(Reuters) - Japan's Emperor Akihito celebrates 20 years on the throne this week, but the occasion is being marked quietly amid concerns over his health, which doctors say is being affected by stress.
Below are key facts about the 75-year-old emperor and Japan's monarchy.
* Akihito, born in 1933, has made efforts throughout his reign to reconcile Japan with its former colonies in Asia and to help it project an image as a peace-loving nation.
* His father, Emperor Hirohito, headed Japan's empire during its relentless expansion across Asia in the early 20th century. But he renounced his divine status after Japan's WWII defeat in 1945. Under the current Japanese constitution, drafted by U.S. occupation forces, the emperor became the "symbol of the state and the unity of the people."
* The small size of the modern imperial household has led to a dearth of male heirs. Akihito's eldest son Crown Prince Naruhito has only one child -- a daughter, Princess Aiko. Under current law, women may not accede to the throne, so upon Naruhito's death it will pass first to his younger brother, Prince Akishino, then to Akishino's son Prince Hisahito.
* A debate over whether to change the law to allow women on the throne was shelved when Akishino's wife became pregnant.
* Traditionalists believe Japan's imperial institution is the world's oldest hereditary monarchy. Eighth-century chronicles say the sun goddess Amaterasu Omikami bequeathed her grandson a mirror, jewels and a sword, which he gave to the first emperor, Jimmu. The chronicles give Jimmu's reign as 660 B.C.-585 B.C., but there is doubt as to whether he ever existed.
* For most of the imperial institution's history, the emperor lacked direct political power and was primarily a symbolic and religious figure. Under the Meiji constitution, promulgated in 1889, the emperor became a constitutional monarch as well as a divine sovereign who was the focus of loyalty for his subjects.
Reporting by Isabel Reynolds, Editing by Dean Yates