How to See the Only Supermoon of 2017

This weekend, sky-watchers can look forward to a phenomenon that hasn't yet graced the skies this year: a supermoon. Before you dig out your telescope, however, here's what you need to know about the only supermoon of 2017 — and the best time to view it. What is a supermoon? The moon makes an elliptical orbit around Earth once every 27 days, while a full moon happens once every 29.5 days. When the moon is full during the portion of its orbit that brings it closest to Earth, Space.