Why has the mother been compared to the 'late winter's moon'?

With the growing age, the poet’s mother has started losing all her vitality and radiance. The poet uses the simile of ‘late winter’s moon’ for her mother to indicate her approaching death. 

Winter, being the last season of the year, is synonymous with lifelessness and dormancy. And, a winter’s moon is also pale-white in colour bearing close resemblance with her mother who, having lost all her strength and beauty, looks ’wan’ and ‘pale’ to the poet. Her mother, too, is in the last phase of her life.