Steve’s Virtual Planet

Viewing the Lunar Straight Wall (archived)

I just happened to notice an event on my 2010 observing calendar a few months ago, something about a “Lunar Straight Wall”. Hmm, I had not heard of that one before, probably because I had not paid too much attention to the finer details of our moon. But since this event corresponded with a normal Thursday open house down at Fernbank Science Center, I decided to use the 36 inch scope and try to find this wall.

Well, good grief, I found it alright!! Can’t miss this thing if you’re looking anywhere near Mare Nubium. It’s quite prominent as an almost straight line. However, it turns out that it is easy to see ONLY on the first day past 1st Quarter, about 8 to 8.5 days phase. That’s why its in the calendar every month. I just had not noticed before. But is one of the best examples of a linear fault, also called a rille. The reason why it’s only listed around this time is because of the shadow that it casts when struck with the sunlight at just the correct angle. For the rest of the month, it’s virtually hidden because it no longer creates a good shadow.

I have found several references online, like the one below from Wikipedia. The link has an image of the feature. The Lunar Straight Wall is also known by it’s Latin name Rupes Recta.

Wikipedia - Straight Wall
“Rupes Recta is a linear fault, or rille, on the Moon, in the southeastern part of the Mare Nubium at 22°06′S 7°48′W / 22.1°S 7.8°W / -22.1; -7.8. The name is Latin for “Straight Fault”, although it is more commonly called the Straight Wall. This is the most well-known escarpment on the Moon, and is a popular target for amateur astronomers.

When the sun illuminates the feature at an oblique angle at about day 8 of the Moon’s orbit, the Rupes Recta casts a wide shadow that gives it the appearance of a steep cliff. The fault has a length of 110 km, a typical width of 2–3 km, and a height of 240–300 m. Thus although it appears to be a vertical cliff in the lunar surface, in actuality the grade of the slope is relatively shallow.”

I’ve now had two convenient views, one on April 22, 2010 and another on October 15, 2010. The first view was a novelty and I was able to show the feature to a number of visitors that came to the observatory that night. On the second occasion, I set up a Meade DSI I camera on a separate refractor to attempt to get some images. I just kept snapping images in the hope of getting the escarpment to slowly come into view as the moon’s terminator advanced. But the images did not show any of the rille as it must have been a little past the optimum time. I could see it fairly well in the 36 inch at 125 power but the contrast was not good enough to also see it in the images.

In any case, I hope to try again when I can next set up my camera on one of my scopes. If I can spend more time during the course of a longer night’s viewing, my hope is to get a good sequence of images to catch the wall at just the right point. More on that in a future post, hopefully.