Chandra Grahan or Lunar eclipse in Hinduism is also known as Kethugrasta Chandragrahana. Kethu is one among the nine grahas or planets in astrology. There is an interesting story in Hindu religion that talks about how Chandra Grahan takes place. This mythology is related to the Samudra Mathana or the Churning of the ocean.
At the end of the Samudra Mathana, when the Amrita, or elixir of life, was retrieved devas (demi gods) and Asuras (demons) fought for it. Lord Vishnu appeared in the form of Mohini, a beautiful damsel, and promised to equally share the Amrut with Devas and Asuras.
The Devas and Asuras sat in two different rows. Mohini started serving the Amruta to Devas first. An Asura who thought that there was something fishy in the whole episode took the appearance of a Deva and sat near Chandra (moon) and Sun (Surya).
At the point when Mohini served the Asura the Amrut, Chandra and Surya found out that the one sitting near them was not a Deva and shouted that he was an Asura in the appearance of a Deva.
Immediately, Mohini took the form of Lord Vishnu and used his Sudarshana Chakra to cut off the head of the Asura. The body of the Asura came to be known as Kethu and the head of the Asura was attached to a snake and this came to be known as Rahu.
To take revenge on Surya and Chandra – Rahu tries to swallow Surya periodically and Kethu tries to swallow Chandra periodically.
When Kethu attempts to swallow the moon the Chandra Grahan or Lunar Eclipse happens. This is the reason why lunar eclipse is also known as Kethugrasta Chandragrahana in Hinduism.