he.li.o.stat

n.（名詞）
An instrument in which a mirror is automatically moved so that it reflects sunlight in a constant direction. It is used with a pyrheliometer to make continuous measurements of solar radiation.
定日鏡：一種儀器，裝有一面會自動調控的鏡子，以便不斷朝某一恒定方向反射日光。與太陽熱量計同時使用，以保持對太陽輻射的不間斷測量