Shadow Analysis: Using the Sun to Check When and Where a Photo Was Taken

A photo taken in sunlight has a clock in it. The direction of a shadow depends on where the sun is, and the sun's position depends on four things: latitude, longitude, date and time. If you know two of them, the shadows let you check the other two.

In practice it's mostly used to test a claim. Someone says a photo was taken in a given city on a given morning. You know the place and the date, so the question becomes simple: could the sun have cast these shadows at that hour? Sometimes the answer is a clear no, and that can be the most useful finding of the whole check.

The geometry

At any moment the sun has two coordinates. Its azimuth is the compass direction it's in, and its elevation is how high it is above the horizon. A vertical object casts its shadow in the opposite direction to the sun, so if the sun is in the south-east, shadows point north-west.

Elevation sets the length. Divide the height of an object by the length of its shadow and you get the tangent of the sun's elevation. A one-metre post with a one-metre shadow means the sun is 45 degrees up. Long shadows mean a low sun: early morning, late afternoon, or winter at high latitudes. Very short ones mean the sun is high, around midday, especially in summer or near the tropics.

Finding a usable shadow

Not every shadow is worth measuring. You want something vertical, like a lamp post, a sign pole, a fence post or a person standing straight, with its shadow falling on flat, level ground. Shadows on steps, slopes or walls bend and stretch, and the measurement stops being reliable.

You also need to know where north is in the picture. The easiest way is to find the exact spot on a satellite map and use the direction of the street or a building's wall. Once you know which way the road runs, you can estimate the shadow's angle relative to it.

Perspective is the main trap. A shadow photographed at an angle looks shorter or longer than it really is, and wide-angle phone lenses make it worse. Work with a range of values rather than a single number.

Checking it against the sun

You don't have to do the astronomy yourself. Free sun-position calculators show the sun's direction and the shadow of a one-metre object for any place, date and time. Move the time until the shadow matches the photo and you have your time window.

If you know the place and the time instead, the same calculation tells you what time of year fits. It also works the other way around: from a shadow's length and a known date and time, you can work out the band of places on Earth where that shadow was possible.

What it can't tell you

Shadows need direct sun, so overcast days, indoor shots and night photos give you nothing. Around midday in summer, shadows are short and change slowly, which makes it hard to tell eleven o'clock from one. And a single shadow almost never points to one place. It gives you a set of possibilities that you narrow down with everything else in the image.

It's also easy to fool yourself. If you already believe a photo was taken at a certain time, you'll be tempted to read the shadow so that it fits. Take your measurement first, then look at the claim.

Where it's used

Open-source investigators use shadows routinely to check the time of photos and videos from conflict zones, and fact-checkers apply the same check to pictures sent in by readers. PixTrace includes a shadow check in its reports and lists the times and places that fit the shadows it finds, but deciding what the result means is still up to you.

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