WP 34S conversion error

[Peter Van Roy]

My WP 34S converts 1 light-year to 9.46073E12 km. This is not quite right: the IAU defines 1 light-year as 9460730472580.800 km (exactly), according to Wikipedia. This standard is defined by the IAU as the product of the Julian year (365.25 days) and the speed of light (299792458 m/s).

Peter

[Paul Dale]

As is documented in the manual, the 34S uses the NIST definitions for units where ever possible:
http://physics.nist.gov/Pubs/SP811/appenB9.html#LENGTH

I guess the IAU and NIST disagree here


Changing this is easy of course but which source is more canonical?


- Pauli

[Gerson W. Barbosa]

(10-09-2014 09:37 PM)Paul Dale Wrote:  As is documented in the manual, the 34S uses the NIST definitions for units where ever possible:
http://physics.nist.gov/Pubs/SP811/appenB9.html#LENGTH

I guess the IAU and NIST disagree here


Changing this is easy of course but which source is more canonical?


- Pauli

NIST appears to be more Physics-minded than IAU. 14 significant digits in the light-year don't make much sense, considering the speed of light is known to only 9 significant digits.

Gerson.

[Dieter]

(10-09-2014 09:37 PM)Paul Dale Wrote:  As is documented in the manual, the 34S uses the NIST definitions for units where ever possible:
http://physics.nist.gov/Pubs/SP811/appenB9.html#LENGTH

Unless printed in bold, the values on that page are generally rounded to seven significant digits. So they are "close enough" but not exact. I think the 34s constants should not be limited in that way, if a more exact value is available.

Dieter

[walter b]

(10-09-2014 09:37 PM)Paul Dale Wrote:  As is documented in the manual, the 34S uses the NIST definitions for units where ever possible:
http://physics.nist.gov/Pubs/SP811/appenB9.html#LENGTH

I guess the IAU and NIST disagree here

Footnote 18 on that NIST site reads: "This conversion factor is based on 1 d = 86 400 s; and 1 Julian century = 36 525 d. (See The Astronomical Almanac for the Year 1995, page K6, U.S. Government Printing Office, Washington, DC, 1994)." So let's see:

299 792 458 m/s * 24 h/d * 3600 s/h * 365.25 d/y = 9.4607304725808e15 m/y

So IAU and NIST agree, though NIST didn't print all the digits. In consequence, I'd vote for setting the conversion factor to the exact value.

(This definition is not consistent with the length of a year being 365.2425 days. But there are even more definitions of a year...)

d:-)

Edited to complete the output unit.

[walter b]

(10-10-2014 03:08 AM)Gerson W. Barbosa Wrote:  NIST appears to be more Physics-minded than IAU. 14 significant digits in the light-year don't make much sense, considering the speed of light is known to only 9 significant digits.

I principle, you're right. The speed of light, however, is defined to 9 digits exactly - meaning there are no more digits.

d:-)

[Marcus von Cube]

(10-10-2014 08:23 AM)walter b Wrote:  I'd vote for setting the conversion factor to the exact value.

Fixes are under way.

EDIT: Both 34S and 31S are rebuilt. The release packages aren't updated yet.

[Dieter]

(10-10-2014 08:23 AM)walter b Wrote:  Footnote 18 on that NIST site reads: "This conversion factor is based on 1 d = 86 400 s; and 1 Julian century = 36 525 d.

Which is correct for a Julian century.

Quote:(This definition is not consistent with the length of a year being 365.2425 days.

Which is correct for an average Gregorian year.

Quote:But there are even more definitions of a year...)

You bet. So the question is: which definition of a year is valid for a light year? A tropical year maybe? This varies permanently so that anything more precise than 365,242... days/year would not make much sense.

Dieter

[walter b]

(10-10-2014 07:04 PM)Dieter Wrote:  

(10-10-2014 08:23 AM)walter b Wrote:  Footnote 18 on that NIST site reads: "This conversion factor is based on 1 d = 86 400 s; and 1 Julian century = 36 525 d.

Which is correct for a Julian century.

As written.

(10-10-2014 07:04 PM)Dieter Wrote:  

(10-10-2014 08:23 AM)walter b Wrote:  (This definition is not consistent with the length of a year being 365.2425 days.

Which is correct for an average Gregorian year.

Quote:But there are even more definitions of a year...)

You bet. So the question is: which definition of a year is valid for a light year? A tropical year maybe? This varies permanently so that anything more precise than 365,242... days/year would not make much sense.

I think that's the reason the light year was defined based on an average year per convention. And it's just a yardstick for measuring distances in the universe, so who cares about deviations less than 10^(-4)? Has anybody an idea, however, why they took the Julian instead of the Gregorian year? The latter would have been more consistent IMHO but I'm no astronomer.

d:-)

[Wolfgang]

http://www.iau.org/publications/proceedi...les/units/


5.15 Astronomical units: .......... The unit known as the light-year is appropriate to popular expositions on astronomy and is sometimes used in scientific papers as an indicator of distance.

The IAU has used the julian century of 36 525 days in the fundamental formulae for precession, but the more appropriate basic unit for such purposes and for expressing very long periods is the year. The recognised symbol for a year is the letter a, rather than yr, which is often used in papers in English; the corresponding symbols for a century (ha and cy) should not be used. Although there are several different kinds of year (as there are several kinds of day), it is best to regard a year as a julian year of 365.25 days (31.5576 Ms) unless otherwise specified.

It should be noted that sidereal, solar and universal time are best regarded as measures of hour angle expressed in time measure; they can be used to identify instants of time, but they are not suitable for use as precise measures of intervals of time since the rate of rotation of Earth, on which they depend, is variable with respect to the SI second.

[walter b]

(10-10-2014 07:44 PM)walter b Wrote:  I think that's the reason the light year was defined based on an average year per convention. ... Has anybody an idea, however, why they took the Julian instead of the Gregorian year? The latter would have been more consistent IMHO but I'm no astronomer.

Any astronomers out there?

d:-?

[walter b]

(12-09-2014 07:03 AM)walter b Wrote:  

(10-10-2014 07:44 PM)walter b Wrote:  I think that's the reason the light year was defined based on an average year per convention. ... Has anybody an idea, however, why they took the Julian instead of the Gregorian year? The latter would have been more consistent IMHO but I'm no astronomer.

Any astronomers out there?

??

d:-?

[Didier Lachieze]

(12-09-2014 07:03 AM)walter b Wrote:  

(10-10-2014 07:44 PM)walter b Wrote:  I think that's the reason the light year was defined based on an average year per convention. ... Has anybody an idea, however, why they took the Julian instead of the Gregorian year? The latter would have been more consistent IMHO but I'm no astronomer.

Any astronomers out there?

d:-?

I'm not an astronomer and this may not fully answer your question but here is a hint about usage of Julian dates in astronomy:

Quote:Astronomers, unlike historians, frequently need to do arithmetic with dates. For example: a double star goes into eclipse every 1583.6 days and its last mid-eclipse was measured to be on October 17, 2003 at 21:17 UTC. When is the next? Well, you could get out your calendar and count days, but it's far easier to convert all the quantities in question to Julian day numbers and simply add or subtract. Julian days simply enumerate the days and fraction which have elapsed since the start of the Julian era, which is defined as beginning at noon on Monday, 1st January of year 4713 b.c.e. in the Julian calendar. This date is defined in terms of a cycle of years, but has the additional advantage that all known historical astronomical observations bear positive Julian day numbers, and periods can be determined and events extrapolated by simple addition and subtraction. Julian dates are a tad eccentric in starting at noon, but then so are astronomers (and systems programmers!)—when you've become accustomed to rising after the “crack of noon” and doing most of your work when the Sun is down, you appreciate recording your results in a calendar where the date doesn't change in the middle of your workday. But even the Julian day convention bears witness to the eurocentrism of 19th century astronomy—noon at Greenwich is midnight on the other side of the world. But the Julian day notation is so deeply embedded in astronomy that it is unlikely to be displaced at any time in the foreseeable future.

(Source: Julian day).

Herschel originated the use of the Julian day system in astronomy.

[Dieter]

(01-05-2015 10:13 PM)Didier Lachieze Wrote:  I'm not an astronomer and this may not fully answer your question but here is a hint about usage of Julian dates in astronomy:

Caveat!
There are two different things that should not get confused:

On the one hand there is the Julian calendar. Unlike the Gregorian calendar that is used today in most parts of the world, every 4th year is a leap year. According to the Julian calendar, Christmas day is today, on 7 January. In the Julian calendar, a year on average has 365,25 days.

On the other hand there are Julian day numbers. Unlike the usual notation with day, month and year, a certain point in time here is defined by a single number, which allows easy calculations by simple addition/subtraction. These Julian day numbers usually are based on the Gregorian calendar which on average has 365,2425 days per year.

Walter wondered why the light year was defined on a year being 365,25 days (i.e. like it was in the now mostly defunct Julian calendar) and not 365,2425 days (like in the Gregorian calendar) or others, e.g. 365,2422 days (= 1 tropical year).

Dieter