Word in haiku - sugara


This is a well known haiku: Meigetsu ya Ike wo megurite Yo mo sugara

I cannot understand the meaning and grammatical form of "sugara". Is it from
"sugaru" (to finish)? Why does it end in "a"?




In this context,
[すがら](http://dictionary.goo.ne.jp/jn/117460/meaning/m0u/%E3%81%99%E3%81%8C%E3%82%89/)
roughly means "throughout". Therefore,
[夜{よ}もすがら](http://dictionary.goo.ne.jp/jn/228442/meaning/m0u/) means something
like "throughout the night" or "all night long".

So, the subject of the haiku was walking around the pond the entire autumn
night.

