Much to my disbelief (no, I'm kidding, it happens all the time), I found out that my handbook was wrong to tell me we must use 静けさ, because 静かさ doesn't exist.
Wiki(tionary) says "For degree, corresponding to the English quietness, both 静かさ (shizukasa) and 静けさ (shizukesa) can be used, but 静けさ (shizukesa) may be more common." On the other hand someone on chiebukuro says:
「静かさ」といった場合「静かである程度」を 意味し、「静けさ」といった場合「静かであること(様子)」を意味しているように 思います。
meaning shizukasa is for degree, shizukesa is for appearance.
But I can't help feeling unconvinced. I've looked on weblio. In the ruigo jisho I've found 3 meaning for 静かさ (for reference 静けさ):
- 音がないこと サイレンス ・ 物静かさ ・ 静寂さ
- 静かな状況(例えば、話し声がしない状況) サイレンス ・ 音無 ・ 森閑 ・ 沈黙 ・ 無音 ・ だんまり ・ 黙 ・ 音無し ・ 黙り ・ 静寂 ・ 無言 ・ しじま ・ 無声 ・ 静けさ ・ 静寂さ
- 穏やかな静寂 静寂 ・ しじま ・ 静けさ
So the first meaning, apparently, is the real difference with shizukesa, right?
But then again, if 1 is 音がないこと why 音無, 無音 and 音無し aren't also in the first group? And why 静けさ can be in the second group with these one, but not in the first group?
Side question: why 静寂さ for 1st and 2nd group, but just 静寂 in the 3rd?
For reference, instead of the 1st meaning of 静かさ, with 静けさ we have another group:
風のない穏やかさ 黙 ・ 沈静 ・ 無風 ・ 静寂 ・ 平静 ・ 静止
Isn't it overly specific? Can't this definition easily overlap with another group's definition (same of 2 and 3 for 静かさ).