SQL & PL/SQL

What is the best way to get Weekdaynum for a date?
The algorithm should be independent from nls-parameters.

This solution depends from nls_teritory and works differently when
nls_territory='AMERICA'
and when it is ESTONIA:

select decode (
   to_char(sysdate,'DY','nls_date_language=english'), 
   'MON', 1,
   'TUE', 2,
   'WED', 3,
   'THU', 4,
   'FRI', 5,
   'SAT', 6,
   'SUN', 7
) WeekdayNum
from dual;