Where is the sun? (sun path diagram, solar iPhone apps, and more)

Ever wondered how to calculate the exact position of the sun at a given date and time? Not something everyone worries about – but maybe you’ve thought about how to best shade an overheated room… or where to install solar panels on your roof for the best outcome.

As you have probably gathered, this is actually quite tricky to work out! I’ve been asked this question a few times so I thought I’d post the options I know of here.

1. Use a Sun-path Diagram

The easiest way that I can think of is to use a sunpath diagram. This will tell you where the sun will be on any given date and time for a given locality. A sunpath diagram for Sydney, Australia (latitude of about 33 degrees South, longitude about 151 degrees East)  is shown below (click to enlarge).

How to read it for 5pm on the 1st January (as an example):

Follow the solid line for the 1st January up from bottom left of the chart till you hit ’16’ for 4pm (because we’re in Daylight Savings Time). You will come to a point with an ‘Azimuth’ of (very roughly) 265 degrees from North and an ‘Elevation’ or ‘Altitude’ of 35 degrees.

Source: Photovoltaics CD ROM, Honsberg and Bowden.

2. Use a pre-developed calculator

Another more accurate and probably even easier way is to use a pre-developed calculator. It’s important to understand the fundamentals first though, otherwise you won’t appreciate what you are looking at. A great free tool is provided here: www.susdesign.com/sunangle/

This calculator works really well, but it is important to note that the Azimuth referred to here is not the same as the standard definition as used on the sunpath diagram above. This might be because the tool was developed for the Northern Hemisphere. Choose Azimuth Zero = North and you should be able to work out what the results mean (if not, click the names for their definition).

UPDATE: Or you can simply use an application on your phone. See the comments section below about available iPhone applications.

3. Crunch the numbers!

Not for the faint hearted, but if you like numbers and want to appreciate all the goings-on in these and other solar equations I suggest you buy a book such as ‘Applied Photovoltaics’ (authors: Stuart R. Wenham, Martin A. Green, Muriel E. Watt, Richard Corkish).

Steplight plug-in appliance power meter

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2 Responses to “Where is the sun? (sun path diagram, solar iPhone apps, and more)”


  1. 1 Tony Hollingsworth 16 January 2010 at 8:38 AM

    Hi Ryan

    I’d recommend you try my Twitter pal Graham Dawson’s (@gpdawson) iPhone application – it’s been recommended to me, however I don’t think either of us are iPhone users. It’s called SunSeeker. According to Graham’s website, it is described as follows:

    “Sun Seeker shows you exactly where the sun is for any time on and day of the year, as well as its path throughout the day, both on a flat compass dial as well as in an Augmented Reality view. This is a must have app for all real estate hunters and agents, gardeners and landscapers, architects and photographers.”

    It might come in handy for your field operatives who are using iPhones – I’m going to tweet Graham now to introduce you guys.

    There’s more on Graham’s blog:
    http://ajnaware.wordpress.com/2009/10/06/sun-seeker-seeing-the-light-with-augmented-reality/

    Go straight to the video demonstration here:

    I remember Graham speaking at BarCamp, about his well-knows Oz Weather application for the iPhone, in 2008. I wonder is he is coming to the Canberra BarCamp (something I would recommend you come to, and host a talk on what you’re up to) See #bcc2010 on Twitter and the page http://bcc2010.eventbrite.com/

    Cheers
    Tony

  2. 2 rpmccarthy 16 January 2010 at 3:53 PM

    Hi Tony,

    That app looks excellent – but you are right, I don’t have an iphone!

    I came across an iphone application shortly after I wrote this post and had been meaning to come back and update it with this information. It was one by Energy Matters, and can be found here:

    http://www.energymatters.com.au/iphone/

    Having said that (and not having tested either) Graham’s does look even more versatile…

    http://ajnaware.wordpress.com/2009/10/06/sun-seeker-seeing-the-light-with-augmented-reality/

    Thanks,

    Ryan.


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