With the Mercury transit taking up so much of yesterday I didn’t even mention another bit of news. It’s the full Beaver or Frosty Moon. Because the moment of full moon occurred this morning at 7:34 a.m. CST we’re fortunate to get two nights of full moonness — last night and tonight. Last night, the brilliant lunar disk rose a little before sunset. Tonight, because it’s moved further to the east, it rises about 25 minutes after sunset. Click here to find out your moonrise time.
That means we’ll see it come up in a darker sky, making it easy to tell where it will rise along the northeastern horizon. First, find a location with a good view in that direction and scan back and forth about 5 minutes before moonrise looking for a telltale glow-spot that gives away the moon’s rising point. I always bring a camera and pair of binoculars along to capture images of the squished celestial fruit and its lumpy, jagged outline. The moon’s edge is rarely smooth at rising because variations in the temperature and density of air pockets along our line of sight distort its outline.
These variations may not be apparent with the naked eye, but binoculars and telephoto lenses show them clearly. They’ll also reveal how the atmosphere acts like a prism and spreads the white light of the moon into a slight rainbow, an effect called dispersion. The top edge of the moon glows blue-green and the bottom red. Imagine watching a moonrise without the atmosphere in the way. Luna would crest the horizon at full brightness and look perfectly circular. No thank you.
As you can see in the photo (above), last’s night moon was not 100 percent full. The shadow line, called the terminator, defines the border between sunlight and darkness on the moon. Earth also has a terminator as viewed from orbit. On the one side the sun is shining; on the other it’s still night. Last night’s terminator wrapped around the top or northern edge of the moon. You can tell it’s there because shadow-filled craters — where the sun was only starting to rise — are visible. These were not apparent with the naked eye but an easy catch in 10x binoculars.
Tonight the moon will be slightly past full, and the terminator will appear along the opposite (west) side of the moon. The moon’s phase waxes from new to full and then wanes from full to new. Before full moon the terminator marks the line of advancing sunrise. After full, it’s the line of advancing sunset.
The full moon always rises around sunset because it shines directly opposite the sun in the sky. But the moon never stands still. As it orbits the Earth, it keeps moving east (left in the northern hemisphere), rising about an hour later each night while waning from full to gibbous (“three-quarter” phase) to half to crescent. That’s why we have to stay up late or rise in the morning hours to best appreciate the moon’s slow vanishing act.