The Astro Compass


Let’s say you’re hiking in the woods, it’s 4pm, and you don’t have a compass or a GPS but want to know which way is north. You could use the analog watch method where you point the hour hand at the sun and bisect the angle between the hour hand and the 12-o’clock mark on the watch to find north and since it’s afternoon you    go clockwise (before noon, go counter clockwise). That’ll get you pretty close, especially if you’re lost.

Well, let’s say you’ve got a digital watch while on your hike, or are in an airplane in an area known as magnetic unreliability which is a place where a magnetic compass doesn’t work very well as you get closer to the earth’s poles. Why wouldn’t a compass work? Well, consider flying over the North Pole. Which way will the compass point? Possibly down. You could use your aircraft’s gyrocompass (I doubt you’ll carry one into the woods, however) though they do suffer from drift due to precession  which means they periodically needed to be reset where the reference was typically a . . . . magnetic compass. Hmmmm . . . . These were the kinds of problems aircraft navigators had prior to Global Positioning Systems (GPS). So, are there any other navigation tools available to help you find your way? Well, yes. One of them is an Astro Compass. Generally, this is an impractical instrument for most everyone, which is why you may not have heard of it before. However, it is an interesting tool and learning about it was a fun exercise in understanding the solar system and aspects of celestial navigation.

A friend sent me a link to an Astro Compass on ebay. That one sold, but I looked around and found one that had a mounting base and also seemed within reach so, I bought it. It is a Sperti Astro Compass Mark II probably made in the 1940s. Unfortunately, it was dropped and consequently broken by the USPS in shipping, so I did need to fix it before I could use it. If you decide to get one, be mindful of the fact that some of the components are made of Bakelite so ask the seller to pack paper or other materials around the compass itself prior to shipping.


The instructions on the side of the compass reads, 1) LEVEL, 2) SET LAT., 3) SET L.H.A BODY, 4) ALIGN ON BODY, 5) READ TRUE COURSE.

Seems simple, right?

Level it – You need the base or something similar in order to level it using the two leveling wheels/screws for adjustments and observing the bubble levels.

Set Latitude – What is your latitude? I wasn’t sure, so I went to the NOAA Magnetic Field Calculator website , chose Declination, entered my ZIP code and then chose Get & Add Lat / Lon button. This will give you latitude and longitude. My latitude is about 38.5 degrees North, so I used the micrometer adjustment for latitude turning it until I was at 38.5-ish, North. Note: you can also get your magnetic declination by clicking the Calculate button.

Set L.H.A. – Uh-huh, right. What is L.H.A.? This is where it got interesting. L.H.A. stands for Local Hour Angle. Imagine that you drew a line from the North Pole to the South Pole right through your location. This is your meridian. The 12 hours prior to noon, that is, from midnight to noon are known as Ante Meridian (A.M.) while the 12 hours after noon, that is, from noon to midnight are known as Post Meridian (P.M.).


At noon, the sun is at its zenith. If you drew a line from the center of the earth to the center of the sun, it would be noon and your Local Hour Angle would be zero when your meridian crosses that line as a result of the earth’s rotation. If the 360 degrees of a circle (earth) are divided by the 24 hours of a day the result is 15 degrees per hour. So, 11 Ante Meridian would be 360-15, or 345 degrees, while 1 Post Meridian would be 15 degrees. I did my testing today at 5PM in June where we are on Daylight Savings Time so, I subtracted 1 hour and multiplied by 15 which is 60 degrees, post meridian. I then used the hour angle knob to set the L.H.A to 60 degrees, that is, 4PM.

Declination – For some reason the instruction to set declination is excluded, but I think it helps to do it. Solar Declination is an angle of the sun relative to the tilt of the earth’s axis.  The declination range is between -23.45 and +23.45 degrees. It reaches +23.45 on June 21st, the summer solstice, which is the longest day of the year in the northern hemisphere. So, today it is roughly 23 degrees North and you really don’t need to be too accurate here as the Astro Compass scale has two-degree increments. Tilt the sight assembly until the proper declination is set by the index arrow relative to the index scale.


Align on Body – This means that while holding the base still (assuming you haven’t fixed it to anything), and pointing the True Heading arrow to the direction you want to go, rotate the Azimuth Circle until the sun casts the shadow of the shadow bar center post between the two lines on the sight assembly’s white plastic plate opposite from it.

Read True Course – Now take a look at the Azimuth Circle and read your True Heading.

Compare your results to a magnetic compass. What I did was place a length of painter’s tape on the pavement aligned with the true north heading on the Astro Compass. Then, I set the magnetic compass down on the tape and aligned it as well. I measured 8 degrees east. If you used the Calculate button at the NOAA site I provided earlier you will also be presented with your magnetic declination. Magnetic declination is the difference between geographical True North and Magnetic North and this difference (declination) varies depending on where you are on the earth. The magnetic declination for my location was . . . . 8 degrees, 5 minutes. Looks like it works.


At this point you might be asking yourself, “Why is this thing called an Astro Compass instead of a Sun Compass?” Good question. The reason is that, just like travelers hundreds of years ago, celestial objects, other than the sun, can be used for navigation. The U.S. Navy produces an Air Almanac in pdf format which you can download for free. It looks something like this segment below which I used in the demonstration above.

Astro_table_Snip for 11JUN17

So, what do you do if it is dark? Notice that the columns after the Sun are Aires, Venus, Jupiter, Saturn, Moon, hence Astro Compass. It was around 10:20 P.M. and I took the Astro Compass outside after setting it up for the location of Jupiter. Or, so I thought. I looked through the lens (there’s no shadow from Jupiter), but there was nothing there. The compass was off for some reason. I went back to the chart and looked again. Oh, yes. The declination for the sun was ~23 . . . North, but the declination for Jupiter was 3-1/2 degrees South. I adjusted the declination and there it was. Very bright, too. Neat.

Astro_table_Snip for Jupiter 11JUN17

There are some other interesting things I learned while looking up the terms used in celestial navigation. Here are some of them:

Did you know that the magnetic poles are moving? Check out Magnetic Declination

Did you know your magnetic compass reads differently in different parts of the world? Check out this interactive map.

Did you ever hear of the Equation of Time?

So far, learning about the Astro Compass has made me think again about the solar system.

Now . . . if I could just find a good place to mount this thing in my truck. HA!

Thanks for visiting my blog.


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