Glossary

Short explanations of the terms used across Heliotempo, in alphabetical order.

Analemma. The figure-eight shape traced by the sun if you note its position at the same clock time every day for a year. Its width comes from the equation of time, its height from the changing declination.

Astronomical twilight. The darkest part of twilight, when the sun is between 12° and 18° below the horizon. See twilight explained.

Azimuth. The compass direction of the sun, measured clockwise from true north: 90° is east, 180° south, 270° west. See azimuth and elevation.

Blue hour. The period of deep blue twilight when the sun is between 4° and 6° below the horizon.

Civil twilight. The brightest part of twilight, from sunset until the sun is 6° below the horizon (and before sunrise in the morning).

Clear-sky irradiance. The sunlight power reaching a surface under a cloudless sky, in watts per square metre. The solar panel tool does not assume clear skies: it uses typical weather, cloudy days included.

Dawn and dusk. The start of morning twilight and the end of evening twilight. Each twilight stage has its own dawn and dusk.

Day length. The time between sunrise and sunset.

Daylight saving time. Moving clocks one hour forward in summer, so that sun events happen an hour later on the clock. See daylight saving time and sunrise.

Declination. The sun’s angle north or south of the celestial equator, from +23.4° at the June solstice to −23.4° at the December solstice.

Elevation (altitude). The sun’s height above the horizon in degrees; negative below the horizon.

Equation of time. The difference between sun time and clock-average time, up to about 16 minutes. It makes solar noon drift through the year. See why the earliest sunset is not on the shortest day.

Equinox. One of the two moments each year (around 20 March and 22 or 23 September) when the sun crosses the celestial equator and day and night are nearly equal everywhere.

Golden hour. The period of warm, low light when the sun is between 6° above and 4° below the horizon. See golden hour and blue hour.

Horizon. Here, the ideal flat horizon at sea level used for standard sunrise and sunset calculations.

Meridian. The imaginary line in the sky from due north through the zenith to due south. The sun crosses it at solar noon.

Midnight sun. Days when the sun does not set, north of the Arctic Circle or south of the Antarctic Circle around their summer solstice.

Nautical twilight. The middle stage of twilight, with the sun between 6° and 12° below the horizon.

Polar night. Days when the sun does not rise, inside the polar circles around their winter solstice.

Refraction. The bending of light in the atmosphere, which lifts the image of the sun by about 0.57° at the horizon.

Solar noon. The moment the sun crosses the meridian and reaches its highest point of the day. See what is solar noon.

Solstice. One of the two moments each year when the sun reaches its northernmost or southernmost point, around 21 June and 21 December.

Subsolar point. The place on Earth where the sun is directly overhead at a given moment.

Sun path diagram. A chart of the sun’s path across the sky for a location, usually for several dates. See how to read a sun path diagram.

Terminator. The line on Earth dividing day from night.

UTC. Coordinated Universal Time, the reference time scale from which all time zones are defined.

Zenith. The point in the sky directly overhead, at 90° elevation.

Last updated September 23, 2026