The Google Earth Polar Alignment Method
by Joseph Cali

If you know exactly where you will be setting up your gear you can use Google Earth (not Google Maps) to align your mount even during the daytime. I tried this one afternoon last week. Sky was predicted to clear at 2am and I wanted to photograph the Kreutz comet 2020 X3 (ultimately unsuccessful) at 4am low on the southeast horizon if it was bright enough. My front deck is floored in slightly bouncy thick-sheet (fibre-cement sheeting) however the concrete landing at the top of the stairs with open sky overhead is 4" solid concrete topped on a brick staircase structure. Quite solid! It also gave me a clear view to that horizon.

First step was to launch Google Earth (GE), find my house, and activate the ruler tool. Google Maps does not have this ruler tool which is why GE is used. I started the ruler right at the landing and drew a line due south 180 degrees until it intersected with some landmarks that were easily recognisable. The edge of a small shed on an adjacent property some 150m away was precisely due south. This method is independent of magnetic interference. My house is all steel construction, cladding, roof and structural components for termite resistance and so any magnetic method won't work.


Note: I am in Australia, the southern hemisphere, obviously in the northern hemisphere you point north. If however, there is a convenient landmark in the opposite direction to your pole, you can always align the mount simply by aligning and looking from the opposite direction but the polar axis must always point to your pole.

I set up my EM200 mount and aligned the straight edge of the mount with the gap between the two shed buildings and exactly on the edge of the smaller shed on my neighbour’s property. This task is even easier with a small tracker if the tracker has a polar finder. If you have something like a Polarie and no polar finder, you could use the long straight rear casing and align them on something due east or due west. If the landmark doesn't precisely align, consider changing your telescope / tracker location so that you have a precise N, S, E, W reference.

After setting the azimuth, I used a clinometer app on my iPhone to set the polar axis elevation to my latitude. After dark, I checked using the polar finder and I was almost exactly on the south pole. So close that I didn't bother to adjust.

Using ground structures

Another variation of this method is if you have a driveway or other ground structure visible on GE, you can measure the orientation of the structure on GE then cut yourself a triangular template with the angle between that feature and due south or north. Place the template against the side of your mount and align the other edge with the driveway edge.

I first devised this technique when observing a total solar eclipse from near Shanghai in 2009. We set up on a long straight concrete sea tidal / storm wall near the Donghai Bridge south of Shanghai which we had scouted and selected prior to arrival in China using Google Earth and Google Maps street view.



The site looked good. Sitting & set up on the seawall, it was unlikely anyone would be able to stand in front of us. We were sure the massive concrete structure and the nearby bridge would have a lot of steel reinforcing causing problems with compasses.

Using the triangular template, the polar axes could be set at 64.5 deg to the wall. There were four mounts in our group requiring alignment. Using the template and a clinometer, I worked my way around each member of the group and used the template technique, new to everybody to align all four mounts over a period of 15 mins. Being able to sight along a very long length of the wall cancelled out any small variations in the alignment of the wall.

These methods can be applied equally well to a location you are very familiar with, to one that you have never visited and scout from the web beforehand.