After all the turbulence that happened at the time of Ganesha's birth, everything started to become normal again. But since Ganesha was made the first one to be worshipped before starting any auspicious ritual, some of the Gods were not happy with this.
The Gods thought that the only reason why Ganesha was declared to be the one for first worship, was the anger of Goddess Parvati when Lord Shiva severed Her boy's head and the Goddess demanded Her son to be brought back to life or else She would end the entire Creation and finding no other new head available to be placed on the boy, an elephant's head was placed.
Making sure that no one mocks Ganesha for having an elephant's head, it was declared that Ganesha will be the first one to be worshipped in rituals from now on.
Read: The Story Behind Ganesh Chaturthi
However, many Gods felt that they were far superior to a little boy with an elephant's head and deep down inside, they were not comfortable with the idea of Ganesha being worshipped first.
But, since this provision of first worship was made by the Tridevas or the Holy Trinity (Lord Brahma, Lord Vishnu & Lord Shiva), they quietly accepted it.
Lord Ganesha's Curse to the Moon
Once it happened that on Ganesh Chaturthi, Lord Ganesha was going somewhere on his vehicle and interestingly Lord Ganesha's vehicle is a mouse!
While Lord Ganesha was on the move on his mouse, the Moon was watching him from above and laughed at him mockingly.
He made fun of Lord Ganesha for having an elephant's head and a mouse as his vehicle. This was certainly not a display of humbleness on part of the Moon.
This unjust and uncalled for mocking from the Moon did not go well with Lord Ganesha and he got angry at the Moon.
Read: जानिए गणेश चतुर्थी की पूजा और मुहूर्त का समय
In his anger, Lord Ganesha cursed the Moon that light will never fall upon him and he will loose his illumination forever.
Hearing this, the Moon realized what a mistake he has done by mocking Lord Ganesha and begged for mercy.
Watching the Moon repent, Lord Ganesha out of mercy, modified his curse. Lord Ganesha said: "The Moon will endlessly go through the phases of waxing and waning to remember the virtue of being humble".
Lord Ganesha also declared that if anyone looks at the Moon on this day, unjust & false blame and social criticism will fall upon that person.
Hence, there is a tradition of not looking at the Moon on Ganesh Chaturthi and people also remind themselves of staying humble and polite to others in life.
Read: How to celebrate Ganesh Chaturthi at home?
Another story that goes like this:
Everyone knows that Lord Ganesha is very fond of sweets and once a devotee offered him lots & lots of sweets. Lord Ganesha was such pleased with the devotee's devotion that he stayed there the entire day and kept on eating the sweets. But when the night time approached, Lord Ganesha took the remaining sweets with him and headed for home.
But with all that overeating, his stomach became full and while he was walking back he stumbled and fell down.
This made Lord Ganesha feel embarrassed and he got up, dusted himself and picked up all the sweets that he was carrying for home. Lord Ganesha carefully looked around in hope that no one had seen him.
But the Moon who was up in the sky saw everything. It must be noted that in those days the Moon was always full throughout the month and the was no phenomenon of waxing and waning of the Moon.
Watching Lord Ganesha take a stumble and fall due to overeating, the Moon did not help Lord Ganesha and mocked him by laughing at him. The Moon was very proud of his beauty and considered himself as someone very handsome & good looking.
Lord Ganesha became extremely angry when he saw the Moon laughing at him and making fun of him rather than helping him as any gentleman should have.
Read: जानिए क्यों किया जाता है गणेश विसर्जन और क्या है इसका महत्व
It was then, Lord Ganesha cursed the Moon that light will never fall upon him and he will loose his illumination forever. He cursed that the Moon will disappear from the sky, leaving him with no chance to showcase his beauty.
The Moon immediately realized his mistake and begged for mercy to Lord Ganesha to which Lord Ganesha said that he can not take back his curse but since the Moon has learnt his lesson, he sure can modify the curse.
Lord Ganesha told the Moon that: He will endlessly go through the phases of waxing and waning to remember the virtue of being humble and since he committed this mistake on a Ganesh Chaturthi, hence anyone who would look at the Moon on this day, will face some problems in life such as unjust & false blame and social criticism.
A scientific approach:
Ganesh Chaturthi occurs on the 4th day of the waxing period of Moon. Interestingly on this day, the cosmic alignment of the Sun, Moon and Earth is such that the Moonlight which falls on Earth bears a negative charge.
It is for this reason, one is advised not to see the Moon or spend time in the exposure to moonlight on this day.