index arm（指标臂）是什么意思？音标发音+图解场景

指标臂 /ˈɪndɛks ˈɑːm/ n

English: index arm · 指标臂

释义：Moving arm on the sextant that measures the displacement angle on the graduated arc to determine the height of the observed heavenly body.

这个词出现在以下图解场景（1）