Celestia Forums

Is it just me, or is the value of the constant LY_PER_PARSEC (defined in astro.h) incorrect?

from astro.h:


Using the other constants in astro.h

And [tex]\mathrm{1 parsec} = \frac{\mathrm{1 AU}}{\tan 1^{\prime\prime}}[/tex] gives
LY_PER_PARSEC = 3.26156378

(which is inconsistent with the value of LY_PER_PARSEC)

According to the IAU,

IAU wrote:One AU is exactly 149,597,870.691 kilometres

So KM_PER_AU has used a less precise value. Using the IAU value, the parsec is 30856775812815.5 km.

IAU wrote:Although there are several different kinds of year, the IAU regards a year as a Julian year of 365.25 days (31.5576 million seconds) unless otherwise specified.

Which would make the year equal to 365.25 * 86400 seconds = 31557600 s, and since c is defined as 299792458 m/s, this gives 1 light year = 9460730472580.8 km, so KM_PER_LY is correct.

This would then give LY_PER_PARSEC = 3.26156378

[tex]Distance = 3.26167 / (PA / 1000 )[/tex]

PA = Parallaxe

celestia
HIP 1 => 921.37570 LY (3.26167)
HIP 1 => 921.34570 LY (3.26156378) -0.03

ajtribick wrote:Which would make the year equal to 365.25 * 86400 seconds = 31557600 s, and since c is defined as 299792458 m/s, this gives 1 light year = 9460730472580.8 km, so KM_PER_LY is correct.

This would then give LY_PER_PARSEC = 3.26156378

31556736 sec = 365.24 DAY

149597870.691 km = 1AU

149597870.691 km / TAN(00°00'01") = 30856775810346,990 km

149597870.691 km / TAN(0.0002777777778) = 30856775810346,990 km

30856775810346,990 km x 1 au / 149597870.691 km = 206264,8062 AU = 1 PC

63241.07709 = 1 Ly

206264.8062 = 3.261563776 Ly

3.261563776 ly = 3.261563776/(1000/1000) = 1 PC

[tex]PA[/tex] = Parallaxe

3.261563776 ly = [tex]3.261563776/(PA/1000)[/tex]

math ...

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