NASA reports this photo was taken 19 Sep 1977 by Voyager 1:



It doesn't strike me as obvious how to determine which celestial body is closer to Voyager 1 as the photo was taken.  My (not very scientific) guess is that the Earth is closer, since the moon seems more out of focus.  It also seems more probable, since it was launched from earth.
Question: In this image taken by Voyager 1, which is closer: the earth or the moon?

I used JPL Horizons to get the position vectors of each relative to the SSB on Sep 19, 1977.
Voyager X = 1.547492527774134E+08 Y = 2.045859856469853E+06 Z = 8.442122223290936E+05
Earth   X = 1.503771470116906E+08 Y =-9.323322057091754E+06 Z =-1.007092461168021E+04
Moon    X = 1.502869015003825E+08 Y =-9.680376320152178E+06 Z = 1.932598843803350E+04

Subtracting Voyager's position from each, and computing $ \sqrt{x^2+y^2+z^2}  $ yields:
Earth 1.22E+07km
Moon  1.26E+07km

So, the Earth is closer.
