oracle - why am I getting ORA-00936 error -

i can run following statements , correct results:

select moon_phase_text (sysdate)    dual;  select date_day3    aday3import t1,         aasum_report t2   t1.date_day3 = t2.game_date; 

this statement gives me list of days ie 22-feb-03

however when trying update field in table following statement error ora-00936: missing expression

update aday3import    set moon_phase = select moon_phase_                      (select date_day3                          aday3import t1,                               aasum_report t2                         t1.date_day3 = t2.game_date )                     dual; 

instead of going dual. try concatenating moon_phase_ in select statement

 update aday3import set moon_phase =                   (select 'moon_phase_'||date_day3                      aday3import t1,                           aasum_report t2                     t1.date_day3 = t2.game_date )