Why has my auth.log file emptied - is this normal?

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http://serverfault.com – Just took a look at my server auth.log file for the first time in a few days, and noticed that it has no entries before 6:47 this morning. This is really odd because I dumped that logfile to my laptop when I looked at it a couple of days before, and it was well over 2400 lines of cron jobs, break-in attempts etc. When I ran cat /var/log/auth.log just now - I was expecting thousands of lines, and got about 50 lines of cron jobs, one break-in attempt and my latest ssh in. Why has this happened? - Does Ubuntu periodically empty this file? Have I been hacked? (HowTos)