helio-, heli-

1. The point in the orbit of a planet or comet at which it is nearest to the sun.
2. The point nearest the sun in the orbit of a planet or other celestial body.

The art or process of taking photographs of the sun.

1. An apparatus for measuring the heating effects of the sun.
2. An instrument that measures the intensity of direct solar radiation.
3. A general term for the class of actinometers (name for any instrument used to measure the intensity of radiant energy, particularly that of the sun) that measure the intensity of direct solar radiation.
4. An instrument that measures the total intensity of direct solar radiation received by the earth.

Actinometers may be classified, accordingly to the quantities that they measure, in the following manner:

• Pyrheliometer, which measures the intensity of direct solar radiation.
• Pyranometer, which measures global radiation (the combined intensity of direct solar radiation and diffuse sky radiation)
• Pyrgeometer, which measures the effective terrestrial radiation.

1. An image of the sun produced using a narrow wavelength band of the radiation it emits.
2. A photograph of the sun made with a spectroheliograph taken in a narrow wavelength band centered on a selected wavelength.

1. An instrument for taking photographs (spectroheliograms) of an image of the sun in monochromatic light or over a narrow band of wavelengths.
2. An apparatus for making photographs of the sun with a monochromatic light to show the details of the sun's surface and surroundings as they would appear if the sun emitted only that light.

An instrument used to directly observe solar radiation or spectrum.