8 minutes for the Sun's light to reach the earth?

[dstebbins]

It is usually taken as a matter of faith that it takes eight minutes for the Sun's light to reach the earth.

 

But, my math says otherwise.

 

The average distance between the earth and the sun is called an "Astronomical Unit."

 

According to the International Astronomical Union (the same guys who said that Pluto isn't a planet), define an Astronomical unit as 149,597,871,000 meters. Let's call that a (for "astronomical unit").

 

The speed of light in a vacuum © is 299,792,458 meters per second.

 

Therefore, the time it takes for the Sun's light to reach the earth (t) is computed by the equation t = a / c.

 

Well, by that logic, t comes out to 499.004784836849 seconds; round it down to 499 seconds.

 

That comes out to eight minutes and nineteen seconds.

 

Now, I understand that that would normally round down to eight minutes, but nineteen seconds is a good bit to slice off. That's almost a third of a minute; almost four percent of the accuracy is taken off.

 

Why do we slice off nineteen whole seconds for the sake of convenience? Isn't it worth learning to say "eight minutes and nineteen seconds," instead of simply saying "eight minutes" if it would give us a more accurate statement of time? Seriously, it's just five more syllables!

 

For example, we often say that the speed of light is "three hundred million meters per second," but that is because it is a lot less of a mouthful to say "two hundred ninety-nine million, seven hundred ninety-two thousand, four hundred fifty-eight meters per second." THAT is where an estimate would better.However, is it really that much more difficult to tack on "and nineteen seconds" to "eight minutes?"

 

Besides, the speed of light estimate is still about 99.9% accurate; not 96%.

 

What do you think?

[insane_alien]

eight minutes sounds better than 8 minutes 19 seconds.

 

its really only the popsci shows that call it 8 minutes. some even use " just over 8 minutes"

 

seriously is it that big a deal? anyone who might actually need to use that information for something important would already know this.

 

and for a truely accurate answer you'd include the variation between perihelion and aphelion

[J.C.MacSwell]

It is usually taken as a matter of faith that it takes eight minutes for the Sun's light to reach the earth.

 

But, my math says otherwise.

 

The average distance between the earth and the sun is called an "Astronomical Unit."

 

According to the International Astronomical Union (the same guys who said that Pluto isn't a planet), define an Astronomical unit as 149,597,871,000 meters. Let's call that a (for "astronomical unit").

 

The speed of light in a vacuum © is 299,792,458 meters per second.

 

Therefore, the time it takes for the Sun's light to reach the earth (t) is computed by the equation t = a / c.

 

Well, by that logic, t comes out to 499.004784836849 seconds; round it down to 499 seconds.

 

That comes out to eight minutes and nineteen seconds.

 

Now, I understand that that would normally round down to eight minutes, but nineteen seconds is a good bit to slice off. That's almost a third of a minute; almost four percent of the accuracy is taken off.

 

Why do we slice off nineteen whole seconds for the sake of convenience? Isn't it worth learning to say "eight minutes and nineteen seconds," instead of simply saying "eight minutes" if it would give us a more accurate statement of time? Seriously, it's just five more syllables!

 

For example, we often say that the speed of light is "three hundred million meters per second," but that is because it is a lot less of a mouthful to say "two hundred ninety-nine million, seven hundred ninety-two thousand, four hundred fifty-eight meters per second." THAT is where an estimate would better.However, is it really that much more difficult to tack on "and nineteen seconds" to "eight minutes?"

 

Besides, the speed of light estimate is still about 99.9% accurate; not 96%.

 

What do you think?

 

eight minutes may be less precise, but it is always correct.

 

eight minutes 19 seconds is wrong, most of the time/year.

[StringJunky]

A constant accurate figure is not possible because the Earth's orbit round the Sun is elliptical, therefore, the time it takes light to reach us varies at any given moment in time to the next. The eight minute figure is probably a ballpark average of the nearest and furthest distances in the Earth's orbit to the Sun.

 

http://www.physicalgeography.net/fundamentals/6h.html

Edited February 23, 2010 by StringJunky

[swansont]

Indeed. Aphelion is 152,097,701 km and perihelion is 147,098,074 km, so it varies from ~490 to ~507 seconds. Rounding isn't that big of a sin if you are ignoring these other details.

[Akhenaten2]

Indeed. Aphelion is 152,097,701 km and perihelion is 147,098,074 km, so it varies from ~490 to ~507 seconds. Rounding isn't that big of a sin if you are ignoring these other details.

I agree with you entirely on this Swansont. Rounding to 8 mins sounds all right to me, but I'm sure you'd want me to point out that all the distances you quoted are between centres of mass and the light actually comes from the suns surface. You need to subtract about 2.4 seconds.

Light coming from the suns centre of mass takes thousands of years to "wriggle" out.

[swansont]

I agree with you entirely on this Swansont. Rounding to 8 mins sounds all right to me, but I'm sure you'd want me to point out that all the distances you quoted are between centres of mass and the light actually comes from the suns surface. You need to subtract about 2.4 seconds.

Light coming from the suns centre of mass takes thousands of years to "wriggle" out.

 

Ah, but we don't know if the observer is measuring at noon, sunset or sundown, or where on the surface the light is coming from.

[Akhenaten2]

Ah, but we don't know if the observer is measuring at noon, sunset or sundown, or where on the surface the light is coming from.

I stand corrected Swansont I didn't think it worth bothering with adjustments for tiny earth/sun differentials - I'll amend my 'estimate' to "about 2.4 seconds + or - about a millisecond. OK?

Your comments regarding the varying positions and times of observers and where the light originated from, have no bearing on the original concept of the discussion.

Edited March 5, 2010 by Akhenaten2

[swansont]

I stand corrected Swansont I didn't think it worth bothering with adjustments for tiny earth/sun differentials - I'll amend my 'estimate' to "about 2.4 seconds + or - about a millisecond. OK?

Your comments regarding the varying positions and times of observers and where the light originated from, have no bearing on the original concept of the discussion.

 

They have as much bearing as your radius vs center arguments. Exactly as much, one might say.

[Sisyphus]

Surely we can be more pedantic than that. Doesn't anyone want to talk about reference frames or something? Solar flares?

[insane_alien]

or we could go to the fluctuations in the size of the sun which are big enough for a small change in the time taken. Then there is the orbit of the earth around the barycenter of the moon,

[Iggy]

Surely we can be more pedantic than that.

 

My vote is for 9,255,858,616,106,894,221,940,637,969,821,537,730,502,421,652 Planck times.