<p>I know that it's an old post, but there is an app available online which might help in similar situations:</p>
<p><a href="https://shadowmap.org" rel="noreferrer">https://shadowmap.org</a></p>
<p>It calculates and visualizes sun light and shadows in a 3D sense anywhere on the globe.</p>
<p><a href="https://i.sstatic.net/brz53.gif" rel="noreferrer"><img src="https://i.sstatic.net/brz53.gif" alt="Screenshot of app from the site mentioned above." /></a></p>

CC BY-SA 4.0
2022-09-03T10:29:47.740Z
2022-09-03T10:29:47.740Z
pesta
<p>In short: If you know length of shadow and object height (assuming vertical object casting shadow of an horizontal surface), you can tell time if you know date and you can tell date if you know time. Additionally, if you know direction of shadow (e.g. relative to north) you can tell date and time without knowing any of them.</p>
<p>The most tricky part is to measure objects and shadows from a photograph - because you need their real length and perspective distorts object and shadow lengths -. To do this you need to make some assumptions about the geometry of the objects in the photograph. In fact, if you know the geometry of the settings, after some geometrical calculation you can find the Sun position and therefore date and time.</p>
<p>For example, with an image like <a href="https://commons.wikimedia.org/wiki/File:Aerial_view_of_Washington_Monument_04799v.jpg" rel="nofollow noreferrer">this one</a> and a detailed map, you can find date, time, position of camera and even focal length of lens - with a lot of measurement and geometry calculation, of course.</p>
<p>However, if the only objects casting shadows in your image are trees and people, measuring their exact position and size would be impossible and date and time estimations will be quite rough, specially if the image doesn't show any other identifiable and measurable features - and even worse if you can't tell the north from features in the photo. In general, finding date and time from shadows works better with buildings, specially with sharp shaped buildings with known orientation.</p>

CC BY-SA 3.0
2017-12-25T18:22:15.077Z
A look at the algorithm for computing a sundial of arbitrary orientation and tilt might be helpful. Both parameters affect the location of the point where the shadow of the gnomon hits the sundial in addition to the ones like date, time and gnonom height for a horizontal sundial. Therefore: slope and orientation of the slope of the ground must be known as well.
laune
CC BY-SA 4.0
2022-09-04T14:24:50.080Z
“If you know direction of shadow”: I suppose you mean in addition to length of shadow and height of object, right?
jcaron
CC BY-SA 4.0
2022-09-04T23:09:06.673Z
I guess that six years ago I assumed that that was obvious, but I edited the answer to make it clearer just in case.
Pere
CC BY-SA 4.0
2023-09-14T16:23:42.190Z
I'm right now looking for formula to calculate time of day from shadow direction on Earth and latitude. Do you know it? My Q: https://astronomy.stackexchange.com/questions/54711/formula-to-calculate-time-of-day-from-shadow-direction-on-earth-and-latitude
Alex Martian
CC BY-SA 4.0
2016-12-06T22:54:34.350Z
2022-09-04T23:07:56.017Z
2022-09-04T23:07:56.017Z
Pere
<p>I have a photo where objects and their shadows (e.g. trees, people) are visible, the Sun is not visible directly, but it is clearly neither dusk nor dawn.</p>

<p>Let's assume that coordinates of this place are known more or less precisely so that I can find it on google maps.</p>

<p>I also know the date when this photo was taken but don't know <em>the time of day</em>. Is it possible to estimate it from the photo?</p>

<p>I have a debate with a friend of mine about this.
My friend states that a reasonable enough estimate is to calculate the Sun's elevation above the horizon by</p>

<p>$$
\text{elevation} = \arctan \frac{\text{object's height}}{\text{shadow's length}}
$$</p>

<p>and then use one the myriad websites on the internets to look up the time for that date and location.</p>

<p>I doubt it because</p>

<ol>
<li><p>The length of a shadow on the photograph depends on the relative position of the Sun, the object <em>and the camera</em>.</p></li>
<li><p>Most importantly, the slope of the ground can't be estimated neither from the photo, nor from google maps. <a href="https://upload.wikimedia.org/wikipedia/commons/0/04/Apollo14Shadows.jpg" rel="nofollow noreferrer">This picture</a> is a good example.</p></li>
</ol>

<p>Who's right? Am I missing something?</p>

<p>Also, it would be interesting to understand the role of uncertainties in this calculation. Assuming that the surface is reasonably smooth and the slope is close to zero, what uncertainties (location, object's height, shadow's length, the slope, camera position) would play the major role? Does it depend on the time of year?</p>

CC BY-SA 3.0
2016-12-07T04:34:21.330Z
The part that You missed is the fact that; and referring to Your example photo:
there is no day and night, Only Day. So the "time" of day would only be relevant to the astronauts clock.
Leslie Jenkins
CC BY-SA 3.0
2016-12-07T13:10:48.033Z
I use this photo from the Moon only to illustrate my point about the role of the surface. Actual photo I'm talking about is taken on the Earth.
xaxa
CC BY-SA 3.0
2016-12-07T13:12:57.350Z
Your comment is irrelevant - nobody claimed an attempt to achieve a 24-hour clock.
Carl Witthoft
<observational-astronomy><amateur-observing>
Is it possible to tell the time of day by shadows on the photo
xaxa
# Is it possible to tell the time of day by shadows on the photo

(Asked by: username_0 on 2016-12-06)

I have a photo where objects and their shadows (e.g. trees, people) are visible, the Sun is not visible directly, but it is clearly neither dusk nor dawn.

Let's assume that coordinates of this place are known more or less precisely so that I can find it on google maps.

I also know the date when this photo was taken but don't know *the time of day*. Is it possible to estimate it from the photo?

I have a debate with a friend of mine about this.
My friend states that a reasonable enough estimate is to calculate the Sun's elevation above the horizon by

$$
\text{elevation} = \arctan \frac{\text{object's height}}{\text{shadow's length}}
$$

and then use one the myriad websites on the internets to look up the time for that date and location.

I doubt it because

1. The length of a shadow on the photograph depends on the relative position of the Sun, the object *and the camera*.
2. Most importantly, the slope of the ground can't be estimated neither from the photo, nor from google maps. [This picture](https://upload.wikimedia.org/wikipedia/commons/0/04/Apollo14Shadows.jpg) is a good example.

Who's right? Am I missing something?

Also, it would be interesting to understand the role of uncertainties in this calculation. Assuming that the surface is reasonably smooth and the slope is close to zero, what uncertainties (location, object's height, shadow's length, the slope, camera position) would play the major role? Does it depend on the time of year?

**Tags:** observational-astronomy, amateur-observing

> **Comments:**
> **username_1**: The part that You missed is the fact that; and referring to Your example photo: there is no day and night, Only Day. So the "time" of day would only be relevant to the astronauts clock.  
> **username_0**: I use this photo from the Moon only to illustrate my point about the role of the surface. Actual photo I'm talking about is taken on the Earth.  
> **username_2**: Your comment is irrelevant - nobody claimed an attempt to achieve a 24-hour clock.  

---

## 2 answers

### Answer by username_3 on 2022-09-03. Score: 8

I know that it's an old post, but there is an app available online which might help in similar situations:

<https://shadowmap.org>

It calculates and visualizes sun light and shadows in a 3D sense anywhere on the globe.

[![Screenshot of app from the site mentioned above.](https://i.sstatic.net/brz53.gif)](https://i.sstatic.net/brz53.gif)


---

### Answer by username_4 on 2016-12-06. Score: 6

In short: If you know length of shadow and object height (assuming vertical object casting shadow of an horizontal surface), you can tell time if you know date and you can tell date if you know time. Additionally, if you know direction of shadow (e.g. relative to north) you can tell date and time without knowing any of them.

The most tricky part is to measure objects and shadows from a photograph - because you need their real length and perspective distorts object and shadow lengths -. To do this you need to make some assumptions about the geometry of the objects in the photograph. In fact, if you know the geometry of the settings, after some geometrical calculation you can find the Sun position and therefore date and time.

For example, with an image like [this one](https://commons.wikimedia.org/wiki/File:Aerial_view_of_Washington_Monument_04799v.jpg) and a detailed map, you can find date, time, position of camera and even focal length of lens - with a lot of measurement and geometry calculation, of course.

However, if the only objects casting shadows in your image are trees and people, measuring their exact position and size would be impossible and date and time estimations will be quite rough, specially if the image doesn't show any other identifiable and measurable features - and even worse if you can't tell the north from features in the photo. In general, finding date and time from shadows works better with buildings, specially with sharp shaped buildings with known orientation.

> **Comments:**
> **username_5**: A look at the algorithm for computing a sundial of arbitrary orientation and tilt might be helpful. Both parameters affect the location of the point where the shadow of the gnomon hits the sundial in addition to the ones like date, time and gnonom height for a horizontal sundial. Therefore: slope and orientation of the slope of the ground must be known as well.  
> **username_6**: “If you know direction of shadow”: I suppose you mean in addition to length of shadow and height of object, right?  
> **username_4**: I guess that six years ago I assumed that that was obvious, but I edited the answer to make it clearer just in case.  
> **username_7**: I'm right now looking for formula to calculate time of day from shadow direction on Earth and latitude. Do you know it? My Q: https://astronomy.stackexchange.com/questions/54711/formula-to-calculate-time-of-day-from-shadow-direction-on-earth-and-latitude  

---

