By Jacob Gronholt-Pedersen COPENHAGEN (Reuters) -Denmark’s Queen Margrethe II, Europe’s longest-serving monarch, will abdicate on Jan. 14 after 52 years on the throne and will be succeeded by her eldest son Crown Prince Frederik, she announced on Sunday. The 83-year-old queen, who ascended the throne in 1972, made the surprise announcement on live TV during her traditional New Year’s Eve speech, which is viewed by many in the country of 5.9 million people. Referring to a successful back operation she underwent in February, she said, “The surgery naturally gave rise to thinking about the future…