The sun and the moon mean that Nepal will last as long as the two remain in the sky. The flag has been in use in Nepal for centuries, from the time of the Malla dynasty that preceded the Shahs and the Ranas. As an example, see this photo of the Golden Gate in Bhaktapur (1753 AD) which is ornamented with a metal Nepal flag. Such flags are installed at the doorways of many temples in Kathmandu; some have the sun and moon design and some don't.