What does this model represent?
Interactive teaching model. Solar coordinates use the linked UTC instant. The daily curve, maximum and daylight duration use the displayed apparent solar date and account for changes in declination during the day. Daylight is expressed in apparent solar hours, without atmospheric refraction, terrain or horizon obstructions; it is not a sunrise or sunset forecast. Seasonal buttons are date presets, not exact equinox or solstice instants.
Solar positions are teaching approximations, recommended for exploring phenomena in 1900–2100.
Clock time and apparent solar time
Both times represent one UTC instant: apparent solar time = UTC + east longitude × 4 minutes + equation of time. The equation of time is apparent minus mean solar time. Clock time adds the selected time-zone offset; daylight saving changes clock time only. Dates may differ, and full dates are retained across midnight. Changing location or time zone preserves the instant. Confirm the time zone for custom coordinates. Skipped DST times cannot be selected; repeated times offer earlier and later occurrences. Time-zone rules use the browser’s built-in data.
Three views, shared simulation settings
Earth view: select locations and explore the graticule, day and night, and subsolar point. Orbit and seasons: observe Earth relative to the Sun while its axis keeps a fixed direction. Solar angle: examine the angle above the horizontal and the longer geometric atmospheric path at lower elevations.
How should I read the ecliptic and the axis?
The ecliptic is the Sun's apparent annual path on the celestial sphere. Its plane matches Earth's orbital plane; it is not a terrestrial latitude line. Earth's axis makes an angle of about 23.44° with the orbital-plane normal, or 66.56° with the plane itself. The orbit is illustrated as a circle. Sizes, distances and the atmosphere are not to scale.
Light-path geometry and UV
Relative geometric path is calculated through a uniform spherical shell with radius 6371 km and thickness 100 km. The zenith path is normalized to 1. It describes length only, without realistic atmospheric density, scattering, ozone or spectral effects. It cannot be converted into UVI, UVB transmission, vitamin D synthesis or safe exposure minutes. At night, the model simply has no direct sunlight.
SUNANSWER's 30° seasonal prompt
When the local daily maximum solar elevation is strictly below 30°, pay more attention to dietary vitamin D sources. Exactly 30° does not trigger it. This is a SUNANSWER product prompt, not a cutoff for synthesis, a medical threshold or a diagnosis of deficiency. Food and sunlight-related synthesis are complementary; the model does not provide individual doses.
Public scientific sources
Earth texture: NASA Visible Earth《Blue Marble: Land Surface, Shallow Water, and Shaded Topography》, credited to Reto Stöckli, Robert Simmon and others. Rendering: Three.js (MIT). NASA does not endorse this website; the model does not provide a live UV Index.