How did Sirius find Harry in Little Whinging?

Question

At the beginning of Prisoner of Azkaban, Harry gets a fright when he sees Sirius in dog form on a street near his aunt and uncle’s house. In the letter Harry gets from Sirius on his way back to London at the end of the school year, he writes:

I would also like to apologise for the fright I think I gave you, that night last year when you left your uncle’s house. I had only hoped to get a glimpse of you before starting my journey north, but I think the sight of me alarmed you.
_{Prisoner of Azkaban, ch. 22: “Owl Post Again”}

This description makes it sound like it was sort of a spur-of-the-moment thing to go and see Harry—something he just thought he’d do on the way. He doesn’t make it sound like a major thing he’d spent lots of time on, at any rate.

But how did he know where to look for Harry to get a glimpse of him?

There is an older question on here which asks how Sirius found Harry in Privet Drive when it was supposed to be protected from Voldemort and his supporters, but the answers to that question mostly just point out the flawed premise: Harry wasn’t in Privet Drive when he saw Sirius,^{note 1} and Sirius wasn’t a Voldemort supporter (even if he was widely believed to be one at the time).

What puzzles me more is how Sirius knew where to look in broader terms. After all, Sirius would have had to know to go to Little Whinging somewhere in Surrey to find him at all.

DVK’s answer to the older question says that “He'd have known Lily extremely well (being James best friend and Harry's Godfather) and would have likely known that Lily's sister got married and to whom”, which seems like an unlikely answer to me. It requires several things to all hold true:

• That Hagrid actually told Sirius that Harry was being sent to live with his mother’s sister before borrowing his motorcycle
• That Lily ever mentioned her sister’s husband’s last name to Sirius, despite not having seen her sister for a long time
• That Sirius remembered this last name (which, at the time it was mentioned, was presumably quite unimportant to him) for however long it was between him hearing it and the Potters dying, and then for twelve more years in Azkaban
• That, once he escaped from Azkaban, he managed to locate the present whereabouts of the Dursleys armed with nothing but a name remembered from at least twelve or thirteen years ago

Even if the first three points held up, which would be unlikely but not unthinkable, Sirius would still have had to locate two specific people in a country of about 58 million people. This was in 1993, before the common availability of the Internet, so he would likely have had to resort to phone directories. As far as I can tell (please correct me if this is wrong), there were no national phone directories for the UK in 1993, only local and regional ones, so it wouldn’t be as easy as just looking under D and finding Dursley, Vernon and Petunia. He’d have had to do that in dozens of different phone books to get lucky (if they were listed at all, that is).

Granted, Dursley isn’t a particularly common name (there are apparently just under 200 of them in the UK), but even so, finding the family that lived on Privet Drive would not have been an easy task for a worn-out wizard who’s never lived among Muggles and likely doesn’t know how to use a phone.

Surmountnig all these practical difficulties seems like a lot of overkill just to “get a glimpse of [Harry] before starting [the] journey north”, which makes me think perhaps there’s some other way he knows where Harry lives.

Is there?

How did Sirius know where in the country to look for Harry at the Dursleys’ place?

 

---

^{note 1 It is a bit too much of a coincidence for my taste that Harry just happens to drag his suitcase to the exact street and sit down for a breather at the exact place where Sirius is standing at that moment (if Sirius has narrowed it down to the correct neighbourhood of Little Whinging, why isn’t he looking for him on Privet Drive itself, rather than on some other street?), but I’ll let that slide for now.}

Answer 1

Probably from Hagrid.

In the absence of any direct canon information, it is certainly possible that he got the address from Lily, though we are told in the very first page of the very first book:

Mrs Potter was Mrs Dursley's sister, but they hadn't met for several years.

That implies, however, that Lily and Petunia had met not too many years ago, likely recently enough that the Dursley's were already living at their house in Privet Drive, and recently enough that it was after Lily married James and became friends with Sirius. (It was probably before Harry's birth, though, as "they had never even seen him".)

Sirius might have occasionally given Lily a lift to Petunia's house on his motorbike.

We can assume that Vernon's hatred for magic didn't go quite so far as to refuse to allow his wife's sister to visit (preferably while he wasn't there) but he certainly wouldn't want her arriving by broomstick, apparition, or Floo powder. A motorbike, while not ideal from his perspective, might have been an acceptable compromise. (He didn't need to know it could fly!)

On the other hand, Sirius might have simply seen the return address on a letter from Petunia one time, and happened to remember it - or at least enough of it to find the right phonebook. If there's one thing a skilled wizard must have it's a good memory, and whatever one might say about Sirius's personality (not to mention that of his extended family) they were certainly skilled wizards.

All that said, there's an even simpler possibility: Hagrid.

'Hagrid,' said Dumbledore, sounding relieved. 'At last. And where did you get that motorbike?'

'Borrowed it, Professor Dumbledore, sir.' said the giant, climbing carefully off the motorbike as he spoke. 'Young Sirius Black lent it me. I've got him, sir.'

Sirius would certainly have wanted to know where Harry was going, and at that point, Hagrid knew of no reason not to tell him. It was important to Sirius, so he'd have wanted to remember it, but it wasn't a happy memory, so the Dementors couldn't have sucked it out of him.

Edit: on re-reading the question I see you already mention that Sirius and Hagrid talked. But I'm not sure why you think it unlikely that Hagrid would have told Sirius the address; it seems to me Sirius would certainly have asked for Harry's address so that he could visit, and Hagrid would have had no reason not to tell him.

Answer 2

It is pretty simple in the Wizarding World to track someone's whereabouts.

When Harry receives his initial letters from Hogwarts, the person writing the address was able to know where he was at a given time. They knew when he was in the cupboard under the stairs, they knew when he moved to Dudley's second bedroom, they knew when they went to a hotel and even knew which room they were in, and they knew that Harry was on the floor of a hut on a rock in the sea.

Additionally, we find that Harry is able to send letters to Sirius all the time despite having no idea where Sirius is. If an owl can track someone, then presumably a wizard can too, or at the very least a wizard can follow an owl to track someone.

In later books we also find out that underage wizards have a "trace", and that taboos can be created that give people's locations.

Thus, while we are never told the precise method that Sirius used to track Harry's whereabouts, and some methods are probably beyond the ability of a single wizard on the run with no wand, the idea of tracking someone's whereabouts is hardly a novelty and it is therefore not very surprising that Sirius was able to figure out some way to locate Harry.