Finding True South the Easy Way


Inside this Article

Magnetic Declination (2005)
Magnetic Declination (2005)
Many things can negatively affect the accuracy of a compass
Q: Which of the elements in this scene can negatively affect the accuracy of a compass? A: Lots of them.
Recognizing the Big and Little Dippers can help you identify Polaris, the North
Recognizing the Big and Little Dippers can help you identify Polaris, the North Star.
Aligning an array to true south by siting the North Star.
Aligning an array to true south by siting the North Star.
Magnetic Declination (2005)
Many things can negatively affect the accuracy of a compass
Recognizing the Big and Little Dippers can help you identify Polaris, the North
Aligning an array to true south by siting the North Star.

Proper layout of passively heated homes, photovoltaic (PV) systems, and solar thermal systems is important for getting the most out of your solar resource. Not only is it imperative to use the proper tools for evaluating shading, but also important is the proper orientation of a home or solar collector relative to true south.

PV systems sited within 10 degrees of true south lose a maximum of only 2 percent of their generating capability. (For math-oriented people: The cosine of 10 degrees is approximately 0.98.) This doesn’t sound like much, but considering the cost of PV modules, finding that extra “free” 2 percent can be worthwhile. Passive solar homes and solar thermal systems can show similar gains from accurate orientation.

But few people I encounter—even those with lots of solar energy experience—understand the differences between true, magnetic, and compass north (or south). And many solar energy system designers use only magnetic declination (a location’s difference between magnetic and true north) to determine true south, without realizing that this gives only part of the picture—and that the solar orientation might be farther off than they think.

As a professional mariner, I work intimately with the difference between true north and magnetic north on a daily basis, and know that, in the northern hemisphere, there is an easier and more accurate way of determining true south than is usually discussed.


Two components determine the difference between true north and what the compass reads: magnetic declination and magnetic deviation. Magnetic declination is the difference between true north and magnetic north based on geographic location, and is approximated in commonly found magnetic declination maps. Current theory is that the spinning, molten iron core at Earth’s center creates an electromagnetic field. Since the magnetic field is not exactly lined up with Earth’s axis (North and South Poles), there is a geographic difference between the true poles and the magnetic poles. Compasses are basically magnets that point as closely as they can toward the magnetic poles.

Magnetic declination changes slightly over time as the magnetic pole moves, but is easy to determine by using the National Oceanographic & Atmospheric Administration Web site (see Access). For example, my house’s magnetic declination is 9 degrees 34 minutes east (E), and has been changing by 0 degrees 7 minutes west (W) per year (1 minute of arc equals 1/60 of a degree). Magnetic declination can either be east of north or west of north, which further complicates the procedure—be sure to get the direction correct or else your orientation could be off doubly far.


Magnetic declination is only one of the potential orientation errors. Magnetic deviation is the difference between magnetic north and what the compass reads, or compass north, and is induced in a compass by local magnetic fields. Deviation must be taken into account along with magnetic declination if accurate bearings are to be calculated.

Just like magnetic declination, magnetic deviation can either be east of north or west of north. And it can be the same or opposite of magnetic declination. Local magnetic fields that can contribute to deviation include:

  • The metal parts of the compass or the ship or vehicle it is traveling in
  • Variations in Earth’s magnetic fields caused by differences in Earth’s crust and mantle
  • Variations caused by mountains, iron ore deposits, etc.

While geologic variations like iron ore deposits near your site can cause deviation, it is most commonly caused by iron, steel, or magnets near where you are measuring. And deviation calculations can change from measurement to measurement! Most likely, you are causing the deviation. How far away is your vehicle? Are you carrying a wrench or hammer? Most audio speakers have a magnet in them; do you have a cell phone or radio at hand? Is the solar array’s post made of steel? Steel, a magnet, or any magnetic field from electrical equipment can deflect the needle of a compass you are using.

Luckily, it is easy to address most deviation by removing the source or bypassing it. Move your truck farther away. Leave all your metal tools several meters from where you are measuring. Stand at least a few meters from the steel post the array is mounted on.

To determine magnetic north, you must apply deviation effects to what your compass reads. Then you apply magnetic declination to magnetic north to get true north. Again, each correction can be east or west, so be sure to add or subtract correctly. As you can see, determining true south can become quite complicated!

Comments (2)

Grey Chisholm_2's picture

Good Morning Mike Reed,

Yes, I like the daytime methods also. For true local apparent noon, all you have to do is calculate the time of local apparent noon. For a sailor like myself, with an almanac in hand, a piece of cake. For the highest arc, it takes at least a day to mark the sun's shadow. But once figured out, True South does not change. Both methods work well. Both methods are also written about in many sources. I like the Polaris method because you do not need to calculate anything, you do not need to mark shaddows over a time period. Polaris is just there. The disadvantage to Polaris is first, you have to be able to identify the star; and second, Polaris is not very bright so city lights or even a full moon can wash it out. But, on a clear dark night, it is the simplest method around.

Again, thank you for your words,

Fair winds,
Capt Grey Chisholm

solarmike's picture


I've always liked the Polaris - night Sky approach.

For daytime applications it is easy to find "Solar Noon". At Solar Noon, a shadow cast by a vertical pole will point either directly north or directly south, depending on the observer's latitude and the time of year. At Solar Noon the Sun is at the "highest point" in the sky for that day and when measured in time, it is halfway between Sunrise and Sunset.

A nice calculator is found here:


Mike Reed

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