Solar position
The sun's apparent position is computed from the standard low-precision solar series: mean anomaly, equation of centre, ecliptic longitude, obliquity, then a conversion to right ascension and declination and finally to the local horizontal frame using the observer's latitude and the local sidereal time. The result is the altitude and azimuth shown throughout the site, with azimuth measured clockwise from true north.
Displayed altitudes include atmospheric refraction using the Saemundsson formula, which lifts an object on the horizon by about 34 arcminutes under standard conditions. Real refraction depends on temperature and pressure and can vary by a few arcminutes, which matters only within a couple of minutes of sunrise and sunset.
Rise, set and twilight
Event times are found from the hour angle at which the sun reaches a defined altitude, evaluated about the solar transit for that day and then refined once with the declination at the event itself. The thresholds are the conventional ones:
| Event | Sun altitude |
|---|---|
| Sunrise / sunset (upper limb, refracted) | −0.833° |
| Sunrise end / sunset start | −0.3° |
| Golden hour boundary | +6° |
| Blue hour | −6° to −4° |
| Civil twilight | −6° |
| Nautical twilight | −12° |
| Astronomical twilight | −18° |
Above the polar circles these events can be absent. The tools detect that case and report midnight sun or polar night rather than printing a misleading time.
The moon
Lunar position uses a series with the main periodic terms — the equation of the centre, evection, variation, the annual equation and the parallactic equation — giving roughly 0.05° in direction and a distance good to a few hundred kilometres. Phase and illuminated fraction come from the geocentric elongation between sun and moon, so the computed new and full moon moments land within about an hour of a full ephemeris. Moonrise and moonset are found by scanning altitude across the day in ten-minute steps and interpolating the crossing.
This is the right level of precision for photography, tides at a glance or planning a moonlit walk. It is not suitable for occultation timing, eclipse contact times or navigation.
Irradiation and the PV model
The solar energy tools model a full year of sun positions and convert each one into irradiance in two steps. First a clearness index Kt — the share of extraterrestrial radiation that reaches the ground — sets the global horizontal irradiance. Then the Erbs correlation splits it into a direct and a diffuse component, because a cloudy sky delivers a very different mix from a desert sky at the same total. Both components are projected onto the tilted plane, and ground reflection is added with an albedo of 0.2.
Typical yearly clearness values are about 0.42 for cloudy maritime climates, 0.50 for temperate ones, 0.58 for Mediterranean and up to 0.68 for deserts. Because a single value is applied across the year, the model reproduces annual totals and the shape of the seasonal curve well but cannot capture a local microclimate, a foggy valley or a monsoon.
Estimated production applies a performance ratio of 0.8, a common figure for a well-installed roof system covering inverter losses, temperature, wiring, soiling and mismatch. Use the results to compare orientations and to sanity-check a quote; use PVGIS or NREL data for the contract.
Shading and shadows
Shadow length is height divided by the tangent of the sun's altitude, and shadow direction is the sun azimuth plus 180°. The obstacle tool converts a placed marker into a bearing, a distance and an angular width, computes the elevation it blocks as arctan(height ÷ distance), and marks every five-minute step of the day as shaded when the sun sits inside that window and below that elevation. Terrain elevation is not modelled: the calculation assumes both points sit on the same level ground.
Time zones
The map shows times in your device's zone by default, including its daylight saving rules. You can switch to UTC, to true local solar time, or to the zone estimated from the pin's longitude. That last option is an estimate — political time zone borders do not follow meridians — so for travel planning check the actual zone of the country you are visiting.
Data and credits
- Map tiles: the standard OpenStreetMap tiles, © OpenStreetMap contributors under the ODbL. No other base layer is used.
- Place search and reverse geocoding: the OpenStreetMap Nominatim service.
- Map engine: Leaflet.
- Astronomical algorithms: the classical low-precision solar and lunar series in the tradition of Meeus, Astronomical Algorithms.
- Irradiance decomposition: Erbs et al., diffuse fraction correlation; Kasten–Young air mass.
Nothing on this site tracks you. See the privacy note for what the search box sends and what stays on your device.
Questions
Frequently asked questions
How accurate is the sun position?
Better than about 0.01 degrees for the sun's direction over the years 1900 to 2100, which is far below the half degree width of the solar disc. Sunrise and sunset times are typically within a minute at mid latitudes, degrading near the polar circles where the sun crosses the horizon at a very shallow angle.
Does the site need an internet connection?
Only for the map tiles and the place search. All astronomy runs locally in JavaScript, so once the page is loaded the calculations keep working offline and no coordinates leave your device unless you type in the search box.
Can I use the figures for engineering or legal work?
Use them for planning and comparison. For a right to light assessment, a planning objection or a bankable PV yield report, verify with the appropriate authority: a national almanac for times, and PVGIS or NREL measured datasets for irradiation.