels at 300,000,000 meters per second. A lightyear is this rate multiplied by the 31,536,000 seconds in a year. This is $9.46 x 10^{15} meters$. So a kilowatt is a rate of change of energy, a speed is a rate of change of position. The kWh and Lightyear are just ways of dealing with very huge numbers in more manageable ways while having some important physical connection in mind.

Recognitions:      Homework  Help  No worries - some questions turn out to be very big. Remember, lots of weighty books have been written on these topics. To get the most out of these forums, refine your question. The more specific you can be the better quality your answer will be. It helps us to help you.

Dear Brothers, Thanks all for your support and patience to answer my question. @Simon Bridge: Can you guide me to some good books about cosmology (Particularly about stars) which explains in simple terminology with examples.  In other words, a man like me can understand from that.   @Cosmic Eye: Yes, I am clear of what is Light year.   @Pengwuino : Thanks for your effort to make me understand about the logic.

Recognitions:      Homework  Help  Almost anything on paper will be fine at your level - browse a bookstore science section you'll find dozens. But I can save you money:  At the level you are asking questions, you want a FAQ like these: http://www.astro.ucla.edu/~wright/cosmology_faq.html http://preposterousuniverse.com/writ...rimer/faq.html http://supernova.lbl.gov/~evlinder/umass/faq.html ... these allow you to get a quick overview of the concepts and refine your questions. (I've tried to rank them as to difficulty, so start at the top - don't worry if you don't understand something, make a note of it and push on: chances are it will become clear in a bit.)  http://www.phys.vt.edu/~jhs/faq/stars.html ... same sort of thing but for stars.  A lot of video sources, like you'll find on TV, look good but tend to emphasize the sensational aspects at the expense of the science which is a bit sad.  The TED talks are pretty good though - for example: http://www.ted.com/talks/lucianne_wa...her_stars.html  Notice that some of these sites are from quite good Universities, and the TED talks are by experts in their fields. One of the beauties of the internet is this sort of access - the trick is to find it! Thus: practice your search skills. After a while you'll get good at sorting the wheat from the chaff.

Kahnacademy.org is a tremendous resource that will answer a lot of your questions. It has a huge collection of short videos (10-15 minutes) on a wide range of subjects including cosmology and physics. They range from very basic to intermediate complexity and build on each other. It's like a whole series of mini lectures where you can skip the classes about things you already know, jump to the interesting bits, and then jump back if you find you need firm up your foundations.

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Quote by optical mouse 					 				 				1. If sun is having such enormous magnetic power to hold all the 9 planets and make revolving around sun then why the moon is revolving around the earth instead sun?
1. the force holding the planets in is gravity not magnetism
2. the moon is orbiting the Sun along with the Earth
3. the moon is more strongly attracted to the earth than the sun because it is much closer.
Correction:
1. Correct.
2. Correct.
3. Incorrect! Do the math. The gravitational force of the Sun on the Moon is more than twice that of the Earth on the Moon.

So why do we say that the Moon is orbiting the Earth rather than the Sun? First of all, this begs the question regarding the meaning of "orbiting." It assumes that it is an exclusive relationship. The Moon orbits the Earth. And the Sun. And the Milky Way. And the Local Group. And the Virgo Supercluster. "Is in orbit about" is not an exclusive relationship.

That doesn't explain why we can say that the Moon is orbiting the Earth. The answer is that force is the wrong metric. A better metric is that the Moon is gravitationally bound to the Earth. That isn't quite good enough due to perturbations from the Sun. An even better metric than being gravitationally bound is the concept of a gravitational sphere of influence. One such is the Hill sphere. There are others, but none is "perfect." It's a bit hard to be "perfect" in the N-body problem.

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Quote by D H 					 				 				A better metric is that the Moon is gravitationally bound to the Earth.
This is quite correct -

The moon and the Earth form a gravitationally bound system that, in turn, is bound to the Sun as part of the Solar System which in turn...

In terms of potential, the moon has a hump to get over before it can be claimed by the Sun. Plot the gravitational poltential of the earth-sun-moon system, you can clearly see the moon is well inside the dimple around the Earth and this is what we mean by "bound".

Bottom line: we say the moon orbits the earth because it is a useful approximation. It is good enough to get spacecraft to land on it or to bounce a laser beam off a mirror.

Recognitions:      Gold Member  To bounce off what DH said, both the Moon and Earth are in "free fall" around the Sun. In addition to that the Moon is also in free fall around the Earth as well. The moons orbital velocity around the Sun varies between 28-30 km/s, and it has an average orbital speed of 1 km/s around the Earth.  If it helps, think of the Moon and Earth as one system that orbits the Sun.

Quote by optical mouse 					 				 				Dear Brothers, Thanks all for your support and patience to answer my question. @Simon Bridge: Can you guide me to some good books about cosmology (Particularly about stars) which explains in simple terminology with examples.  In other words, a man like me can understand from that.   @Cosmic Eye: Yes, I am clear of what is Light year.   @Pengwuino : Thanks for your effort to make me understand about the logic.
Go watch the PBS TV series from the 1970s called "Cosmos" hosted by Carl Sagan. Informative AND fun to watch.

Recognitions:      Homework  Help  Sagan is a good showman but tends to emphasize the "mystery" of the cosmos. I don't like to suggest anything that draws it's appeal partly from ignorance. Of course - with cosmology thats a bit of a big ask.  One of the signs you may be a scientist (IMO :) ) is if you don't lose the sense of wonder when you know how something works.

Also I think Cosmology has already moved on in some areas since the 80s.

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