# Light year, explanation required?

I am a lay man but having lot of interest in understanding about out space & universe. In connection to that, I need a explanation for my confusion regarding light years, which is denoted for mentioning the distance of an object located far away from the earth.
Let say a star located a light year distance.. that means a light travels all along the whole year is the distance of that star, right?
That means if I seeing that star glowing now is the one year old light i am seeing????

Simon Bridge
Homework Helper
You got it.

A light-year is the distance light travels in a year.
The star 1ly away that we see is what the star was doing a year ago.

It's not clear where you need an explanation.
Imagine you reliably walk at 2m/s ... then 2m would be your "walk-second". You are quite happy using time as a way to describe distance ... if it is 5mins walk to the shops, the shops are 5 walk-minutes away and downtown is a half-hours drive away then it is 30 drive-minutes distant. Making sense?

Dear Simon Bridge..
First, I would like to thank you for your quick reply.. and I can understand the logic behind a light year measurement. But where I need explanation is, If a star emits a light, how long it takes to reach us?, if that star is away from us a light year distance?.

Thanks

Ibix
One year.

The Sun is eight-point-something light-minutes away. Light takes eight-point-something minutes to get here, so the Sun you observe now is actually how it was eight-point-something minutes ago. The nearest star after the Sun is Proxima Centauri, 4.2 light years away. The light we see today was emitted 4.2 years ago (mid-August, 2007).

Simon Bridge
Homework Helper
If a star emits a light, how long it takes to reach us?, if that star is away from us a light year distance?.

If the star is n light-years away, then it takes light n years to reach us from that star.
You were right when you said:
Let say a star located a light year distance..
That means if I seeing that star glowing now is the one year old light i am seeing.
:)

Similarly (thanks Lynx) light from the Sun takes nearly 500 seconds to reach us
- the Sun is 500 light-seconds away

IBix & Simon.. Thanks a lot for your explanation. Meanwhile, I have lot of doubts and require explanation for that.. can I continue in the same thread.. or do i need to open up different discussion topic?
Once again thanks to you both.

Simon Bridge
Homework Helper
Depends if you are continuing a theme or starting on a new topic.

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?
2. In nomenclature, a magnet means, it absorbs material within its magnetic filed, if so, why other planets not moving closure to the sun (I assume, the sun is having enormous magnetic power)?

Pengwuino
Gold Member
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?
2. In nomenclature, a magnet means, it absorbs material within its magnetic filed, if so, why other planets not moving closure to the sun (I assume, the sun is having enormous magnetic power)?

The magnetic field is not responsible for keeping the planets in orbit. It is the gravitational force that is responsible for this.

Simon Bridge
Homework Helper
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.

All masses attract each other. The strength of the attraction is bigger according to the amount of mass and smaller according to the square of their separation. Work out the distances.

2. In nomenclature, a magnet means, it absorbs material within its magnetic filed, if so, why other planets not moving closure to the sun (I assume, the sun is having enormous magnetic power)?

The planets are moving too fast for the Sun to catch them. If they were slower, they'd spiral into the Sun and faster and they'd spiral away. Technically, they will eventually do one or the other ... but not for a very long time.

You can see this on the earth.

If you just release a stone in the Earth's field, it just drops down and hits the ground. If you throw it, it goes along for a bit before it hits - staying up for longer. Throw it harder and it stays up for longer. It is possible to throw it hard enough that it flies all the way around the Earth before it hits. Harder? You can get it to go many times around the Earth before it hits.

Harder still, and its trajectory will match up with itself and it won't come down... as with satellites and the Moon.

Harder than that ... it keeps sailing on away from the Earth.

Great explanation..
but, I learnt that earth is having north and south pole as like magnet.. if so, what makes the difference between gravitational force and magnetic force.

Drakkith
Staff Emeritus
Great explanation..
but, I learnt that earth is having north and south pole as like magnet.. if so, what makes the difference between gravitational force and magnetic force.

There are 4 fundamental forces of nature. The two most obvious ones in your day to day life is Gravity and Electromagnetism. Gravity is responsible for pulling you towards the Earth while Electromagnetism is responsible for the effects of electricity, magnets, and all things related to those.

Here's the wiki article that explains it: http://en.wikipedia.org/wiki/Four_Forces_of_nature
If you can't make heads or tells of it, just click on the links to get to the articles on each force and go from there.

Simon Bridge
Homework Helper
What Drakkith said - the wiki article is good.

At some point you will end up with things being the way they are just because that is the way they are. Gravity is a result of mass, even objects that are not magnetic will attract each other. But the magnetic fields of planets, moons, suns, etc are too weak to attract or repel each other the way you are thinking. Things like planets have a lot more mass than they have magnetism - so their gravity is stronger.

Dear Drakkith & Simon Bridge..
Excellent guidance.. thanks a ton..
Another one.. How the lightening is happening..?
is this happening only during the rainy season or even in the normal day?

Simon Bridge
Homework Helper
http://en.wikipedia.org/wiki/Lightning
... then get back to us.

This sort of forum works best when you google your initial questions and come ask questions when you get lost.

Lightning is a big spark - it occurs when the electrostatic potential between the clouds and the ground is bigger than the ability of the air to act as an insulator. Just the same as you get sparks normally but on a big scale.

Dear simon,

Dont feel bad that I am throwing a lot of basic questions..
After doing a google, I found this forum and thought that this is the right place to take clarification.

Its ok, you have lots to learn little one :) . If you can understand how to read some of the equations like how R^3 in the denomenator of magnetism makes it a weak force at farther distances, you can see it better. Also a basic understanding of what electricity, magnetism, and gravity are and how function and how they differ may be more important first before asking questions.

Are you still unclear about a light-year? Your thinking of it exactly right.

Pengwuino
Gold Member
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.

Simon Bridge
Homework Helper
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,
@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.

Simon Bridge
Homework Helper
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/writings/cosmologyprimer/faq.html [Broken]
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_walkowicz_finding_planets_around_other_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.

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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.

D H
Staff Emeritus
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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Simon Bridge
Homework Helper
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.

Drakkith
Staff Emeritus
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.

Dear Brothers,
@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.

Simon Bridge