An instrument used to measure the angular separation of two stars that are too far apart to be included in the field of view of an ordinary telescope. The instrument was originally designed for measuring the variation of the Sun's diameter at different seasons of the year.
General term for the class of → actinometers that measure the intensity of solar radiation received on the surface of the Earth. It functions by converting the heat of the sunlight into a voltage using a device called a thermopile, and a recording voltmeter.
From pyr-, from → pyro- + → heliometer.