Japanese: Why does the は particle sound like "wa"?

I’ve been working on some very basic Japanese grammar, and managed to come across a point of confusion in my first ten minutes. When learning Hiragana, you learn that the character は produces a “ha” sound:

は = ha

However, when learning the most basic Japanese sentences, you’re taught instead that は should be pronounced as “wa”:

わたしはミイクです。 = Watashi wa Maiku (Mike) desu. = I’m Mike.

So which is it, “ha” or “wa”? I was disappointed to see that my textbooks didn’t even try to explain the inconsistency. I call this phenomena “the curse of knowledge” – when something is so obvious to you, you stop being able to even imagine that someone else might not know it, and don’t bother talking about it.

Luckily, we have the internet, and I was able to find this great explanation on JapanesePod101’s forums:

That’s right: there are two particles that are written with different characters to what you’d expect. Wa, which is written with the は (ha) character and e is written with the へ (he) character. You’ll perhaps notice they’re both from the h row of the kana table, and in the old days the kana in the h row had more than one pronunciation (a bit like c in English being either an s sound or a k sound). Back in those days the particle wa was always written with は and the particle e was written with へ; when Japanese spelling was reformed in the late 1940s, the use of は for the particle wa and へ for the particle e was retained because they were so well established.

So not too surprisingly, it’s historical baggage in the language. The は character (along with others in the “h” row) used to have multiple pronounciations before the alphabet was revamped, but when used as a particle, retains its old pronounciation of “wa”. So:

• は used as particle = “wa”
  • e.g. わたしは = “watashi wa”
• は used in word = “ha”
  • e.g. 花 = はな = “hana”