# From km/s to kpc/Gyr

1. Jun 26, 2009

### florian101

Hay
Can anybody tell me how to change from km/s to kpc/Gyr? I found a facto of 0.96 but I need it very precise and I am not sure about the exact definition of a Gyr because people use different definitions...

kpc/Gyr = 3.08568e16km/3.2e16s = 0.964275???

thanks a lot for help
regards
florian

2. Jun 26, 2009

### Nabeshin

A quick type into google: 1 km/s in kiloparsecs per 10^9 year yields 1.02268944.

I assume Gyr is giga year = 10^9 years?

3. Jun 26, 2009

### D H

Staff Emeritus
Astronomers use the Julian year for such things. One Julian year is defined to be 365.25 days or 31,557600 seconds, exactly. This yields a conversion factor of 1.02271128 rather than 1.02268944.

4. Jun 26, 2009

Staff Emeritus
Is the conversion between the parsec and the kilometer known to this level of precision? The AU varies by about 2%, which, unless the AU is defined to be a certain number of kilometers, puts a constraint on how well we can ever hope to know the length of the parsec.

5. Jun 26, 2009

### D H

Staff Emeritus
The AU does not vary. You are conflating the instantaneous distance between the Earth and the Sun with the AU. In lay terms, one AU is the mean distance between the Earth and the Sun. That was the scientific definition up to 1976. Now the AU is "the radius of an unperturbed circular Newtonian orbit about the Sun of a particle having infinitesimal mass, moving with a mean motion of 0.017 202 098 95 radians per day (known as the Gaussian constant)." (http://www.bipm.org/en/si/si_brochure). The published value, 149 597 870 691 meters, has an uncertainty of 6 meters.

The parsec, being derived solely from the AU, will have the same relative accuracy as the AU.

6. Jun 26, 2009