Are there really 9,460,000 km in a light year?

  • Thread starter Physicist50
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In summary, the original book stated that there are 9,460,000 km in a lightyear, but this number is off by a factor of one million. The correct calculation for the distance in km per lightyear is 9,467,000,000,000. The OP's initial calculation was incorrect due to a transcription error, but their follow-up calculation was accurate to three significant figures. Additionally, the length of a tropical year should be taken into account when making this calculation. It is important to always double check information sources, as even books can have errors and rounding.
  • #1
Physicist50
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I was wondering if my measurements book that states there are 9,460,000 km in a lightyear is correct. I raise this question because I have recently done some calculations that resulted in my answer being 90,450,800,000,000 km in a LY. How I got this; a light year is how far a light beam travels in a year, and there are 31,536,000 seconds in a year, (1x60x60x24x365). The typical speed light travels at is 300,000,000 m/s. So the obvious calculation to do is 300,000,000 x 31,536,000 ÷ 1,000, which is 9,460,800,000,000. Have I done something wrong in my calculations, or am I right?
 
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  • #2
You are right.
 
  • #3
Thank you, and I thought books were the information source you can always rely on!
 
  • #4
The speed of light is not actually 300,000 km per second, it is 299,792,458 m/s. (299,792 km/s) And your book is probably rounding anyways, which is perfectly fine with such a large number. It is not wrong.
Edit: Just noticed that your first number is missing like 6 zero's. Yeah, that's a BIT off...
 
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  • #5
If we are to take the original posting as an accurate reflection of the book then the book is off by a factor of about one million. Possibly it was intending to report a distance in millions of km.

On the other hand, OP reports two results, one of which is wrong due to an apparent transcription error and the second of which is good to three significant figures.

9.46 x 10^6 quoted from book
9.045 x 10^13 first reported by OP
9.46 x 10^12 next reported by OP
9.467 x 10^12 per my calculations.

The length of the tropical year is somewhat greater than 365 days. I used 365.25 as a better approximation.
 
  • #6
jbriggs444 said:
...
9.46 x 10^6 quoted from book
9.045 x 10^13 first reported by OP
9.46 x 10^12 next reported by OP
9.467 x 10^12 per my calculations.

...

Without units these numbers are meaningless.
 
  • #7
Integral said:
Without units these numbers are meaningless.

Good point. All figures were either intended or reported to be km/light-year.
 
  • #8
Whoa, I just noticed the missing zero's in the OP's first post lol. Yeah, if that's what is quoted from the book then it's WAY off. Now I feel like a moron lol.
 

1. How many kilometres are in a lightyear?

A lightyear is a unit of measurement used to describe the distance that light travels in one year. This distance is equivalent to approximately 9.46 trillion kilometres.

2. Why do we use lightyears instead of kilometres to measure astronomical distances?

Lightyears are used in astronomy because the vast distances between objects in space are better expressed in this unit. Using kilometres would result in extremely large and impractical numbers.

3. Can we travel a kilometre in a lightyear?

No, a lightyear is a measure of distance and not time. It represents the distance that light travels in one year, which is approximately 9.46 trillion kilometres. It would take many years for an object to travel this distance.

4. How many lightyears away is the closest star to Earth?

The closest star to Earth is Proxima Centauri, which is approximately 4.24 lightyears away. This means that it would take light 4.24 years to travel from Proxima Centauri to Earth.

5. Is a lightyear a standard unit of measurement?

Yes, a lightyear is a standard unit of measurement used in astronomy and astrophysics. It is recognized by the International Astronomical Union (IAU) and is commonly used to describe the vast distances between objects in space.

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