How to use it
- Choose a place, a date and a time in the local time of that place.
- Read the azimuth, the altitude and the shadow length. The sky chart shows the day's path; the map shows the direction of the sun.
- Press Now for the present moment.
5 practical uses
- Place solar panels or a sundial.
- Check which windows and rooms get sun at a given hour.
- Plan shade for a garden, a terrace or a playground.
- Set a photo so the sun is behind or beside the subject.
- Work out where a shadow falls at a given time.
The maths behind it
From the Julian day the sun's ecliptic longitude gives its declination δ and right ascension α; the local sidereal time θ gives the hour angle H = θ − α. Then sin h = sin φ sin δ + cos φ cos δ cos H for the altitude and tan A = sin H / (cos H sin φ − tan δ cos φ) for the azimuth. A shadow of a pole of height 1 has length 1/tan h and points away from the sun.
The numbers
Azimuth is the compass direction of the sun, clockwise from north. Altitude is its height above the horizon in degrees; 90° is straight overhead, which only happens in the tropics. The shadow length is the shadow of a vertical pole one unit tall, one over the tangent of the altitude, and it points away from the sun. Useful for placing solar panels, planning a photo, or seeing which windows get the afternoon sun.
Times are computed in the browser with the SunCalc library, which follows the astronomical almanac formulas to within a minute or so, and are shown in the local time of the place, including summer time, from its time zone. Sunrise and sunset are the moments the top edge of the sun meets a flat horizon, with the usual allowance for the bending of light in the air; a hill to the east delays your sunrise.