See the mysterious Lunar X

The Lunar X is a relatively-unknown feature that comes and goes on the surface of the Moon. It’s visible for about an hour each month when light from the Sun is at a very particular angle. The X (as well as a nearby V) happens when shadow fills valleys between craters with steep walls, meaning only the ridges of the craters are lit. It’s known as a clair-obscur effect, meaning it’s caused by bright light and deep shadows.

People who buy telescopes, especially young astronomers, often ask me what they can see with the scope, apart from the Moon.

Of course, there are loads of things to see, like star clusters, globular clusters, nebulas, (I could go on), but the Moon has a couple of hidden little gems that you might not know about.

Shafts of light

Somewhere in the desolation that is the Moon’s highlands, there is a group of craters that have come to be in a very peculiar pattern. For most of the lunar “day” (which lasts two weeks in Earth terms), these craters are indistinguishable from any others. But just as the sun rises, its light grazes the rim of one crater, crosses the valley, and strikes just the very top of the rim of two more craters.

So what? This happens everywhere else in this meteorite-blasted area.

In this area, the play of light illuminates only the very top of the razorback between the craters, while everything below these rims, both left and right, stays pitch black. With no atmosphere to scatter the light, shadows on the Moon are absolute. If you were standing on the ridge between the craters, you would be able to see the sun. But if you walked down into the craters left or right from you, you would quickly find yourself in total darkness.

Again, so what?

What this looks like from Earth

From above you, from Earth, the brightly-lit rims of the two craters makes a cross shape, surrounded by darkness. This cross floats in the black that is the unlit part of the Moon, looking like a pier sticking into the void.

This is called the Lunar X.

About an hour after the effect becomes visible, the light from the rising sun floods the valleys between the rims and ends the show.

Only one other place has anything like this, called (perhaps not surprisingly) the Lunar V. Strangely enough, you can see this at about the same time as the Lunar X, and it disappears just as quickly.

This is a photo I took of the Lunar X and Lunar V last month. I’ve marked them with arrows: V on the left, X on the right.

Lunar V and Lunar X

This next photo is an extreme close-up of the first one. Confession: it would have been a much better shot if I’d used my telescope. I didn’t have time to set up the scope before the effect was gone, so I used my 500mm birdwatching lens on a tripod. A mate’s dad has a saying “poor planning leads to poor performance”.

Close crop of the Lunar X

At other times of the lunar cycle, the Lunar X is completely invisible. In this next photo I’ve used a yellow circle to highlight the area where it occurs. Meh, it looks just like any other part of the moon when the light isn’t at the right angle.

Gibbous moon with location of the Lunar X marked

When can I see the Lunar X?

Now you know where to see the Lunar X (duh, the Moon?), the trick is to know when it’s visible. If you get clouded out, the weather will make you wait at least a whole lunar cycle. Also, because the sun doesn’t always rise over the same spot each month, sometimes the X doesn’t appear at all. What’s worse, the Moon mightn’t even be up for Australian observers at the time.

There are computer models around that can take a stab at telling us when the Lunar X is visible. The table below shows times and dates (both for UTC and Melbourne). I’ve also used Stellarium to figure out where the Moon would be at those times. Unfortunately, for most months, it’s below the horizon for the critical time.

UTC Melbourne time View from Melbourne
14-Dec-2018 10:46 PM 15-Dec-2018 9:46 AM below horizon
13-Jan-2019 12:35 PM 13-Jan-2019 11:35 PM very low
12-Feb-2019 2:13 AM 12-Feb-2019 1:13 PM below horizon
13-Mar-2019 3:26 PM 14-Mar-2019 2:26 AM below horizon
12-Apr-2019 4:10 AM 12-Apr-2019 2:10 PM very low
11-May-2019 4:25 PM 12-May-2019 2:25 AM below horizon
10-Jun-2019 4:17 AM 10-Jun-2019 2:17 PM very low
9-Jul-2019 3:58 PM 10-Jul-2019 1:58 AM below horizon
8-Aug-2019 3:43 AM 8-Aug-2019 1:43 PM low
6-Sep-2019 3:47 PM 7-Sep-2019 1:47 AM below horizon
6-Oct-2019 4:17 AM 6-Oct-2019 3:17 PM high
4-Nov-2019 5:18 PM 5-Nov-2019 4:18 AM below horizon
4-Dec-2019 6:44 AM 4-Dec-2019 5:44 PM high

(Source: https://www.cloudynights.com/topic/598806-2018-lunar-x-predictions/ and https://www.cloudynights.com/topic/630098-lunar-x-start-times-2019/)

How do I get a photo of the Lunar X?

You’ll be desperate to get a shot yourself by now if you’re anything like me. What will you need to get one? Actually, it’s not a difficult one to take.

Look back at the photo I took last month (er, November 2018). I got that with my DSLR and a 500mm birding lens on a tripod. Because the Moon is so bright, I was able to use a fast exposure, about 1/1000s. So any camera with lens longer than 500mm would be able to get a better photo than that one. There are a few tips though.

  • You need a solid mount
  • Use mirror lockup or a camera without a mirror
  • Use a telescope

The most important tip is that your camera has to be absolutely still while taking the photo. Any movement will blur the image and ruin it. At the very least, use a tripod.

If you’re using a DSLR, lock the mirror up before taking the shot. Most DSLRs can do this, and it prevents “mirror bounce”, which is a tiny shake at precisely the wrong moment. (I’m not into pointing out my own faults, but have a close look at the blow-up.)  If you have a mirrorless camera with a long optical zoom, this is probably better here.

Of course, a proper telescope would be the best solution. The Skywatcher Star Discovery 127/1500 Wi-Fi Enabled scope would be a good scope for the job. It’s a Cassegrain design which gives it a good long focal length. This means it has plenty of magnification for a bright target like the Moon. If you wanted more from the optics and were willing to sacrifice the go-to mount, the Saxon 150mm Maksutov would be a great instrument for this. Not only does it have a larger aperture, it’s got an 1800mm focal length, so you won’t have any trouble zooming in on the area.

Finally, I’ve put a short clip of this onto the Optics Central YouTube channel. You can see a transition between the photos you’ve already seen in this blog, along with captions.

Your turn

Do you have any photos of the Lunar X (or the Lunar V)? Let us know in the comments section below, and if you can post a link, we’d love to see them. Include the details of how you got the shot, what instrument you used, and if you processed the photo, how you did it.

Do you have any photos of other weird things on the Moon?

Further reading

Of course, Wikipedia has something on this.

A nice Italian shot of the Lunar X featured on NASA’s Astronomy Photo of the Day page for 10 December 2016, including a blow-up inset.

David Chapman in Nova Scotia (Canada) wrote a very comprehensive paper in 2007 on the effect. He calls it The Lunar X Files: a Fleeting Vision Near the Crater Werner. I’m attaching it with his kind permission.

Bill is Optics Central’s expert on astrophotography, telescopes and bird watching. You’ll find him in the Mitcham store on Fridays and Saturdays. Come in for advice on how to get the best out of your current telescope, what your next telescope should be, how to take photos of the sky, or even how to see some rare birds.

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