With the 50th anniversary of the Apollo 11 lunar landing this month everyone’s talking about the moon. By happy circumstance it’s also shining in the southern sky every clear night. If you’ve been following its progress, you’ve noticed at least two things: the moon sets later each night and its phase has waxed or increased from crescent to gibbous.
As the moon orbits the Earth it moves about a fist eastward (to the left) each night. That eastward movement changes the angle the moon makes with the sun and Earth, which causes the phase to change. Each night brings a new slice of lunar landscape into view, one of the reasons it’s such a joy to observe the moon in a telescope. Tonight’s moon features some big-time impacts. We’ll focus on three.
The first is Copernicus, one of the most picturesque of lunar craters with a diameter of 58 miles (93 km) and surrounded by a rayed halo of impact debris 435 miles (700 km) wide. It’s so big you can see the whole of it as a fuzzy, white blob with the naked eye. Through a small telescope it closely resembles a scoop of vanilla cream that fell out of a cone and hit the ground with a splat.
Copernicus is a relatively young crater. Rocks believed to have been ejected during its formation were collected by the Apollo 12 astronauts and dated at 800 million years old. Now I know that sounds old, but when it comes to the moon, where many craters date back 4 billion years, that’s young! Copernicus lies along the southern “shores” of the Sea of Showers, also known by its Latin name Mare Imbrium.
Notice anything about the shape of the sea? It’s round just like a lot of lunar craters. That’s because it is one! Just a bigger version from a much larger impact. Astronomers call these giant craters basins. The asteroid that gouged out the Imbrium Basin is estimated to have been 150 miles (250 km) across and struck 3.9 billion years ago.
What a titanic wallop! The impact was so powerful that it uplifted the lunar crust, creating the several mountain ranges that ring the basin. Deep fractures in the basin floor allowed magma from below to bubble up and fill it with blazingly hot liquid rock like a swimming pool. Between the impact and lava flows, the cratering rate dropped, the reason the mare’s floor looks relatively smooth compared to surrounding areas.
What about that romantic Bay of Rainbows? It is one of the most striking features on the moon when seen at the right phase, and that phase is tonight. Also known as Sinus Iridum, it’s one half of a 162-mile-wide (260 km) crater, the near wall of which was breached by Mare Imbrium magma. The magma flowed over the crater and filled it up, burying half the wall and all the hills and canyons inside the crater — a lunar version of ancient Pompeii.
The Sea of Showers is huge and easily visible with the naked eye. Extremely keen-eyed moon watchers might glimpse the arc of Sinus Iridum without optical aid, but you’ll have no problem seeing it in binoculars. While Copernicus and likely the Bay of Rainbows were strictly impacts, the giant hit that made the Sea of Showers went beyond, mixing impact with widespread volcanism, the result of which we can see today during any walk in the moonlight.
If you live in the Duluth area and want to see all this cool stuff through a telescope, I’ll be down this evening at Ursa Minor Brewing in Duluth’s Lincoln Park neighborhood sharing the moon with the pub’s patrons from 9 till 10:30 p.m. weather permitting. Maybe see you there!