Today I received an interesting question in email from Katty Geltmeyer, a great fan friend of the Lexicon:
“In PA, Lupin told Harry, Ron, and Hermione that he found out they were in the Shrieking Shack by using the marauders Map. But Harry and Hermione traveled back in time and were lurking around the Willow. How is it possible that Lupin didn’t see the extra Harry and Hermione on the map?”
Here’s my opinion, based on canon.
First of all, it’s important to remember that we don’t know how the Map actually works, what it actually looks like. The version we see in the film doesn’t match what’s described in the books. We can be pretty sure that it isn’t just a two-dimensional map with little moving feet.
Second, we have to assume that Lupin DIDN’T see the extra Harry and Hermione for the purposes of this discussion. We can’t say “This couldn’t have happened!” We have to say “It happened, let’s figure out why.”
Here’s what Rowling says about the Map, in response to the often-asked question of why Fred and George didn’t spot Pettigrew on there:
“Fred and George used the map for their own mischief-making, so they concentrated, naturally enough, on those portions of the map where they were planning their next misdeeds. And finally, you must not forget that hundreds of little dots are moving around this map at any given time… Fred and George did not know everyone in school by name, so a single unfamiliar name was unlikely to stand out.” (JKR)
So let’s make some inferences from what she said.
It would seem that Fred and George see only what they’re concentrating on seeing. I suspect that the map appears somewhat out of focus except for where the user is intentionally centering his or her gaze, with various levels of the castle, area details, and moving names (not footprints, that’s just in the film) coming in and out of clear view as the user looks around.
Even if the Map isn’t out of focus, it is incredibly complex and abuzz with activity, depicting a three-dimensional, dynamic world in real time. What a user sees when looking at the map is a view that is complicated and confusing, with “hundreds of little dots moving around” through multiple floors and spaces. Any user would have quite a time finding any single detail on the map without a lot of practice.
But wait, Harry uses it easily the very first time he opens it, right? Well, he does see things, certainly, but that doesn’t mean that he sees everything perfectly at the same time. He casts his eye around and various things come into focus. And he’s looking for a way to get out of the castle, so that’s what appears. That’s what comes into focus for him.
This whole focus thing is likely another example of a witch or wizard’s intention directly affecting their magic: in other words, the Map focuses on and shows a person only what they’re intentionally looking for, and they’re only going to see something else by chance as they move their focus around or if that thing happens to be in very close proximity to what they were already looking at. Fred and George wouldn’t have been looking for Pettigrew, since they didn’t even know who he was, and Lupin wouldn’t have been looking for more than one Harry and Hermione.