At the age of nine, I bought a mail-order telescope. I practically lived at the post office until it arrived. I soon dropped a marble down the telescope’s barrel (don’t ask), and the mirror broke. But with help from the local science teacher, I soon discovered plenty to explore in the sky without the benefit of any magnification.
Wintertime is ideal for stargazing. Cold air holds less moisture than warm air, so skies are less hazy than in summer. With some extended attention, you can discern fascinating truths about, for example, the relationship between the paths of the sun and the moon.
It seems poetic that the full moon always rises at nightfall; imagine the romantic songs that would not have been written if the full moon rose at 8 A.M. and traversed the daytime sky unnoticed. But there’s a logical explanation.
As the Earth revolves around the sun, and the moon around the Earth, the sun illuminates the moon relative to the moon’s place in its orbit. When the moon is “behind” Earth relative to the sun, the moon is fully illuminated, or full. When the moon is beside Earth (again, relative to the sun) it is either lit on its left or right side; we call this waxing or waning. When the moon is between Earth and the sun, its illuminated side faces away from Earth, and is nearly invisible to us, or “new.”
You can demonstrate this with the help of two friends, a flashlight, and a ball. You are Earth, standing in the middle of a dark room. One friend holds the ball just above and out from your head at one end of the room and walks around you slowly; the ball is the moon, and you should face it. The other friend aims the flashlight (the sun) at the ball. You’ll see that the moon can be full only when you are positioned between moon and sun – that is, when the moon is rising and the sun is setting. Conversely, when the moon moves between you and the sun, its dark side faces you.
You’ve noticed that the sun appears high in the sky in summer and low in winter; the reverse is true of the moon. While no feasible exercise can fully illustrate this phenomenon, some observations help us understand the apparent changes in the positions of the sun and moon throughout the year.
The Earth rotates once daily on a polar axis tilted at 23 degrees; this makes the sun appear high in the sky in summer (Earth tilted towards the sun) and low in winter (tilted away from the sun). The Earth does this 365 times in its annual journey around the sun.
The Earth, moon, and sun all occupy nearly the same plane in space. The plane of the moon’s Earth orbit is about 5 degrees different from that of Earth’s orbit around the sun.
Imagine yourself as the Earth, strapped to a pole and tipped at 23 degrees, rotating 365 times as you walk once around the sun; your friend moves the moon ball around you on a plane 5 degrees different from the sun/Earth plane. The moon does this about 12 times for every trip you take around the sun. You’ll begin to see why the sun and moon occupy different places in our sky at different times of the year. (You’ll also understand, as the uninjured one of you calls 911, why you shouldn’t have tried this.)
Because the moon’s monthly cycle is not mathematically synchronized with the length of the Earth day or year, the moon rises approximately 50 minutes later each day than the day before. This varies seasonally and with the latitude at which moonrise is observed; other heavenly cycles contribute to this variation as well. All these variables contribute to the sun’s and moon’s appearance in different parts of the sky at different times. A full moon, rising at nightfall, foreshadows the sun’s position six months hence. Test this by recording the place where the full moon rises around the winter solstice; at the summer solstice, the sun will rise in roughly the same spot on the horizon.
Long before the telescope’s invention, stargazers discerned much about the movements of the sun, moon, planets, and stars. While early astronomers’ understandable assumption that all celestial movements had the Earth as their center was later proven wrong, many of their observations were detailed, astute, and astonishingly accurate.
Unlike me, the impatient child, they weren’t waiting for a telescope to arrive; they had ample time on their hands and brilliantly recorded the patient observations they made. Today, on any clear night, we may commune with their exploring spirits, confirm their wisest observations, and come to know a whole new meaning of the expression “the music of the spheres.”