The full Moon is a wondrous sight in its own right, but it can also lead to unexpected insights into the relationships among the Sun, Moon, and Earth. Photo by Gary Seronik
One of the most enjoyable aspects of backyard astronomy is that you can participate in the hobby with no equipment at all. Sure, there are sights only a telescope can show, but there are also spectacles best appreciated with your eyes alone. The northern lights, meteor showers, and the grand sweep of the Milky Way are a few examples. But one of my favourite naked-eye activities is simply keeping track of the comings and goings of the Moon. Watching its cycles of phases and movement across the sky can lead to a deeper appreciation of the solar system’s intricate clockworks. One incident from childhood stands out as a particularly memorable demonstration of this.
I remember going downstairs late one sleepless night in December, and looking out our south-facing kitchen window. The yard was awash with light from a full Moon — it almost seemed like daylight. I could easily make out details in the trees and bushes that adorned the yard and the patio furniture on our deck. But there was more to it than that. For some reason, the scene brought to mind warm, lazy summer days. It was an odd sensation — both striking and undeniable. Here it was, a cold winter’s night, yet I couldn’t shake the sense that outside our window summer awaited. Why was that? Years later, I finally figured it out.
Regular sky watchers know the full Moon is always positioned opposite the Sun’s location in the sky. The most obvious manifestation of this fact is that the full Moon rises as the Sun sets. But this opposite positioning has another, less obvious feature. It also means in winter, at midnight, the full Moon is roughly in the same place in the sky as the midday Sun is in summer. In other words, the December full Moon is high overhead in Gemini or Taurus, which is where we find the Sun in June and July. So, the winter Moon casts stubby shadows — just like the summer Sun. No wonder the scene outside my window that December night long ago brought to mind the warmth of a summer’s day.