The moon, big and bright, shines nearly full tonight and fully full tomorrow night. It clears the eastern horizon around sunset, looms upward during twilight as the sun sinks deeper below the world’s western rim, and shines ever higher as evening advances.
If you know why the full moon behaves this way, you know more practical astronomy than many do, and perhaps even as much as your great-grandparents did.
Back when people lived, worked, and slept closer to the cycles of nature, the reason why the full moon was full was obvious. This is the time of the month when the moon is on the opposite side of the sky from the sun, so we look into its full sunlit face. Whenever you gaze at the full moon, the sun is behind your head. So, of course, the full moon rises in the east as the sun sets in the west, shines highest in the middle of the night when the sun is farthest down, and sets in the west when the sun rises in the east.
The position of the full moon opposite the sun has other interesting effects that are less well known. For instance, it is now the coldest time of the year, not long after the winter solstice. The reason why it is winter and why winter is cold is that the sun is far south in its seasonal peregrinations. So, for us living in the Northern Hemisphere, the sun travels low across the sky during the day. January is cold because the sun’s low, slanting rays heat the ground less effectively than the sun’s straighter-down rays in summer.
If the winter sun travels low through the day, that means the winter full moon rides high through the night. And sure enough, if you stay up into the middle of the night this weekend, you’ll see the bright moon passing almost overhead, just as the sun does on the hot middays of June and July.
That is why, when Santa arrives on the rooftop at midnight in “The Night Before Christmas,’’ “The moon on the breast of the new fallen snow/ Gave the lustre of midday to objects below.’’
That was published in 1823 when more people grew up knowing how the sky worked - even if they had no idea of black holes, extrasolar planets, colorful star-birth nebulae, or other aspects of modern astronomy that we take for granted but rarely experience.
Similarly, when the sun rides high in June, the full moon stays low all night, where earth’s thick atmosphere turns it yellow. That’s why the full moon of June is the “honey moon.’’
Bright moon, bright Orion
Tonight the moon shines between the constellations Orion and Gemini. Look for Orion to the moon’s lower right. Although moonlight washes out faint stars nearly as much as city light pollution does, Orion is bright enough to shine through both.
Orion is the centerpiece constellation of winter. Its main shape is formed by seven unusually massive, giant stars burning furiously. They’re only several million years out of their nebular star-birth nests. Six of them are blue-white. The flagrant exception is fire-colored Betelgeuse in Orion’s shoulder, the sky’s brightest example of a red supergiant.
If we could see Betelgeuse close up, it would not be round and clean like the sun and most stars. Betelgeuse is an irregular, puffy, slow-boiling ball of turbulence made of big convection cells like yellow-hot mushroom heads. And you’d see it embedded in gassy, dusty surroundings extending far into the distance.
Because Betelgeuse is so big, the mushroom heads take months or years to rise, swell, cool, and sink while new ones push up from below. These goings-on show up to the naked eye as slight changes in the brightness of Betelgeuse, which you can track from your back yard if you take careful note over a span of years.
Lower left of the moon tonight and directly left tomorrow, shine Castor and Pollux, the heads of the Gemini twins. These are lesser stars just 52 and 34 light-years away, compared with several hundred light-years for Orion’s giants.
Far below the moon is much closer Procyon, 11 light-years distant. And below Orion is brighter Sirius, at only 8.6 light-years. That makes Sirius the nearest thing outside our solar system that you can see from our latitudes with the unaided eye.Easy-to-use maps of stars and constellations across the entire evening sky are available at SkyandTelescope.com/getting started. Alan M. MacRobert is a senior editor of Sky & Telescope magazine. His Star Watch column appears the first Saturday of every month.