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#!perl

...

say join(' ', $_, @{$merged{$_}}) for sort keys %merged;

ken@Miranda ~/c/_/tmp
$ cat gravity
...
2010-10-03 00:00:07 lat3 long3 bath1
2010-10-04 00:00:09 lat4 long4 bath2
2010-10-05 00:00:01 lat3 long3 bath3

ken@Miranda ~/c/_/tmp
$ geo_file_join.pl
...
2010-10-03 00:00:07 lat3 long3 grav3 mag2 bath1
2010-10-04 00:00:09 lat4 long4 null mag3 bath2
2010-10-05 00:00:01 lat3 long3 null null bath3