You may be asking yourself – why is there an article about
magnetic declination and the Earth’s magnetic field on a web site about Milky Way Photography? Isn’t that a bit too nerdy for a photography website?
Well, if you use a star tracker, you’ll occasionally be at a location where you can’t see Polaris, the “North Star.” (Or the constellation Octans if you’re in the southern hemisphere.) I ran into this situation recently when I was setting up my Sky-Watcher Star Adventurer tracker for the Total Lunar Eclipse. I had a row of houses blocking my northern view, and I could not align with Polaris.
For situations like that, a great trick is to use a compass to align your star tracker to true north. But to do that, you need to know your magnetic declination. Magnetic declination is the number of degrees and direction between the Earth’s magnetic field lines at your site and the Geographic or True North Pole.
What is Magnetic Declination?
Magnetic declination varies not only with a location on the surface of the Earth, but it also changes with time. The same charged particles in the solar wind which create auroral displays also cause tiny fluctuations in magnetic declination. These changes vary slightly over milliseconds due to induction with the ionosphere. It also varies every year due to currents in the Earth’s molten metallic core. Over centuries, this can cause the declination to change tens of degrees. The North and South Geomagnetic Poles can even reverse. It doesn’t happen very often. The last time was about 780,000 years ago.
Your Local Declination
Last I looked in about 2010; my local magnetic declination was just over 16 degrees east of true north. At that time, I was surprised it had changed so far from what I remembered it was just a few decades earlier. I was a budding large format landscape photographer in the early 1980s. I amassed an extensive collection of printed USGS maps so that I could plan out photography locations. Each one of those maps listed the declination. At that time, the magnetic declination for my home was 20 degrees east. Today it’s nearly 15 degrees east!
That’s a difference of nearly 5 degrees! That’s going to be far enough off when using a compass and the old value that I’m not going get proper polar alignment with my star tracker. And this change in declination value can be even more significant at latitudes above 55 degrees than it is at my 45 degrees north.
Because the magnetic declination changes over time, the National Centers for Environmental Information (NCEI) publishes updated maps (and models for computerized use) every five years. But this cycle, they decided that they could not wait for five years. So they just released a new contour map showing the Earth’s Magnetic Declination as of 2019. NCEI states that this out-of-cycle release “will ensure safe navigation for military applications, commercial airlines, search and rescue operations, and others operating around the North Pole.”
Back to Milky Way Photography
If it’s changing so fast, how can you find out what the updated magnetic declination is for where you are? NCEI has a convenient tool to figure this out for you. This handy calculator will tell you the declination for anywhere on the Earth.
The NOAA NCEI magnetic declination calculator can be found here:
You enter the latitude and longitude for the location of interest, or if you are in the USA, you type in a street address or even a ZIP Code, and it will autofill the latitude and longitude for you. Then select the computer model based on the year. (Did I mention it can calculate historical declination values as well?).
Testing the NCEI Magnetic Declination Calculator
Comparing the calculator with the United States Geological Survey (USGS) maps I have going back to the 1960s, my published declination was 21 degrees east. The NCEI declination calculator said it was 20 degrees 57′ east. That’s pretty good for government work! My maps that are from 1981 list the declination as 20 degrees east – NCEI says it was 19 deg 49′. Again, that’s pretty good.
Today, NCEI says my declination is 15 degrees 14′. If I had used one of the declination values that I’d remembered from a few years ago, I’d have aligned my tracker about five degrees off from true north.
As it happened, I ended up misaligning my tracker by not checking if it was level… I was in such a hurry to get set up after the rain clouds broke just minutes before totality, I overlooked leveling my tripod! Doh!
Turning of the Screw
Many compasses have a set screw for setting an offset into your compass display to the declination of your location. Look for it on the backside of the compass, under the ring that surrounds the face of the compass. Often there will be a small metal screwdriver on the lanyard of the compass for adjusting the declination. Take the screwdriver, insert it into the set screw, and turn it until the declination indicator is the correct number of degrees east or west for your location.
When you travel, check and see if your magnetic declination has changed from the last place you used your compass. You’ll probably need to adjust the declination setting on your compass whenever you travel a significant distance.
Sometimes You Can’t Rely On Your Memory
So whether you’re hiking around using your compass (probably because your phone died) or you are setting up your star tracker somewhere, and you can’t see Polaris to visually align it, then be aware that the declination value you’ve stored in your head could be several degrees off from what you think it is. Look up your current value. It will help you get a more accurate alignment with your tracker and then get better photos!
After publishing this article, I had a question about whether phone apps are affected by the changing magnetic declination. Well, the answer is yes. I checked out several phone apps that determine your magnetic declination based on your GPS coordinates. Some of these are compass apps, while some are specific to calculate your magnetic declination.
Some apps did not mention which model they used or even if they did use a model to correct for declination. But a few, like Declination, list the model – and it uses the older 2015-2020 model from NCEI. I suspect it will take a while for these apps to update the model they use to calculate declination. You can check your app update log to see if they have upgraded to the newer model.
Get Involved – Become a Citizen Scientist
I’m a big fan of Citizen Science projects – the first one I participated in was the Google MegaMovie project for the 2017 Total Solar Eclipse. (Check at 2:28, and you can see my name in the movie’s credits!)
NCEI has a Citizen Science project to improve the accuracy of its magnetic declination model. The project is called CrowdMag. The CrowdMag app collects data on the Earth’s magnetic field as you do everyday activities like walking, running, or cycling. Then NOAA will create maps and computer models of the Earth’s magnetic field based on the collected crowdsourced data. For more information, check out this YouTube video showing how this app works.
The CrowdMag app is available for both iOS and Android.
NCEI has an article about how the magnetic declination has been moving so quickly recently and why they updated the declination maps a year ahead of their planned 5-year update. If you’d like to read the original news article, you can find it here: https://www.ncei.noaa.gov/news/world-magnetic-model-out-cycle-release
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