Queen Elizabeth marks six decades on the throne in 2012, becoming one of a very few monarchs to reach that milestone.

But there have been three other rulers, equally dedicated to serving their people, who have reached the 60-year mark during Elizabeth's reign.

King Bhumibol Adulyadej of Thailand celebrated 60 years on the throne in 2006. Emperor Hirohito of Japan marked his diamond jubilee in 1986 and Sultan Ibrahim II of Johor, now a part of Malaysia, celebrated his in 1955.

King Bhumibol Adulyadej of Thailand

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Thai King Bhumibol Adulyadej lights a candle during a Buddhist ceremony to commemorate the 60th anniversary of his accession to the throne in Bangkok on June 10, 2006. (Thailand Public Relations Department/Associated Press)

Bhumibol is the ninth king of the Chakkri dynasty, which has ruled Thailand since 1782, and is Thailand's longest-serving monarch.

He was born in Cambridge, Mass., while his father, Prince Mahidol of Songkhala, was studying at Harvard University.

His older brother, Ananda Mahidol, took the throne in 1935 but was found dead from a gunshot wound in 1946. It was never proven whether his death was a murder, but two royal pages and the king's personal secretary were convicted in connection with his assassination.

Bhumibol became king immediately after his brother's death, but was not crowned until May 1950. He was still a student in Switzerland at the time, and was not prepared to take the throne.

Bhumibol, his name meaning ''Strength of the Land, Incomparable Power," became a beloved figure to the Thai people. As king, he did not posses much political power, but he acted as a living symbol and a focus of unity for the nation.

In an interview with the New York Times in 1989, the king spoke about his rise to the throne. The Times reporter asked Bhumibol why he chose to be so involved in the lives of his people, and he replied: "I did not choose; it was chosen for me."

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A Thai royal barge paddles in the Chao Phya River during a rehearsal for celebrations for the 60th anniversary of King Bhumibol Adulyadej's accession to the throne in Bangkok on June 6, 2006. (Sakchai Lait/Associated Press)

In 2006, Bhumibol celebrated his Diamond Jubilee. In the days leading up to national celebrations, United Nations Secretary General Kofi Annan presented the king with the UN's first Human Development Lifetime Achievement Award during a ceremony in Bangkok. He was honoured for the more than 3,000 initiatives he implemented to improve problems relating to individual regions in the country.

The Thai people wore royal yellow t-shirts to commemorate the occasion and show unity amongst the people. The shirts had a special emblem on them chosen by the king for the celebration. The Thai people also wore orange rubber wristbands that said: "We love the King."

Twenty-five countries headed by monarchs sent representatives to the ceremonies.

The longtime leader is now permanently hospitalized in the Siriraj Hospital in Bangkok because of a variety of ailments. He continues to rule with the help of advisers and the heir to his throne, his son Crown Prince Maha Vajiralongkorn.

Emperor Hirohito of Japan

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Emperor Hirohito waves to cheering crowd as he makes the annual New Year appearance on the glass-encased balcony at the Imperial Palace in Tokyo on Jan. 2, 1986, the year he marked 60 years as emporer. (Associated Press)

Michinomiya Hirohito was emperor of Japan for 63 years, from 1926 until his death in 1989.

Before becoming emperor, he had an interest in marine biology and travelled to Europe to study. He was the first crown prince of Japan to travel abroad, and became emperor after the death of his father.

His reign was called Showa, or Enlightened Peace. He possessed supreme authority, but often was only called upon to ratify policies that were put forth by his ministers and advisers.

He publicly opposed Japanese involvement with Germany and Italy during the Second World War.

Hirohito celebrated his 60th year on the throne in 1986. He was considered sacred in Japan and was referred to as Tenno Heika, which means "son of heaven." A special 20-gram gold coin was produced by the Japanese government in honour of the 60th anniversary of his reign.

A short ceremony was held in the national sumo wrestling arena. The emperor spoke in front of about 6,000 Japanese and foreign dignitaries who attended the event.

Hirohito addressed the crowd and said, ''I firmly believe we can open the way to a prosperous future if we only put to use, with ingenuity, the lessons learned from past experiences.''

When he died of cancer in 1989, Hirohito was succeeded by Emperor Akihito. 

Sultan Ibrahim II of Johor

Ibrahim II was the 22nd Sultan of Johor, and the second Sultan of Modern Johor, which is now a part of Malaysia. He was the longest reigning sultan in Malayan history, holding the throne for 64 years from 1895 to 1959.

After the death of his father, Sultan Abu Bakar, he became sultan at the age of 21.

Ibrahim II immediately faced financial difficulty in dealing with the late payment of salaries to state employees. Later, he became heavily involved in overseeing the transactions of the state treasury.

The Sultan largely opposed the idea of Johor being seen as a state within the Federation of Malaya, what is now Malaysia.

At one event set to mark his 60 years in power, he publicly called for Johor's secession from the federation.

The sultan spent the last two years of his life in London, England, with his sixth wife, Marcell Mendl, and their daughter, Tunku Meriam. He died in May 1959 from natural causes.

After his death, Ibrahim's son, Tunku Ismail was appointed the Sultan of Johor.

With files from The Associated Press