The moon’s been waxing for more than a week and will finally round into a Full Thunder Moon on Tuesday the 19th. Given the season there’s a real chance for thunder at moonrise. But for the sake of seeing a beautiful full moon rising I hope the skies remain clear and quiet for you and me both. July’s big moon also goes by the Full Buck Moon, named by Native Americans for the time when bucks begin to grow their new, velvety antlers.
We enjoy the cycle of watching the waxing moon. It’s easy and leisurely, taking about two weeks for the moon to fill out from an impossibly thin crescent to full. Each day, the terminator, the moving line of sunrise on the moon’s globe, creeps to the left or east revealing a new wedge of lunar landscape. If you lived on the moon, you would welcome each sunrise with great passion, since they only happen about once every two weeks. Bereft of sunlight for so many days, lunar nights can get mighty cold with temperatures sinking to around –250°F (–157°C). Any lander sent to the moon has to be well-heated or it won’t make it to the next sunrise.
No special tools are needed to watch the phases though binoculars and telescopes show individual craters, mountains and shadows in luscious detail. As the moon waxes, the angle between it and the sun increases. As a crescent, it’s only a fist or two (10°-20°) from the sun. At half phase, that angle increases to 90°, and at full moon, 180° or directly opposite the sun in the sky. Opposite means that the full moon rises in the east when the sun sets in the west just like two people on opposite ends of a playground teeter-totter. Remember those?
Being opposite the sun, a full moon is directly illuminated by it like someone shining a light directly in your face. Shadows disappear and the moon has a pasty look. Not for long though. The terminator returns immediately after full moon during the its waning phases; instead of marking the advancing line of sunrise, it now traces the advance of sunset. The first hint of shading appears along the moon’s right side. Slice by slice, the terminator creeps to the east as the moon dwindles from gibbous to half (also called last quarter) to crescent and finally new. All the while the angle it makes to the sun decreases, from 90° at last quarter to 10°-20° at waning crescent and finally to 0° at new moon, when the moon and sun are lined up together in the daytime sky.
As seen from the moon, Earth undergoes identical but opposite phases. An astronaut standing there would see a full Earth during the time of new moon and a new Earth (invisible or nearly so) when the moon was full. This month, Earth joins Venus and Mercury in Cancer in that imaginary astronaut’s gaze. You’d have to block the sun with your hand to see them, but all three planets will be lined around the time of full Earth. It’s fun to turn the tables, isn’t it?
If you’re out later Tuesday night (10 p.m. on), point your binoculars at the moon and look in the upper left of the field of view (about 5° northeast) for the optical double star Alpha 1 and Alpha Capricorni in the Capricornus the Sea Goat. The two stars form a neat little pair north of Beta Capricorni, a true double star you’ll need a small telescope to split. Though the dual alphas appear double, it’s only a line-of-sight illusion. Though unrelated they still look cool.