This year, there will be a "blue moon" for Halloween. So for today's show, we're asking: What IS a blue moon? Is the moon ever blue? And are they as rare as the phrase "once in a blue moon" implies?For additional info and fun links, check out the episode page.Follow Maddie and Rebecca on Twitter for more science nuggets. Maddie's @maddie_sofia and Rebecca's @rebeccalramirez. Email the show your celestial musings and inquiries. We're at shortwave@npr.org.