Answer: The phases (and more!) of the moon

So… what’s the story here?

Why did all of the NASA missions land on the side facing the Earth?

It struck me the other day: ALL of the NASA landings on the Moon were all on the side of the Moon facing towards the earth. Why did they neglect the other side? Was it a kind of strange conspiracy?

Not my photo, but very similar to what I saw on Jan 1, 2017. P/C NASA

The US sent six missions to the moon (Apollo 11, 12, 14, 15, 16, 17). Here’s the map of the landing sites I found with the query: [ Apollo landing sites map ]

Apollo landing sites. P/C NASA

Given this background, our Challenges for this week start with this question of “why only on one side?”

1. If you look at a map of the Apollo landing sites, they’re all visible from Earth–none are on the back side (that is, the side of the moon that faces away from the Earth). Why were all of the landing sites on THIS side? (You’d think the back side would have been more interesting.Why didn’t we go to the back side?)

I thought about different strategies for this search, and started with:

[ nasa landing sites on the earth-facing side of the moon ]

I used these terms in a long query because I wanted fairly specific documents (in particular, ones that would use words like “landing sites”). I used the phrase “earth-facing” because I wanted documents that included that terminology (and not “dark side of the moon,” which I already knew many people get wrong… there is no dark side, except in albums by Pink Floyd).

This query worked reasonably well, and I (like others) found the Quora discussion “Which side of the Moon did the Americans land on?” Although I’m always suspicious about QA (Question-Answering) sites, this post looks pretty good. It has lots of citations, illustrations, and it’s easy to go from this Quora page to NASA web pages that say why they landed on the near (visible / earth-facing) side: They needed constant communications access, and terrestrial radios don’t make it to the other side of the Moon.

And, of course, NASA had much better imagery of the Earth-facing side, the better to plan the missions.

Regular Reader Jon found the Quora site with the query:

[apollo missions did not land on the far side because]

While Ramón used:

[Why Apollo missions never went to dark side of the moon]

Both of these queries are fairly long, but include words that are important for getting the right kind of results. We’re searching for a complex concept–I’m not sure you could succeed with a much shorter query.

After checking out a few other links on the SERP, I was pretty convinced that this was the story.

However, Regular Reader (and professional library) Debra and her colleague Anne did something much better–they limited their search to site:.gov and found some NASA documentation of their site selection process. In their words:

“…We didn’t think this was definitive enough so we did an advanced search limiting our search to .gov sites { using [ site:.gov …] }and got this result {a document on the } Operational Constraints on Landing Sites. It gives a very detailed explanation of why the landings needed to be on the near side – radio communications were key as was having some knowledge of the topography, and much more was known about the near side than the far side.

This second article also points out to how the site was selected – NASA definitely wanted to know as much about the site as possible and the near side was what they had information on…”

These days, of course, we have superb lunar images thanks the the Lunar Reconnaissance Orbiter (LRO). You can check out these fantastic images at the LRO image archive site. This is sufficiently high-res that you can see the trails (and debris) left by the astronauts.

LRO image of the Apollo 11 Landing Site

The LRO took this beautiful image of the Apollo 11 landing site in 2013 at 24 km (15 miles) above the surface.

You can see the remnants of their first steps as dark regions around the Lunar Module (LM) and in dark tracks that lead to the scientific experiments the astronauts set up on the surface. The Passive Seismic Experiment Package (PSEP) provided the first lunar seismic data, returning data for three weeks after the astronauts left, and the Laser Ranging RetroReflector (LRRR) allows precise measurements that can be collected to this day. You can even spot the discarded cover of the LRRR. If you happen to have a large telescope and a gigawatt laser, you can still use the LRRR to measure the distance to the Moon. (If you’re curious about how to do this, check out this episode of Mythbusters where they visited the Apache Point Observatory telescope and bounced some green laser light off of the LRRR. For details about Apache Point, check out the APO website.)

2. Every so often I’d sketch out the moon as I saw it in the night sky. Once, when I was looking at several of my sketches together, I noticed that some of the craters on the Moon’s edge seemed to be in slightly different places. Huh? I know that the Moon always has the same face pointed to us, but when I looked at my sketches, it would seem that it’s not always exactly the same face–especially near the edge. Why would the Moon’s face be slightly different during different times of the lunar month? Is it always showing us exactly the same face at all times?

That same Quora article about why all of the missions were on one side ALSO gave a hint about this second question as well. In his post, Robert Walker says “Of course no part of the Moon is in permanent darkness. But we always see the same side of it, and at full Moon we see it at full phase. Actually, we see a bit to either side because of lunar libration…” (emphasis mine) That’s an intriguing thought–that perhaps the Moon really does wobble a bit, and doesn’t show us exactly the same face at all times.

From here, a search for:

[ libration ]

leads to all manner of useful results. (It’s one of those rare words that seems to have no other meaning, but just the Moon-related one.)

Here’s a wonderful YouTube video from NASA showing what libration actually is, the “rolling” of the Moon in the night sky, exposing and hiding different parts of the Moon throughout the month. This is why my sketches sometimes had significant craters in slightly different places on the Moon’s surface.

This is such a beautiful video, it’s worth clicking on the YouTube logo (in the lower right) and watching this at full resolution. It makes the rocking and rolling motion really obvious. The Moon ends up rolling enough in its orbit that 59% of the total surface area is shown to the Earth during the month.

Search Lessons

I take note of 3 things in this week’s discussion.

1. Long queries sometimes work quite well, especially for complex concepts. All of the queries that worked well for this Challenge ended up having a fair number of words in them. The Challenge concept was fairly complex (having to do with the Moon, the choice of landing sites, and the earth-facing side of the Moon), so it’s not a surprise that we need a fairly long query to get to the right results. Since this was a complex Challenge, I was prepared to do many alternatives to my original query, but it turned out that the Quora forum discussion was reachable by many different queries.

2. QA sites are not always low quality! I subscribe to the Quora posts and get to see the original questions as they fly by. They are not necessarily deep or well-thought-out questions. But, on specific topics, the discussion can be very rich and deep. This one (why NASA landed on the earth-side of the Moon) was especially good. Don’t skip over the QA results, but check them out (and, as always, double-check–second source anything you learn from them).

3. Searching for official government documentation about a process (by using a site: restriction) can be a great thing. I was impressed by Debra and Anne’s strategy of searching for an original process-describing document. I should have known that NASA (of all organizations in the world) would have published such a thing. They found it with a brilliant search strategy (to wit, realizing that such a document might exist and then using site:.gov to search the US government’s repository).

Search on, in the spirit of the Apollo missions!