<p>Your original thought, that you could just specify the Sun's altitude was correct. The shadow will reach the end of your patio at the same altitude of the Sun every day. Considering where I think you live that will only be true for late April through late August. The Sun won't reach 54 degrees at other times: the shadow will come from the southeast corner of the house at the south end of your block (if all the houses on your block are the same).</p>

CC BY-SA 4.0
2021-12-27T19:11:42.437Z
2021-12-27T19:11:42.437Z
stretch
<p>You will need both the <span class="math-container">$SunAlt$</span>, and <span class="math-container">$SunAz$</span> values.  You'll also need to know the height of the part of your roof casting the shadow <span class="math-container">$H$</span>, and the distance of the far edge of the patio from the roof peak causing the shadow, measured along the ground <span class="math-container">$D_p$</span>.</p>
<p>The length of the shadow cast by the roof, is</p>
<p><span class="math-container">$L = H * \cos(SunAlt)$</span></p>
<p>But that length won't be cast directly towards the far edge of the awning, so you need to account for the direction <span class="math-container">$SunAz$</span> also.  <span class="math-container">$SunAz$</span> is usually measured with 0 being North, and 180 being South.  But you want to know the length of the shadow cast from the West, so we need to subtract 270 degrees from <span class="math-container">$SunAz$</span>.  So the final distance <span class="math-container">$D$</span> from the peek of the roof to the edge of the shadow is</p>
<p><span class="math-container">$D = L * \cos(SunAz - 270)$</span></p>
<p>Putting both equations together yields:</p>
<p><span class="math-container">$D = H * \cos(SunAlt) *  \cos(SunAz - 270)$</span></p>
<p>If <span class="math-container">$D$</span> is greater than <span class="math-container">$D_p$</span>, then the patio is entirely in shade.</p>
<p>This applies only to the actual plane of the patio though, if a person is standing/sitting on the far edge, the sun will still be in their eyes, while their feet are in shadow.  To account for this, just subtract the height of the person from <span class="math-container">$H$</span>.  And it should be obvious that there is no shade on the patio if <span class="math-container">$SunAz &gt; 180$</span>.</p>
<p>I tried to draw some diagrams, but my skills just aren't up to snuff.</p>

CC BY-SA 4.0
2021-11-25T18:01:56.013Z
2021-11-25T18:01:56.013Z
Greg Miller
<p>I'd like to pull in my awning if my house's shadow has reached a point where the awning has no use any more. The house is almost perfectly aligned in a North-South direction, the garden with the awning is in the east.</p>

<p>My home automation (fhem) offers a module that gives me a variety of astronomic parameters, based on my location and altitude:</p>

<pre><code> "SunAlt": -3.3,
  "SunAz": 309.5,
  "SunDec": 22.3,
  "SunDiameter": 31.5,
  "SunDistance": 151752013,
  "SunDistanceObserver": 151752409,
  "SunHrsInvisible": "08:10",
  "SunHrsVisible": "15:49",
  "SunLon": 72.7,
  "SunRa": "04:47",
  "SunRise": "05:16",
  "SunSet": "21:05",
  "SunSign": "Zwillinge",
  "SunSignN": 2,
  "SunTransit": "13:10",
</code></pre>

<p>Is it enough to target sun's altitude above the horizon? I would think so, but I'm not sure... Currently I'd withdraw the awning when sun's altitude goes below 54⁰.</p>

<p><strong>Edit:</strong>
Sorry, I seem to have written my question in a very unclear way. Here's more information:</p>

<p>I know my location, at least with GPS precision. Given that location, fhem's astro module gives me all kind of information about sun, moon,...</p>

<p>I have a patio that's to the east of my N-S aligned house. The house is a row house, so for my purpose it's long enough that only the roof is interesting for the shadow.</p>

<p>Sun shines onto the patio in the morning until (currently) something like 15:00. I tend to make shadow with the awning in the morning. After 15:00, the patio is in the shadow of the house and I don't need the awning any more and I would like to remove the awning automatically.</p>

<p>I looked at fhem's astro module exactly at the time when the shadow was far enough on the patio that I could remove the awning. I took the "Sun altitude" value (54⁰) and now the awning is removed when the sun reaches an altitude of 54⁰</p>

<p>This was some days ago and I <em>think</em> this might not be good enough: it seems as if the shadow is extending more and more until the awning is removed. Or the other way round: It seems that I have to adjust the 54⁰ to, maybe, 50⁰ now to get the right timing.</p>

CC BY-SA 4.0
2020-06-03T07:21:17.553Z
Depends on what you mean by "has no use any more"  Perhaps if you showed how you calculated 54 degrees it would be clearer.  Is 54 working for you? Is there a problem?  On another note, It's amazing that someone has put in the time to provide "sun distance" in this software!
James K
CC BY-SA 4.0
2020-06-03T10:53:46.533Z
I think we need the relative dimensions of the awning and the space you're trying to shade with it.
Mike G
CC BY-SA 4.0
2020-06-03T15:09:32.810Z
Maybe a add a photo or two of your patio near the critical time of day?
Mike G
CC BY-SA 4.0
2020-06-03T15:19:54.650Z
Right, indeed I read the (initial) question quite differently. The answer to this question probably is answerd that not only the sun height but also angle it reaches a certain height changes daily.
planetmaker
CC BY-SA 4.0
2020-06-03T17:17:23.833Z
Yep, you need two angles, because the sun's latitudinal position in the sky at a given hour varies throughout the year, and I think both of the angles affect the length of the shadow your roof produces.
Carl Witthoft
<positional-astronomy>
Can I derive the position of my house's shadow from sun altitude?
Markus Grunwald
# Can I derive the position of my house's shadow from sun altitude?

(Asked by: username_0 on 2020-06-02)

I'd like to pull in my awning if my house's shadow has reached a point where the awning has no use any more. The house is almost perfectly aligned in a North-South direction, the garden with the awning is in the east.

My home automation (fhem) offers a module that gives me a variety of astronomic parameters, based on my location and altitude:

```
 "SunAlt": -3.3,
  "SunAz": 309.5,
  "SunDec": 22.3,
  "SunDiameter": 31.5,
  "SunDistance": 151752013,
  "SunDistanceObserver": 151752409,
  "SunHrsInvisible": "08:10",
  "SunHrsVisible": "15:49",
  "SunLon": 72.7,
  "SunRa": "04:47",
  "SunRise": "05:16",
  "SunSet": "21:05",
  "SunSign": "Zwillinge",
  "SunSignN": 2,
  "SunTransit": "13:10",

```

Is it enough to target sun's altitude above the horizon? I would think so, but I'm not sure... Currently I'd withdraw the awning when sun's altitude goes below 54⁰.

**Edit:**
Sorry, I seem to have written my question in a very unclear way. Here's more information:

I know my location, at least with GPS precision. Given that location, fhem's astro module gives me all kind of information about sun, moon,...

I have a patio that's to the east of my N-S aligned house. The house is a row house, so for my purpose it's long enough that only the roof is interesting for the shadow.

Sun shines onto the patio in the morning until (currently) something like 15:00. I tend to make shadow with the awning in the morning. After 15:00, the patio is in the shadow of the house and I don't need the awning any more and I would like to remove the awning automatically.

I looked at fhem's astro module exactly at the time when the shadow was far enough on the patio that I could remove the awning. I took the "Sun altitude" value (54⁰) and now the awning is removed when the sun reaches an altitude of 54⁰

This was some days ago and I *think* this might not be good enough: it seems as if the shadow is extending more and more until the awning is removed. Or the other way round: It seems that I have to adjust the 54⁰ to, maybe, 50⁰ now to get the right timing.

**Tags:** positional-astronomy

> **Comments:**
> **username_1**: Depends on what you mean by "has no use any more"  Perhaps if you showed how you calculated 54 degrees it would be clearer.  Is 54 working for you? Is there a problem?  On another note, It's amazing that someone has put in the time to provide "sun distance" in this software!  
> **username_2**: I think we need the relative dimensions of the awning and the space you're trying to shade with it.  
> **username_2**: Maybe a add a photo or two of your patio near the critical time of day?  
> **username_3**: Right, indeed I read the (initial) question quite differently. The answer to this question probably is answerd that not only the sun height but also angle it reaches a certain height changes daily.  
> **username_4**: Yep, you need two angles, because the sun's latitudinal position in the sky at a given hour varies throughout the year, and I think both of the angles affect the length of the shadow your roof produces.  

---

## 2 answers

### Answer by username_5 on 2021-12-27. Score: 1

Your original thought, that you could just specify the Sun's altitude was correct. The shadow will reach the end of your patio at the same altitude of the Sun every day. Considering where I think you live that will only be true for late April through late August. The Sun won't reach 54 degrees at other times: the shadow will come from the southeast corner of the house at the south end of your block (if all the houses on your block are the same).


---

### Answer by username_6 on 2021-11-25. Score: 1

You will need both the $SunAlt$, and $SunAz$ values. You'll also need to know the height of the part of your roof casting the shadow $H$, and the distance of the far edge of the patio from the roof peak causing the shadow, measured along the ground $D\_p$.

The length of the shadow cast by the roof, is

$L = H \* \cos(SunAlt)$

But that length won't be cast directly towards the far edge of the awning, so you need to account for the direction $SunAz$ also. $SunAz$ is usually measured with 0 being North, and 180 being South. But you want to know the length of the shadow cast from the West, so we need to subtract 270 degrees from $SunAz$. So the final distance $D$ from the peek of the roof to the edge of the shadow is

$D = L \* \cos(SunAz - 270)$

Putting both equations together yields:

$D = H \* \cos(SunAlt) \* \cos(SunAz - 270)$

If $D$ is greater than $D\_p$, then the patio is entirely in shade.

This applies only to the actual plane of the patio though, if a person is standing/sitting on the far edge, the sun will still be in their eyes, while their feet are in shadow. To account for this, just subtract the height of the person from $H$. And it should be obvious that there is no shade on the patio if $SunAz > 180$.

I tried to draw some diagrams, but my skills just aren't up to snuff.


---

