Why can't it be just 'Mihail'? I guess the 'k' is inserted to ensure correct pronunciation, but I don't see how 'Mikhail' and 'Mihail' are pronounced differently.
Since the differences in pronunciation have been thoroughly discussed, I'd like to point one the consistency of transliteration.
The spelling Mikhail simply follows the common convention "х=kh". From this perspective, asking why it is not spelt Mihail boils down to why the Cyrillic х is not rendered as h, and I think there's a fairly straightforward answer here.
The English language (unlike Russian) employs numerous digraphs with the letter h, which has the effect of completely changing the sound of the previous letter. Should one systematically replace every х with h, a lot of words will be impossible to read back correctly, as you would not know whether h came from х or letters like ш (sh) or ч (ch). You could of course devise a workaround to fix that, but the digraph kh seems to be easier than convincing English speakers to pronounce sh as
For example, the Russian word восход
[vɐsˈxot] "sunrise" would be transliterated as voshod, which implies the pronunciation along the lines of
[vɒʃəd]: individual s and h merged to a completely different sound! Similarly, ветхий
[ˈvʲetxɪj] is not pronounced vethiy: the t and h are not only distinct, they are in different syllables.
The "kh" is not meant to be understood as a sequence of "k" followed by "h". The idea is that it is a digraph where both letters together represent the single sound of Russian х. It is used by analogy with the English digraphs "th" and "ph", which also are used to represent single fricative sounds. (Those digraphs are based in part on old traditions about how Greek is transcribed.)
English "h" by itself is not pronounced exactly the same as Russian х. The English sound is categorized as a glottal fricative (written [h] in the International Phonetic Alphabet) while the Russian sound is categorized as a velar fricative (written [x] in the International Phonetic Alphabet). Most English speakers aren't accustomed to pronouncing the velar fricative. The glottal fricative /h/ is probably the English consonant that sounds the closest to the velar fricative [x], but some speakers might use the velar stop /k/ as a replacement instead, either because of influence from the spelling or because the velar fricative sounds like [k] to them.
Going the other direction, I've heard that it is usual for Russian speakers to approximate English [h] with Russian х, but it is an approximation, not an exactly identical sound.
The primary reason is "that's the way it's done". You can't say that "Mikhail" is pronounced correctly while "Mihail" wouldn't be, because all English speakers get both of them horribly wrong. But if you transcribe Михаил as "Mikhail" you are at least consistent with other transliterations.
In theory the Russian letter "х" is pronounced slightly differently from the English letter "h", but neither Russian nor English speakers will ever get confused if you conflate the two, so there's no point in making the distinction. (I think the Russian pronunciation has also evolved over time since the transliteration rules were developed.)
Also in theory, the English "kh" is supposed to sound more like the Russian letter "х", and not as "k" followed by "h", but they forgot to tell the English speakers that.
Based on my experience, if one's name is Михаил, it's safest to just tell English speakers that your name is Misha.
Because the Russian x has a different sound (IPA /x/) from /h/
While many English speakers don't know how to pronounce /x/, many do.
But to some degree, transliteration between languages is arbitrary and follows fashion. We write Tschaikovsky with a T because that's how French and German represent the sound: "Chaykovsky" or "Chaikovsky" would better represent it in English orthography.
On the other hand, to turn your example round, it used to be customary to render non-Russian words with /h/ into Russian using the letter 'г', pronounced /g/. So for example, not only does Herbert Hoover appear in Russian as Ге́рберт Гу́вер (/gerbert guver/), but the words "hymn" and "horizon" were borrowed into Russian as гимн /gimn/ and горизонт /gorizont/.
Historically, when Russian names have been transliterated into the Latin alphabet, the most important target language was French rather than English. For example, the Wikipedia page for Romanization of Russian says that in "Soviet international passports, transliteration was based on French rules (but without diacritics), so all of the names were transliterated in a French-style system".
Another answer mentions Tchaikovsky, where a similar reason explains the "tch" instead of "ch". The digraph "ch" is usually pronounced as /ʃ/ (English "sh", Russian "ш") in French, so the combination "tch" is used instead.
Cyrillic transliteration is usually formulaic, where every Cyrillic letter corresponds exactly to some letter or combination of Roman letters. Most of these are quite sensible and would only rarely cause avoidable mistakes. There are, however, some problematic combinations, one of which you've run into.
- KH = x. Kharkov would really be Harkov or Harkiv. Mikhail would really be Mihail. This is probably the worst offender, since most Americans can probably get closer with the 'h' than the 'kh'.
- SHT = щ. Borsht would really be borsh. There's no 't' sound in the word at all.
- ZH = ж. This one's tricker, but ж has significant impact on the vowel which comes after, so in English you would sometimes want to change a following vowel. This can also get confused with the 'zh' used in Chinese and other East Asian transliteration.
- o's and a's are problematic, since the pronunciation in Russian depends on the position of the stress in the word, but English speakers usually say 'o' is 'o' when confronted by an unknown foreign word. The Russian word for milk is guaranteed to be mispronounced if you write it correctly as 'moloko'. My best guess for American pronunciation would be 'mulahko', although I think Americans would be likely to stress the 2nd syllable with that spelling... Americans aren't big fans of putting stress on a late 'o'.
Despite these problems, such a transliteration pattern is beneficial for Russian speakers and for consistency, since it means that a Russian word in English can almost always be converted back-and-forth to Russian without confusion. Additionally, a set formula removes the tedious debate that might otherwise arise about whether 'muhlahkoh' is better than 'mulahkoh'...
Everyone else here recalled the Russian "h" being different, but I want to add an important detail:
A lot of English speakers don't pronounce lone "h" at all in a lot of places
If you give such person a word "Harkov" with no context, it's likely he would pronounce it as "arkov"
and it's a big deal
So to have at least some sound in those places, "kh" was adopted, as it is at least somewhat guessable due to "k"-"h" parity in the Russian language
English language does not have equivalent sound of Russian Х and simple transliteration does not apply to the sounds. In case of my 'family name' (middle name) was used
French dictionary to be put on legal papers (international passport).
It is sufficient to put
mikhail into Google translator to hear the difference and realize that none of the
pronunciation sounds even close to Russian version pronunciation.
It is unavoidable difference of the languages -- the person should live long enough in the country to
adopt his brain and tongue with proper pronunciation.
Some Russian words most English speaking people can not pronounce even if they try really very hard.
ь ё ъ й ы do not have any similar alternative in English. Just sufficient to ask any English speaking person to pronounce 'хорошо, Андрей, мышь, скользко, Иисус, церковь, жимолость, интервью, и т.д)'.
Very good example is Tim Kerbi who lives in Russia long enough -- video
There is a fantastic example Christopher Heni who works in Kazakhstan -- video
American ambassador in Russia answers on students question -- video
We have a friends, an old couple from Nederlands, man of the family told us that during WWII they used
tongue twister to weed out German spies. It doesn't matter how long and hard was their training -- they could not pronounce
tongue twister properly.
It's not the same sound, English h is usually a glottal fricative, Russian х is usually a velar fricative.
We're used to ⟨kh⟩ being /kʰ/, but remember that ⟨th⟩ is not /tʰ/ but rather /θ/, the dental fricative. There is no "unpronounceable t" in ⟨th⟩. English has precedent for using ⟨-h⟩ to mark fricatives, so the notation makes sense in context.