Why did Lord Ganesha curse the Moon?

Question

When there's Ganesha Chaturthi going on, people say that we shouldn't look at the moon on the day of immersion (visarjan) as its counted as a bad omen.

So why did he cursed the moon and why is that, we shouldn't look at the moon?

Answer 1

Lord Ganesha cursed the Moon god because once he made fun of the form of form of Ganesha (which is elephant-headed). The story is given in the Khanda 4 and Chapter 6 of Mudgala Purana. The relevant verses are:

एतस्मिन्नंतरे तत्र चन्द्र: शंकरभालग:।
जहास विवृतैर्दंतैर्गणेश रूपगर्वित:।।
उवाच मोहितोऽत्यंतं प्रणम्य गणनायकम्।
गजवक्त्रयुतं रूपं किं धृतं विकटं प्रभो।
पुनः पुनरुवाचेदं वाक्यं हास्यपरायणः।
ततस्तं कुपितो देवः शशाप शशलाञ्छनम्।।
ये त्वां विलोकयंते वै चन्द्र ते पापिनः सदा।
भवंतु विघ्नसंयुक्ता मद्वाक्यान्नात्र संशय:।।(Mudgala Purana 4.6)

There that Moon which was situated on the head of Shankara started laughing at Ganesha showing his teeth due to pride in his own form. By becoming deluded he spoke to Ganesha as: 'Your form with this elephant head is so funny!' Saying this again and again he laughed. This made the Lord (Ganesha) angry and cursed: 'Those who will see Moon will always be sinner. They will get full of obstacles.' This word of mine shall not undergo false.

After that incident moon god also became powerless and creation also is effected negatively due to Soma tatwa (Moon element) being powerless. All gods and Moon God also pray to Lord Ganesha and after various austerities Lord Ganesh appears before them and tells Gods to ask for boon:

यदि प्रसन्नभावेन वरदोऽसि गजानन।
तदा चंद्र सुनिर्दोषं कुरू नाथ नमोऽस्तु ते।।
स तानुवाच सर्वात्मा भक्त्या संतोषितः प्रभुः।
भाद्रशुक्लचतुर्थ्यां वै न द्रष्टव्यः सुधाकरः।।
हास्यं कृतं मदीयं वै तद् दिने तेन देवपाः।
दोषयुक्तो विधुस्तस्मान्नान्यथा मे वचो भवेत्।।

If you are pleased with us oh Lord Gajanana give us a boon that Moon should be freed from the defect (of your curse), we pray to you, Lord. Becoming pleased with their devotion the Lord who is self of all spoke then: 'Do not see the moon on the day of Bhadra Shukla Chaturthi (4th bright fortnight of Bhadra month). The moon had laughed at me on that very day and hence that defect (curse) will be applicable on that day only and not otherwise.'

So, the curse is now applicable only on the day of Bhadra Shukla Chaturthi (4th bright fortnight of the month of Bhadra). Also as per Ganesha Purana chapter 72 if someone mistakenly sees moon on this day he should do Chintana (contemplation/meditation) on the form and deeds of Lord Ganesha!