The Japanese monarchy is the oldest hereditary monarchy in the world -- records show the imperial line has been unbroken for fourteen centuries.

Emperor Akihito -- meaning shining pinnacle of virtue -- is the 125th Emperor of Japan and a direct descendant of Japan's first emperor Jimmu, circa 660 BC.

The emperor is a ceremonial but revered figure in Japan's constitutional monarchy.

Born in December 1933, he was a child during Japan's military invasion of China, its foray into World War II and its subsequent defeat and foreign occupation.

He is the world's only monarch with the title Emperor and was the first Japanese Crown Prince to marry a commoner.

IS IT EVEN POSSIBLE TO ABDICATE?

No.

The last emperor to abdicate was Emperor Kokaku in 1817 in the later part of the Edo Period.

The current legal framework prevents abdication of a sitting emperor, so in order for an abdication to go ahead, there would need to be revisions to the Imperial Household Law.

Any changes would require approval by Japan's parliament; a process that could invite a broader debate around the monarchy's relationship to the state, the role of the emperor in modern Japan and succession law.

SUCCESSION LAW?

Japanese constitutional law stipulates the throne must pass down the male line.

56-year-old Crown Prince Naruhito is next in line to the throne. As he only has one daughter, his younger brother, Crown Prince Akishino, would succeed him, followed by Akishino's son, Prince Hisahito.

With calls mounting from politicians and academics to remove what might be Japan's last legally codified gender equality, and Japan's ruling Liberal Democratic Party (LDP) previously saying it would consider legal changes that let a woman inherit the throne, the issue is likely to raise its head again.

This could prove divisive in Japan, as the very thought of a female on the throne is anathema to some conservatives, who in pre-war days saw the emperor as divine.

"Even if the public is fine with the idea, the current Diet [parliament] is disinclined to amend the law," says Jeff Kingston, director of Asian studies at Temple University Japan.

'THE RECLUSIVE PRINCESS'

Prince Naruhito married his wife, Crown Princess Masako, in 1993, but soon after she began to suffer from a stress-related illness.

A multilingual former career diplomat, many hoped the most cosmopolitan princess in Japanese history would liberate the tradition-bound court. But the royal marriage appeared to transform the articulate career woman into a soft-spoken shadow, a change much lamented in both Japanese and foreign media.

The Harvard graduate eventually became more reclusive, with reports she was suffering from depression and anxiety as a result of royal family household pressures.

Questions remain as to how this uptick in official duties could affect the popular princess and what role she would play in public life, especially as media scrutiny and restricted royal life are said to have contributed to her ill health.

"Given her frail health there is obvious cause for concern," says Kingston. "But she has made some appearances recently. One can only hope that she would be able to provide the stalwart support for Naruhito that Michiko has given to Akihito."

WHAT ELSE COULD IT MEAN FOR JAPAN?

Emperor Akihito has repeatedly expressed remorse for Japan's aggression in the first half of the 20th century.

In 1992 -- on the first visit to China by a Japanese monarch -- he said he deplored Japan's treatment of the Chinese before and after the war.

"Naruhito's birthday remarks last year were very similar to Akihito's regarding history and need for contrition about wartime aggression," says Kingston.

"I think he will continue to act as the nation's leading emissary of reconciliation."