One day while travelling round the universe on his rat, Ganesha came to Chandraloka 

(the realm of the Moon). The Moon saw him. The Moon, very handsome, was proud of his appearance. Suddenly, the rat got scared by a snake in the realm of the moon and fell. On seeing the elephantfaced, big-bellied Ganapati falling to the ground, he laughed at Ganesha with contempt.

This was an insult and Ganesha was very angry about the Moon. His eyes grew red. He pronounced a curse "Oh Moon, your handsome appearance has made you too vain. Fool, I am worshipped in all the worlds, but you laugh at me. Receive now the fruits of your foolish pride. Let your beauty, which is the cause for your arrogance and ignorance vanishing! From now on, whoever sees you on the fourth day of the bright fortnight of Bhadrapada month, the day of my birth, will suffer because of unjust accusations."

The curse shattered the Moon's pride. He realized his mistake, and felt sorry. Standing devotedly with folded hands before Ganesha he prayed to him. "Sir, forgive me and my ignorance. Take back the curse and protect me."

Then the kind Ganesha grew calm. He consoled the unhappy Moon. He said, "Moon, you have realized your fault. What is important is the destruction of your pride. Anyhow, my curse cannot be in vain. But those who are subjected to false accusations will be saved and regain their good name if they see you on the second day of the bright fortnight also or listen to the story of the Syamantaka gem." The Moon was satisfied. It was from that day onwards that the moon has been decreasing in size before the no moon day and increasing untill the full moon day.