# Light year, explanation required?

1. Oct 31, 2011

### optical mouse

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????
Please explain me in simple English.. thanks in advance

2. Oct 31, 2011

### Simon Bridge

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?

3. Nov 1, 2011

### optical mouse

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

4. Nov 1, 2011

### 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).

5. Nov 1, 2011

### Simon Bridge

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:
:)

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

6. Nov 1, 2011

### optical mouse

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.

7. Nov 1, 2011

### Simon Bridge

Depends if you are continuing a theme or starting on a new topic.

8. Nov 1, 2011

### 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?
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)?

9. Nov 1, 2011

### Pengwuino

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

10. Nov 1, 2011

### Simon Bridge

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.

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.

11. Nov 1, 2011

### optical mouse

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.

12. Nov 1, 2011

### Staff: Mentor

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.

13. Nov 1, 2011

### Simon Bridge

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.

14. Nov 1, 2011

### optical mouse

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?

15. Nov 1, 2011

### Simon Bridge

Have a read of:
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.

16. Nov 1, 2011

### optical mouse

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.

17. Nov 1, 2011

### CosmicEye

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.

18. Nov 2, 2011

### Pengwuino

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.

19. Nov 2, 2011

### Simon Bridge

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.

20. Nov 2, 2011

### 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.