Passive Solar: polar south and solar south

[Mark Siddall]

I’ve heard that there is a few degrees difference between polar south and solar south. What is the difference, how is solar south calculated and how much bearing does this have upon passive solar design.

Anyone able to make any suggestions/comments?

Mark

[David OLIVIER]

Mark


Do you mean just that local time doesn't equal GMT? The far west of England is 10-15 minutes behind Greenwich and this is equivalent to some 4 degrees.

As a non-surveyor, I obtained help in setting out my own foundations. I assumed it would be straightforward but could this difference explain why my own house faces about 3 degrees east of south as now shown by the aerial photo on maps.google.co.uk? Or was it also influenced by the changing relationship between magnetic north and true north?

Or do you mean the variations arising from the eccentricity of the earth's orbit? These differ from month to month and can also produce a discrepancy of up to 4 degrees. See
http://en.wikipedia.org/wiki/Equation_of_time

David.

[Nick Grant]

... and the effect on passive solar design??? of shall we say 5 degrees...

I'd guess that this would be swamped by other variables and errors?

Of course PHPP allows precise orientation input but I haven't tried to see what difference it makes.

Nick

[Brian]

Solar south is the direction of the sun at solar noon. Solar noon varies from day to day through a range of about 20 minutes around 12.00 GMT (or 13.00 BST). There are solar noon calculators online, e.g. http://www.spot-on-sundials.co.uk/calculator.html.

On horizontal ground, set up a vertical (e.g. a post or a plumb line). At solar noon (on a sunny day!), mark the shadow of the vertical on the horizontal plane. That's the line of solar south.

The current position of the north magnetic pole is about 80 N, 80 W. In the UK, magnetic north differs only slightly from solar north – by about the 3 degrees mentioned. So for building purposes here, the difference between solar south and magnetic south is small. (Canadians are not so lucky!)

On many sites, other factors such as shading and aspect are likely to have a greater influence on choice of building orientation, window placement, and so on. For solar panels, a solar sundial (also on the net) shows that an orientation between 10 degrees east and 10 degrees west of south makes almost no difference to efficiency.

[fostertom]

Not sure if this is on-topic but I've found compass north horribly variable as I move around a site - every site or location I've tried. I always thought a decent compass, away from lumps of iron (cars etc) would be pretty pin-accurate. 5 degree swing ea way seems routine - and that's uncomfortable when designing to maximise capture of mid winter insolation coming thro a 'window' in the southern horizon. Then I really do need to know where solar south is, and that's calculated from magnetic south which in turn I'd want to establish by compass.

Is it just me/my compass, or is this soimething I must live with? Needless to say, mobile-phone based GPS is far far worse, making nonsense of apparently wonderful aps like Theodolite. Other than in wide-open countryside, I don't think 'proper' GPS is that much better at finding north.

[Dave Howorth]

Not sure if this is on-topic but I've found compass north horribly variable as I move around a site - every site or location I've tried. I always thought a decent compass, away from lumps of iron (cars etc) would be pretty pin-accurate. 5 degree swing ea way seems routine - and that's uncomfortable when designing to maximise capture of mid winter insolation coming thro a 'window' in the southern horizon. Then I really do need to know where solar south is, and that's calculated from magnetic south which in turn I'd want to establish by compass.

Is it just me/my compass, or is this soimething I must live with? Needless to say, mobile-phone based GPS is far far worse, making nonsense of apparently wonderful aps like Theodolite. Other than in wide-open countryside, I don't think 'proper' GPS is that much better at finding north.

Hi Tom,

If a five degree error on a magnetic compass were routine, there would be far more wrecked ships around the world than there have been over the past two thousand years! And that's notwithstanding being on a moving deck.

There are a few factors to confound the innocent though. An 'ordinary' or magnetic compass points to 'magnetic north', which is not the same as 'true north'; the difference between the two is called the 'variation' and is known for [pretty much] every point on the earth and printed on the right type of maps. It does vary significantly from place to place and changes slowly over time. It shouldn't change within a plot of land, though, unless there are large lumps of iron or magnetic rocks.

Compasses also have something called 'deviation'. That is the error built in to the compass or into the way it is installed. Again, large lumps of metal or magnets near the compass are the usual culprits.  A favourite trick of sailing instructors is to place a portable radio, which has a big magnet in its loudspeaker, near the compass.

So if you're regularly getting an error of 5 degrees each way, go see an expert. You either need a better compass or you need a lesson in how to read it more consistently. Or get a professional-quality gyrocompass.

Oh, and solar south is calculated from true south rather than magnetic south.