Vaidyanath, or Vaidyanathan, is one among the numerous names of Hindu God Shiva. Vaidyanath means the supreme physician. Shiva got the name after he restored the luster of Chandra – the moon god. Chandra was cursed by Daksha Prajapati to lose his luster.
Legend says that Daksha, the master of civilization, gave twenty seven of his daughters – the Nakshatras, lunar asterisms or birth stars – in marriage to Chandra. But the moon god was enamored by the beauty of Rohini Nakshatra and ignored the other Nakshatras.
The other angry and jealous wives of Chandra went to their father and informed about Chandra’s favoritism for Rohini. This behavior of Chandra infuriated Daskha. He cursed Chandra that his body of which he was so proud will lose its luster and will fade away.
Soon the body of Chandra started loosing its luster and this caused upheaval on earth. Several plants and herbs need the moonlight to survive.
Chandra then approached Srihari Vishnu who directed him to Shiva who as Vaidyanath is the supreme physician and the keeper of the sacred herb Soma.
Left with only one part (Kala), Chandra approached Shiva. Shiva helped Chandra in regaining his lost luster and glory with the herb Soma. But Daksha cursed him again and he took the help of Lord Shiva and this continued for a long period. Finally, Shiva asked Chandra to take refuge in his lock of hair on his head where he will find enough soma to rejuvenate him periodically.
Thus the curse of Daksha causes the moon to wane and Shiva rejuvenates him.
Since Shiva rejuvenates Chandra, he came to be known as Vaidyanathan – the Supreme physician and keeper of the secrets of the herbs.