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.
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).