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# How many hours of direct sunlight comes from north of the east west line? Answered

We all know that the sun rises in the east and sets in the west.  And it is true 2 days a year!   The other 363 days it is untrue.  I found a neat little website sollumis.com that  gives a  visual representation  of the sunlight at any day of the year and at any location and  in summer quite a lot comes from the northeast and northwest in the mornings and evenings. (where I live at about 49 degrees north).
I thought the rays represented 20 minutes of sun travel originally but now I am not so sure because the hours of daylight in a day do not correspond with the number of degrees of sunlight.  Does anybody know how fast the sun goes down? Does it appear to speed up as it goes down in summer or does it appear at its fastest around noon?  If it goes down faster than it goes at noon then there will be less hours of sun north of the east west line but if it goes slower, there will be more hours of sun north of that line.
I would like to know how many hours of light  come from north of the east west line so that I can  give a guesstimate about how much more light my lean away greenhouse collects compared to a lean to.  The various websites are good for giving the sun up and sundown times and angles but none I have found give the corresponding times for the sun passing  the east west line in morning and evening.
It is a tricky question and I can make a physical model to figure it out but maybe someone already knows the answer.
In the diagram the sunrise is 5.11 am and sunset is 9.17 pm and the sun rises at 52 degrees and goes down at 308 degrees.  The degrees divided by 360 do not tally with the hours divided by 24  0.67 for the hours and 0,71 for the degrees.
Thanks for any help

Brian

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## 12 Replies

gaiatechnician (author)2012-08-29

Just a note that in the process of understanding this, I downloaded the stellarium program. This is a star watcher program. You can set your location to anywhere on earth click on the sun, run it for any day of the year and check for yourself. For most people (including me) it is too hard to understand with mental pictures. (Because you are moving tilted globes around in your mind which is a high gpu process.) I have an instructable about how you use stellarium to see where the sun is, how to easily show the time of day and where in relation to the east west line the sun is. brian

gaiatechnician (author)2012-06-30

http://higheredbcs.wiley.com/legacy/college/strahler/0471417416/animations/ch02/animation3.htm is not bad to show the sun's path in 3 dimensions.
Brian

lemonie (author)2012-06-25

How much does the diameter of the sun figure?
I am thinking that sunrise and sunset are measured at the edge of the sun, while the degree measurements are dead-central.

L

gaiatechnician (author)2012-06-25

I don't think it comes into it too much. I know sometimes in the evenings in summer the rising and setting sun is pretty big but I think the differences in daylight time and degrees are mostly accounted for by how close to mid summer or mid winter we are. So at equinox degrees and daylight is really close but in June they tally badly.
Unfortunately I destroyed my wooden model for how the sun moves when I moved. If I had it now, I could probably check it and figure it out myself.
Tried imagining it in my head a couple of nights and didn't get anywhere.
I think it is faster north of the east west line (DARN) but didn't think deeply enough to be certain.

lemonie (author)2012-06-28

The degrees divided by 360 do not tally with the hours divided by 24

It's that I was trying to answer, we expect that they should be the same.

L

gaiatechnician (author)2012-06-28

Hi, lemonie,
I really thought they would be nearly the same too. (degrees and hours) I made a model and it suggested that there is less than 12 hours of sunlight coming from south of the east west line in the northern temperate regions in summer. At first people didn't believe it and I thought it might be a mistake myself but it isn't. A guy ran it through the starry night astronomy program using Nottingham England as the area and a couple of days ago as the date and it came up as about 10.5 hours south of the east west line and about 7 hours north of the east west line. (About 3.5 in the morning and about 3.5 again in the evening.)
The guy is user Usbport on the CR4 engineering forum.
Brian

lemonie (author)2012-06-28

It's tilt of the earth's axis then, in such a way as the degrees aren't really on a true circle, it's warped?

L

gaiatechnician (author)2012-06-28

Pretty much. I think it is really hard to imagine. I spun a laser level on a tripod with degrees on it before I saw it. But it is a heck of a thing to explain in words.
(I cannot explain it even after doing it.) The laser was angled up in its little holder to simulate the summer sun and the whole tripod was angled back to simulate the latitude here.

gmoon (author)2012-06-25

You're not accounting for the tilt of the earth's axis, relative to it's orbit around the sun.

Shine a light on a sphere--unless the light rays are perpendicular to the axis, some areas on the sphere won't receive "50% coverage." If, for example, the sphere's rotational axis is parallel to the light rays, then one half of the sphere never sees any light at all...

gaiatechnician (author)2012-06-26

I set up a preliminary model today and it seems to show that in summer, less than 12 hours of light comes from south of the east west line. (which means that more comes from north than I expected). I might have made a mistake but probably not.
Great if someone could verify it.
Thanks
Brian

gaiatechnician (author)2012-06-25

I am trying to account for it. I had a really good model that could measure the lenght of day anywhere from about 55 north to the Equator. but I threw it out when I moved. The light from the sun is usually seen as a giant spotlight with all the rays parallel. The little picture is my house on June 15th sollumis.com is the site that generates the picture. I made a lean away greenhouse on the south side of an east west fence. Over May June and July it gets quite a bit of this north east and north west light. On the other hand, a lean to will get a lot less of it. Also, if that light does hit a lean to greenhouse, the slope of the roof tends to reflect it whereas the leanaway roof tends to transmit it. (I include an image). This is extremely interesting to me. Really I made the lean away by accident. My windowpanes were already way too high before I even looked at putting on the roof! It is impossible for me to compare this with a real lean-to because one greenhouse is enough. So the next best thing is to model it.
Brian

gaiatechnician (author)2012-06-26

Hi, I set up an experiment to model this and the news seems to be really good (for my greenhouse). In the summer time, the sun spends less than 12 hours south of the east west line! And that means more light comes from north of the east west line that I thought. I could have made a mistake but probably not.