19 Jan 2012
Studying the Moon is a challenge when skies are cloudy. We often have to wait patiently for a quick break in the scudding clouds. Those seconds are precious: we must have our binocs ready to find the sickle moon and scan its face.
At 4 am on the 19th we woke up to find the crescent Moon a little south of east – at a narrow angle to the window of Bedroom Observatory. The Moon was sitting just out of reach of the fangs of Scorpio. The crescent is my favourite phase – it is the one I remember from childhood as the illustration in fairy tales and nursery rhymes.
Taking in the view through the binoculars we were delighted to discover features of the Moon we hadn’t noticed before. During the waning phase, the edge along the terminator is more clearly defined as the slanting rays of the sun emphasise physical features by casting shadows of mountains and rims of craters.
At the lower end of the terminator was what appeared to be a small mare with a prominent rim on its lower bank.
Research showed it to be Sinus Iridium or the Bay of Rainbows, the remains of a 260 km diameter walled plain. The mountainous rim on the northern and western edges is the range called the Jura Mountains. Along the line of the terminator, peaks of this range catch the sunlight. The string of bright points is aptly called the ‘jewelled handle’ effect.
Looking closer, about halfway along this mountain range, we noticed the prominent crater Bianchini.
There are a few craters scattered in the area below; some of which are Mairan, Sharp, Harpalus. The Bay of Rainbows is certainly a prominent feature, but we noticed it only now in the crescent Moon.
Moving up (south) we saw the familiar crater Kepler whose rays are now prominent. Aristarchus was its bright self and Grimaldi its dark self.
But what was that fairly large mare with a prominent crater in the top half of the Moon?
I recognized the mare as the Sea of Moisture, but it is more prominent during this phase of the Moon than when I saw it before. The Sea of Moistureis a small circular mare 825 kilometers (275 miles) across. The frilly edge of the Sea of moisture is formed by the mountains of an old impact basin. This basin has been flooded and filled by mare lavas. We could see that the lavas had flowed through the basin rim particularly in the lower left into the southern part of Oceanus Procellarum or Ocean of Storms.
And that crater on the lower (northern) edge, is the large crater Gassendi which was considered as a landing spot for the Apollo 17 mission.
The rim of Gassendi is pretty circular, but is eroded in parts – a big gap appears in the top (southern) end. From the shadow cast it appears that the rim varies in height. Research tells me it goes from as little as 200 meters to as high as 2.5 kilometers above the surface.
A smaller crater cuts into the northern rim, making Gassendi look like a ring with a single stone. We noticed a few central peaks in Gassendi becoming clear as they caught the light.
Click on the link below for a neat photo of Gassendi taken through a near-infrared filter when the moon was waning.
Here are all the features I identified in the crescent Moon.
The top part of the crescent Moon, where it narrows to a point, looks interesting with a collection craters, but it is really hard to work out what they are. So I’ll leave them for another time – another phase of the Moon.