Answer:
此句是强调句构,不是定语从句.
(1)强调句构"It was/is .that (who)..."其中"that"是固定的不论强调时间还是地点,除非强调人时可以被"who"替代
(2)在强调句中去掉强调框架"It was/is .that(who)..." 后不会破坏句子的完整性
如此句去掉"It was ...that..."如下:
However,I saw the comet last night .
如果理解成是定语从句,那么"night"就应该被看作被定语从句所修饰的先行词,但从句缺状语成分,只能用"关系副词"引导从句,不能用关系代词"that"
P.S.
后面的that从句竟然理解成是last night的同位语,两者根本就风马牛不相及.