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

  • Context: High School 
  • Thread starter Thread starter Physicist50
  • Start date Start date
Click For Summary

Discussion Overview

The discussion revolves around the accuracy of the distance in kilometers represented by a light year, specifically questioning a measurement of 9,460,000 km as stated in a measurements book. Participants explore calculations related to the speed of light and the number of seconds in a year to derive their own values for a light year.

Discussion Character

  • Exploratory, Technical explanation, Debate/contested

Main Points Raised

  • One participant questions the accuracy of their measurements book, which states there are 9,460,000 km in a light year, and presents their own calculation resulting in a much larger figure of 90,450,800,000,000 km.
  • Another participant confirms the original poster's calculation is correct, but notes that the speed of light is actually 299,792,458 m/s, suggesting the book's figure may be a rounded approximation.
  • A different participant points out that the book's figure is off by a factor of about one million and suggests it may have intended to report a distance in millions of kilometers.
  • Several participants highlight the importance of including units with numerical values, indicating that without them, the figures are meaningless.
  • One participant acknowledges the missing zeros in the original post's first reported figure, expressing surprise at the discrepancy.

Areas of Agreement / Disagreement

Participants generally agree that the figure quoted in the book is incorrect, but there is no consensus on the exact nature of the error or the correct value of a light year, as multiple calculations and interpretations are presented.

Contextual Notes

There are unresolved issues regarding the assumptions made in the calculations, such as the exact length of a year and the rounding of the speed of light. Additionally, discrepancies in the reported figures raise questions about transcription errors and the intended units.

Physicist50
Gold Member
Messages
45
Reaction score
0
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?
 
Science news on Phys.org
You are right.
 
Thank you, and I thought books were the information source you can always rely on!
 
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...
 
Last edited:
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.
 
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.
 
Integral said:
Without units these numbers are meaningless.

Good point. All figures were either intended or reported to be km/light-year.
 
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.
 

Similar threads

  • · Replies 11 ·
Replies
11
Views
4K
  • · Replies 21 ·
Replies
21
Views
4K
  • · Replies 11 ·
Replies
11
Views
2K
  • · Replies 11 ·
Replies
11
Views
1K
  • · Replies 10 ·
Replies
10
Views
2K
  • · Replies 7 ·
Replies
7
Views
2K
Replies
15
Views
2K
  • · Replies 20 ·
Replies
20
Views
2K
  • · Replies 24 ·
Replies
24
Views
3K
  • · Replies 12 ·
Replies
12
Views
2K