[QUOTE]Besides defining the "year" as 365.25 days…[/QUOTE]
Don't forget the Leap Year every century year that is divisible by 400.

365.2425

[QUOTE=Xyzzy;285460]Don't forget the Leap Year every century year that is divisible by 400.

365.2425[/QUOTE]But the IAU clings to the auld ways.

From [URL]http://www.iau.org/public/measuring/[/URL]
[quote]... the IAU regards [I]a year[/I] as a Julian year of 365.25 days (31.5576 million seconds) unless otherwise specified. The IAU also recognises a Julian century of 36,525 days in the fundamental formulas for precession ...[/quote]

[QUOTE=Xyzzy;285460]Don't forget the Leap Year every century year that is divisible by 400.[/QUOTE]

The honest truth: my wedding to my (ex-)wife was held on 2000.02.29.

The advantage of getting married on a leap day is you only have to remember your anniversary every four years.

In my case, only every 400....

(Maybe that's why she's my ex-wife?)

[QUOTE=chalsall;285504]The honest truth: my wedding to my (ex-)wife was held on 2000.02.29.[/QUOTE]Same as a relative of mine.
[QUOTE=chalsall;285504](Maybe that's why my ex-?)[/QUOTE]Same as a relative of mine.

[QUOTE=chalsall;285504]The advantage of getting married on a leap day is you only have to remember your anniversary every four years.

In my case, only every 400....

(Maybe that's why she's my ex-wife?)[/QUOTE]

No, because anniversaries are typically celebrated every year, not every 100 years. If they were celebrated every 100 years, [I]then [/I]you could wait 400 years on yours. In your case, even though the wedding day was Feb. 29th 2000, you'd still have to celebrate them every 4 years since there is a Feb. 29th every 4 years except the 1 time per centery (actually 3 times per 400 years) that you could wait 8 years between celebrating them. :smile:

[QUOTE=Xyzzy;285460]Don't forget the Leap Year every century year that is divisible by 400.

365.2425[/QUOTE]

[QUOTE=cheesehead;285502]But the IAU clings to the auld ways.

From [URL]http://www.iau.org/public/measuring/[/URL][/QUOTE]

It seems very odd that the IAU doesn't take into account the 3 times every 400 years where years divisible by 4 are not leap years. Since there are 146,097 days in 400 years, as Mike stated a year should be considered 365.2425 days and a century 36524.25 days when calculating precise measurements of the speed of light.

[QUOTE=gd_barnes;285511]It seems very odd that the IAU doesn't take into account the 3 times every 400 years where years divisible by 4 are not leap years. Since there are 146,097 days in 400 years, as Mike stated a year should be considered 365.2425 days and a century 36524.25 days when calculating precise measurements of the speed of light.[/QUOTE]It's not in the least bit odd. Astrometry began long before the invention of the Gregorian calendar and astronomers are still interested in events which occurred millennia ago and which will occur millennia hence, each of which lie outside the period when the Gregorian algorithm gives adequate precision. Sticking to the Julian calendar has the advantage of simplicity and of requiring no adjustments to the timings of two thousand years of observations.

[QUOTE=xilman;285513]It's not in the least bit odd. Astrometry began long before the invention of the Gregorian calendar and astronomers are still interested in events which occurred millennia ago and which will occur millennia hence, each of which lie outside the period when the Gregorian algorithm gives adequate precision. Sticking to the Julian calendar has the advantage of simplicity and of requiring no adjustments to the timings of two thousand years of observations.[/QUOTE]
Yes, clearly there needs to be a pressing need to alter the definition of a standard measurement with all the inconvience and confusion which that would entail, and there is no such necessity regarding the light year. It is a unit of distance and there is no need to tie that to the exact time taken for the earth to orbit the sun.

The Gregorian calendar itself is hardly optimal in terms of being a natural way of recording dates. The lengths of the months are certainly not the most natural way of dividing up the year (why should February be particularly short?). But there's no need to go through all the hassle of changing the calendar: it serves its purpose perfectly adequately.

However: perhaps this inherent inaccuracy of the light year compared with the distance travelled by light in a sidereal year in a vacuum might be an important reason why astronomers in modern times decided to use the parsec instead? Just my guess.

Using a year of 365.2425 days, I got this factorization:

Rounding up: 5878504662190319=179*256499*128034839
Rounding down: 5878504662190318=2*23*2670653*47851061

[QUOTE=Brian-E;285518]
However: perhaps this inherent inaccuracy of the light year compared with the distance travelled by light in a sidereal year in a vacuum might be an important reason why astronomers in modern times decided to use the parsec instead? Just my guess.[/QUOTE]

Is the value of 1 parsec known to this accuracy? It is the distance
subtended by 1 second of arc at opposite points in the Earth's orbit,
but the Earth's orbit is not a perfect circle. And (although the effect is
very small) it precesses in its orbit (like Mercury). So if one says
"one second of arc as measured on the following two dates of the year:...."
its value would change from year to year. Indeed, even though (again)
the effect is small, the Earth's orbit changes slightly from year to year
owing to the (small) influence of other planets.

[QUOTE=R.D. Silverman;285536]Is the value of 1 parsec known to this accuracy? It is the distance
subtended by 1 second of arc at opposite points in the Earth's orbit,
but the Earth's orbit is not a perfect circle. And (although the effect is
very small) it precesses in its orbit (like Mercury). So if one says
"one second of arc as measured on the following two dates of the year:...."
its value would change from year to year. Indeed, even though (again)
the effect is small, the Earth's orbit changes slightly from year to year
owing to the (small) influence of other planets.[/QUOTE]Bob: time for some Socratic questions.

Do you think that astronomers are unaware of the ellipticity of the Earth's orbit? If not, how might they frame the definition of the parsec in order to take ellipticity into account?

If you wish, I could recommend some good books on the subject of astronomical measurement.


Paul