LENGTH function for a large number returns 40 not the number of digits

Question

In SQL*Plus:
SQL> select length(12345678901234567890123456789012345678901234567890)lngth
2 from dual;
LNGTH
40
SQL> select length('12345678901234567890123456789012345678901234567890')lngth
2 from dual;
LNGTH
50
It seems that the implicit conversion from number to char in the first query causes an unexpected result. From the documentation of the LENGTH function at http://download.oracle.com/docs/cd/B19306_01/server.102/b14200/functions181.htm#i79330:
"If you omit fmt, then n is converted to a VARCHAR2 value exactly long enough to hold its significant digits."
Perhaps Oracle considers that only 40 digits are significant (?)

Answer 1

Additional reason.
There is implicit data conversion as like to_char(number).
SQL> set numwidth 50
SQL> select 12345678901234567890123456789012345678901234567890 num from dual;
                                               NUM
12345678901234567890123456789012345678900000000000
SQL> select length(12345678901234567890123456789012345678901234567890) numlen from dual;
                                            NUMLEN
                                                40
SQL> select to_char(12345678901234567890123456789012345678901234567890) numlen from dual;
NUMLEN
1.2345678901234567890123456789012346E+49
SQL> select length(to_char(12345678901234567890123456789012345678901234567890)) numlen from dual;
                                            NUMLEN
                                                40
SQL> select to_char(12345678901234567890123456789012345678901234567890,
  2                '09000000000000000000000000000000000000000000000000')
  3  num from dual;
NUM
12345678901234567890123456789012345678900000000000
SQL> select length(to_char(12345678901234567890123456789012345678901234567890,
  2                '09000000000000000000000000000000000000000000000000'))
  3  num from dual;
                                               NUM
                                                51
SQL> select to_char(12345678901234567890123456789012345678901234567890,
  2                'fm09000000000000000000000000000000000000000000000000')
  3  num from dual;
NUM
12345678901234567890123456789012345678900000000000
SQL> select length(to_char(12345678901234567890123456789012345678901234567890,
  2                'fm09000000000000000000000000000000000000000000000000'))
  3  num from dual;
                                               NUM
                                                50