Apache2 doesn't find qualified domain name

[Micha Feigin <michf@post.tau.ac.il>]

I just installed apache2 on a machine with a proper domain name
received through dhcp (maybe not properly since I didn't play with the
default settings)

When I start apache it gives me the error:

Starting web server: Apache2apache2: Could not determine the server's fully qualified domain name, using 127.0.0.1 for ServerName

How do I fix this without adding ServerName explicitly?