Banner photo credit: John Chumack
One of the things we amateur astronomers tend to hate the most is the moon. Sure, the moon is plenty interesting when you’re first starting out, and other than the sun, it’s about the only astronomical object to show considerable detail through the scope. Note the word “considerable” there. Yes, you can see some detail on Jupiter, Saturn, and Mars, when it’s close, even in the Great Orion Nebula if you’ve got a large enough scope and dark skies, but not a whole heckuva lot. Not like the moon.
And that’s all well and good, but after you’ve seen the terraced walls of Copernicus and the hummocky terrain nearby:
and the rock-skipping interior craters of Clavius that go in almost an 180-degree turn:
and you’ve tried to catch the Plato craterlets:
what else is there left to see? And more to the point, LIGHT POLLUTION!!! The moon is always fu – – oops, messing up the sky, for almost two weeks out of every four. After an initial fascination for a few months or even years, most advanced astronomers avoid the moon like the plague.
Well, there’s plenty to see on the moon. Of course, there are plenty of craters and mountains galore, but those are obvious. And maybe one looks a lot like the next, but if you get to know them, they start to develop their own individual personalities. There are also transient features that you can only observe during certain times of the lunar month. You just gotta know where to look and what to look for.
This blog post isn’t intended as an exhaustive list of cool things to see on the moon – because there are plenty. To learn more about these, check the Astronomical League’s lunar observing list. If you a member of an astroclub, your membership probably includes a membership in the AL as well. Contact some of the muckety-mucks in your club to get full access to the AL’s offerings. I just wanted to highlight some of the highlights (ahem) to hopefully get you interested.
Rays are an example of these transient features that can only be seen at certain times of the month. Rays are formed when an asteroid impacts the moon and creates a crater. When the asteroid strikes, material, called ejecta, is blasted out of the crater. Most of the ejecta goes out into space, while some of it settles back onto the moon’s surface.
Tycho’s rays are a glorious example of this. The crater Tycho itself is an easily visible, prominent feature, seen towards the moon’s south pole, shortly after first quarter:
It was once thought that tektites – a type of glassy meteorite – found here on earth originated as Tycho’s ejecta. With the collection of lunar samples from the Apollo missions, the consensus has shifted to that tektites were formed as a result of solely terrestrial meteorite impacts. However, the lunar origin hypothesis cannot be fully rejected until we collect some samples from Tycho’s rays.
Tycho’s incredible ray system doesn’t become visible until the moon gets to be waxing gibbous. This simply has to do with lighting effects – the angle of the sun making the light hit the surface differently to highlight things that you couldn’t otherwise see.
Tycho has such a dramatic ray system because the impact that created it is relatively recent – approximately 108 million years ago. The ray system is so bright because the sun’s solar wind – a stream of charged particles constantly being emitted by the sun in all directions – has not yet had enough time to interact with the ejecta and darken it to the same background color as the rest of the moon. This is why not every crater has a ray system associated with it – only the more recent ones.
So, if the moon is full, or almost full, and there’s nothing else to look at because of all the light pollution the moon is throwing off, why not grab a moon filter and take a good look at some of the rays on the moon? Copernicus (the left middle of the above photo), Kepler (to the left of Copernicus), and Aristarchus (the bright spot above and slightly to the left of Kepler) all have notable ray systems as well.
Another couple of these transient features, which are more difficult to spot, are the Lunar X and V. These two show up when the angle of the sun’s light hits them just right, just before first quarter, when the moon is about half lit up.
For Thanksgiving, I travelled back east (well, at least a bit) to visit with friends in Kansas City, and brought the scope along to show them a few things. What I didn’t expect to show them were the Lunar X and V, though!
We literally stumbled upon them. As that great sage, George Costanza, so aptly put it, it was like discovering plutonium by accident. We were just looking at the moon generally, and there they were!
This incredible video from John Moore shows the full progression of the Lunar X as the angle of the light changes. Below is a still video frame from that video that shows the X in the middle, just a smidge to the left of dead center, all lit up:
The X is formed by highlands that were thrown up in between three craters – Purbach, LaCaille and Blanchinus. This area is plateaued above the rest of the local terrain, such that it catches the light first at lunar dawn before the surrounding terrain does, giving us the dramatic Lunar X when the light hits it just right. Because the light has to be “just right”, it is only visible for about 4 hours or so; plus, because of the ever-changing sun-moon-earth angle, it isn’t visible from the earth at all the times it should be, either.
The Lunar V is more easily seen. It is simply a couple of mountain ridges near the crater Ukert. It is visible for a longer period of time than the Lunar X, appearing before the X, and persisting longer.
Even Clavius is subject to these transient lighting features. When the sun is just rising over Clavius, it develops eyes:
But wait, there’s more! Look at that pic of the Lunar X all lit up again. See that large crater at the top left of the photo? What’s that straight feature running up and down? Why, that’s the Straight Wall, another cool thing to see on the moon! The Straight Wall isn’t really a wall, although it sure is pretty straight. The official, Latin version of the name is Rupes Recta. It’s an 80-mile long fault on the surface of the moon – a location where the surface of the moon suddenly shifted. Although it looks extremely sharp, that is a trick of the light yet again.
The Straight Wall is actually just a slope, not a wall or cliff. Different measurements place it as rising between 240m and 450m high from one side to the other, over a distance of about 2-3km. The result is that it could be a “gentle” 8% incline, or a relatively steep – but still not “wall-like” – 23% grade. And, to dissuade any of you “Aliens!” types, if you look closely, it’s really not all that straight. Straight enough, but hardly what you would expect from an advanced lunar civilization. Ahem.
Now how much wouldja pay? But don’t order yet! Take another look at that Lunar X pic, or the Straight Wall pic. What’s that to the left of the Straight Wall? There are also rilles, also referred to by the Latin name rimae. These are skinny, sinuous valleys snaking across the moon. They look like cracks in the surface. The one just to the left of the Straight Wall is called Rima Birt (Ernie is nowhere to be seen). It snakes along for about 30 miles, starting and ending in a pit.
Another of these rilles is Rima Posidonius, travelling its way about 80 miles across the floor of Posidonius crater:
Pretty neat stuff, huh? Alls I’m a-sayin’ here is don’t discount the moon. There is plenty there to be seen. Get an atlas, get a reference (Wikimoon looks like a good one), and get out there and give it a good look!