Shadow Locator
Measure a shadow in a photo. With the capture time, map every place on Earth where the sun matched. With the place, work out the time of day. A satellite dish gives the longitude.
How it works
A vertical object of height 1 casts a shadow of length 1 / tan(elevation), so the shadow ratio gives the sun's elevation. At one instant, every place with that elevation lies on a circle around the subsolar point, the spot where the sun is overhead, with a radius of 90 degrees minus the elevation. The shadow points away from the sun, so its compass direction cuts the circle down to an arc.
- Where scans the whole globe at 1 degree, then refines every hit at 0.25 degrees, once for each minute step inside the time uncertainty.
- When steps through each day at the place in 2 minute steps and lists the windows that fit, with how many minutes one degree of measuring error is worth.
- Dish computes the pointing from each longitude at that latitude to the geostationary satellite and keeps the longitudes that match the dish's direction.
Sun positions use the NOAA solar position algorithm with refraction, ported from geo-sleuth (MIT) and checked against it on 400 random places and times.
Shadow geolocation FAQ
How can a shadow tell where a photo was taken?
The ratio of a shadow's length to the height of the object gives the sun's elevation. At any one moment, every place where the sun stands at that elevation lies on a circle around the point where the sun is directly overhead. If you know when the photo was taken, the location is somewhere on that circle, and the shadow's direction narrows it to an arc.
What is chronolocation?
Working out when a photo was taken. At a known place the sun reaches a given elevation and direction at only a few times a day, so a measured shadow gives the time of day, and across a date range it gives the dates that fit.
How accurate is the shadow ratio from a photo?
It is best for a vertical object, such as a pole or a standing person, on level ground, with the shadow lying roughly across the frame. Perspective, slopes and lenses add error, so keep the tolerance at 2 degrees or more. An error of one degree in elevation moves the circle about 111 km.
How does a satellite dish give longitude?
Fixed TV dishes point at a geostationary satellite over the equator. If you know the latitude and which satellite the dish serves, the direction it faces narrows the longitude to a short range.
Is my photo uploaded?
No. The photo is opened in your browser only to measure the shadow. The sun calculations run in the page; only the map tiles are fetched from OpenStreetMap.
Map data and tiles © OpenStreetMap contributors. Sun geometry ported from geo-sleuth (MIT). Use it on photos you have the right to investigate; locating a private person from their photos can put them in danger.