Bah, Humboldt! Visiting a Fringe Crater

Meet Humboldt, a magnificent lunar crater compromised by its life on the edge.

Biggest Hidden Crater

The large crater Humboldt is next-door neighbors with better known Petavius. Both are located near the Moon's southeastern limb (celestial southwest) and seen here in low, morning sunlight.
Oskari Syynimaa

I don't mean to sound grumpy. I just wish this magnificent crater lay closer to the center of the Moon. It would be an instant magnet, as magnificent as Copernicus or Clavius, but instead it's relegated to the fringe. Rarely do amateurs put the crater on their observing lists — because it sits so close to the Moon's edge, studying Humboldt's interior is like looking into a saucer almost edge-on. To see it best and least foreshortened, we must bide our time until a favorable libration tips the crater our way.

I wanted to share Humboldt with you last winter, but after doing simulations, I discovered that good evening viewing would have to wait till May or June.

Crater-hop to Humboldt

This simulation shows the (celestial) western half of the Moon on June 29th with prominent craters labeled to help you crater-hop to Humboldt. The most distinctive feature of Humboldt at this time are the four dark lava deposits on its floor.
Virtual Moon Atlas / Christian Legrand & Patrick Chevalley

I like life on the limb and eagerly study craters and other features that occupy the land of "Here there be monsters." Examining the lunar circumference at high magnification, you get an "on approach" view of crater walls and mountain peaks, as if you're coming in for a landing. Many of my fellow lunatics also enjoy the challenge of ferreting out features photographed by orbiting spacecraft including Apollo. During their many lunar orbits, the astronauts got to see Humboldt and other edgy craters in their full glory. We can use their photos to help us better identify details here and in other craters nearby.

A Proper View

Now we're talking! In this overhead photo taken by NASA's Lunar Reconnaissance Orbiter (LRO), the unique features of Humboldt come sharply into view: the fractured floor crisscrossed by rills; the peculiar range of hills north of the crater's center; a prominent rayed crater located just below the "d" in Humboldt; crater N; four dark lava patches; a striking concentric crater inside the eastern wall; and the slumps and terraces all around the crater's rim.
NASA / LRO

Let's see what this crater's got going for it:

  • Enormity —  It's 207 km (129 miles) across, or just 18 km (11 miles) smaller than Clavius, the third largest crater on the lunar nearside.
  • Cool cracks — It's host to a spiderweb of radial and concentric rills from faulting and lava flows that testify to the Moon's violent and active past.
  • Linear peaks — It sports a curious linear range of hills that stretch across the central floor.
  • Bull's-eye crater —  Can you find a more beautiful example of an interior concentric crater?
  • Dark spots — Four dark mare patches nestle into areas where the crater floor meets the rim. Lava may have bubbled up through faults and pooled here during the Moon's volcanically active past.

Not that you can see all these things, but there's a surprising amount visible in 6-inch and larger telescopes. During the Moon's waxing phase this month and next, favorable librations put Humboldt's best face on view. June 28th through July 4th are ideal. And though the crater's right on the limb on July 8th, the low sunset light should make the central peaks and hill snap into view. The next good observing window opens from July 24th to August 2nd. While it only takes 50× to find Humboldt, I recommend a magnification of 200× for a good explore.

Wilhelm, the stay-at-home Humboldt brother.
S&T / Public Domain

The crater's named for Wilhelm von Humboldt, a German statesman and brother of Alexander von Humboldt, the famous naturalist and explorer. You'll find it just inside the southeastern lunar limb, east of the prominent peaked crater Petavius — which is worth a look, too! Like Humboldt, it also has a fractured floor.

When the Moon is in a waxing crescent or waning gibbous phase, shadowy, low-angled sunlight highlights the textures of the central mountain chain, delicate rills, and the biggest interior crater, Humboldt N.

The four dark lava patches greatly resemble the smaller, pyroclastic deposits in the interior of Alphonsus and hint at volcanic fountains and lava flows in the crater's distant past. Also look for a small, rayed crater halfway between the center and the south rim, apparent when the Sun stands high over the crater, between first and last quarters.

Foreshortened Challenge

In this photo of Humboldt, taken several days after sunrise at the crater (when we see a waxing crescent Moon), the most prominent crack or rill is shown. You can also see central peaks, a row of hills, terraced walls, and other features.
Wolfgang Paech and Franz Hofmann

I've caught hints of the rills as brighter patches around local noon but have never seen them distinctly. The easiest rill, if any might be considered such, slices approximately east-west 1/3 of the way from the southern rim to the central peak. Look for a short, dark segment in the otherwise smooth floor a day or two after sunrise or before sunset at the crater. Under the right lighting, a 10-inch should show it. So many of Humboldt's rills formed as a result of magma spreading under the floor of the crater, uplifting it, and fracturing it into sections.

High Noon on Humboldt

The Apollo 12 crew photographed Humboldt in the light of local high noon. Shadows are gone, but mountains and the lava deposits are prominent. Cliffs along the rim reach to 16,000 feet. Three separate frames were stitched together to create this view.
NASA / labels by author

Be sure to play your eye around the crater's circumference: Humboldt's weathered, slumped walls are apparent especially to the south, where they resemble those in Copernicus but on a grander scale.

Sunset Farewell

This is Humboldt about a day before sunset (just past full Moon). Shadows really bring out the most prominent rill (right) and details in the central peaks and hills. What a contrast from the full Sun photo above!
Damian Peach

Photos show what appears to be a multi-peaked central mountain that looks like a couple of closely-spaced white dots in high sunlight, as well as a range of hills trending northeast to southwest slightly north of the crater's center. The hills and the bumpy terrain to their north looks suspiciously like an impact deposit — material blasted into the crater from another impact in the region. Just a hunch.


Grab a pair of those red-blue 3D glasses and see Humboldt Crater in three dimensions.

I invite you to explore Humboldt at every phase of the Moon to fully appreciate its many facets. Don't let its shyness stop you from being a repeat visitor.

CATEGORIES
Explore the Night with Bob King, Moon, Observing
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Bob King

About Bob King

Amateur astronomer since childhood and long-time member of the American Association of Variable Star Observers (AAVSO), Bob King also teaches community education astronomy and writes the blog Astro Bob. The universe invites us on an adventure every single night. All we need do is look up. My book "Night Sky with the Naked Eye" was just published and is now available on Amazon and BN. It covers all the great things you can see at night with just your eyeballs. No equipment needed!

8 thoughts on “Bah, Humboldt! Visiting a Fringe Crater

  1. Anthony BarreiroAnthony Barreiro

    Huh. I always assumed Humboldt Crater was named after Alexander von Humboldt. I guess he’s got enough terrestrial features named after him, so Wilhelm gets a lunar crater.

    By the way, I think there is one small error. The paragraph:

    “When the Moon is in a waxing or waning crescent phase, shadowy, low-angled sunlight highlights the textures of the central mountain chain, delicate rills, and the biggest interior crater, Humboldt N.”

    should read, “When the Moon is in a waxing crescent or waning gibbous phase … “.

    We should get some clear weather this weekend, and my astronomy club is holding a public star party Saturday evening, so I’ll have a look!

    1. Bob KingBob King Post author

      Anthony,
      Absolutely true! Thank you for pointing out the error. That’s exactly what I meant, but it came out wrong. Good luck in exploring the crater!

  2. DoctorV

    I would love to see a book of “overhead photos” from the LRO. It would be fun to look at the photo and compare that to what I see, especially when I’m viewing a limb resident that doesn’t show up very often, and only gives me a sideways glance when it does.

  3. Mike-McCabe

    Thanks Bob, this article got me out to have a look at Humboldt the other night. It was a tough observe at 29% waxing crescent, but I was able to see the central peaks chain and the nearest lava flow on the floor of the crater.

    While I was there I also happened to notice something else I’d never seen before, and I documented it here on the bulletin board of one of my astronomy clubs; http://www.assne.org/board/viewtopic.php?f=98&p=45962&sid=7cecfd2c6914f10f97fbffb6550379da#p45962 Please have a look and tell me if you’ve ever seen or heard of it before.

    @ Doctor V; Chuck Woods latest moon atlas uses all LRO images, although I’m not sure if they’re all overhead. It is a nice atlas though; https://www.amazon.com/21st-Century-Atlas-Moon-Charles/dp/1938228804

    1. Bob KingBob King Post author

      Hi Mike,
      Thanks for sharing your observation of Humboldt. Regarding the lava patches, I think if you look now (around first quarter) under a much higher sun (for Humboldt) you’ll see all four with ease. That is a most interesting “X” you’ve found. I’ve not noticed it before. After studying it up close on the ACT-REACT LROC map, the vertical axis appears to be a happenstance alignment of crater walls but who knows, possibly an ancient, eroded fault line. The more horizontal axis is more difficult to fathom.

      1. Mike-McCabe

        Bob, you were right. I got out last night (about 78% waxing gibbous) and was able to see all four lava flows with ease The high sun made all the difference.

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