from cron(8):

Additionally, cron checks each minute to see if its spool directory's

modtime (or the modtime on /etc/crontab) has changed, and if it has,

cron will then examine the modtime on all crontabs and reload those

which have changed. Thus cron need not be restarted whenever a crontab

file is modified. Note that the crontab(1) command updates the modtime

of the spool directory whenever it changes a crontab.