Today I learned vertical on Wanikani. 垂直。According Jisho horizontal is 水平。This word is not learned on WK which makes me wonder if there is a more frequently used translation for horizontal?
Jisho also uses another Kanji when horizontal/vertical is placed before a noun. A vertical line is 縦軸.
Can someone explain when what kanji is used and which are most common?
横 has horizontal as an additional meaning. You can see that in phrases like 横になる (to lie down), 縦横 vertical and horizontal. 横書き and 縦書き are the words for horizontal writing and vertical writing respectively.
水平 to me sounds like the kind of horizontal you need when installing a shelf and want it to be perfectly horizontal. Likewise 垂直 for vertical.
Doesn’t it make sense that if you want to make a long noun like that out of a kanji compound, the onyomi version is going to be more likely to be used?
If you wanted to say “move horizontally” (not in noun form) you could say 横に動く if you wanted.
I can’t imagine using 水平動 unless you were writing technical specifications or something.
横になる, as an expression, means to lie down. Like, if you want to tell someone you are going to lie down and take a rest, that’s an expression you can use. You can’t use 水平になる for this.
横になる, as just the sum of its parts, means to become horizontal. If you were talking about something that wasn’t horizontal becoming horizontal, you could say that. You could use 水平になる for this.
横に動く would mean to move horizontally as opposed to vertically. Your position changes, not the alignment of anything. Depending on your perspective, it could actually mean a few different things. Some of those might be appropriate for 水平に動く, like a pixel moving horizontally on a screen. Others, like you walking left and right, would not. This has more to do with the multiple meanings of 横. 水平 has fewer meanings.
ロッケトは垂直に発射した I’m not sure exactly what you mean by asking how it’s different. Do you mean, can 縦 be used instead of 垂直? I think technically the answer is yes, but if you’re talking about a rocket, it sounds more appropriate to use a “heavier” kanji compound than a kunyomi word like 縦. A small child who doesn’t know a difficult word like 垂直 might say 縦, I suppose.