webspotting 9 - mag dec

First published in the Apr/May 2009 issue of SCOPE, the newsletter of the RASC Toronto Centre. The URL is updated. Republished here with permission.

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I'm assuming you're not the proud bouncing parent of the Orimeadostron UHBRO-10000XTC One-Touch Instamagic Wake-Me-Up-When-Its-Over Adaptive-Aligning Auto-Levelling Butter-My-Toast Self-Collimating Liquid-Mirror telescope with Self-Cleaning Polishing and Red Blinky Lights options. I'm assuming, in fact, your mount does not have a GPS.

I'm assuming you're using an equatorially mounted 'scope, with R.A. motor for tracking. Or you have an alt-az GOTO. You're not working from a permanent observatory. You're in an unfamiliar location and you don't know which is north, exactly. You're not planning an astrophotography session. You're observing from the top half of this planet. You're going to set up early, while it's still daylight. And finally, you have access to a half-decent compass.

Let's say you've been invited to a friend's new cottage. They report tall trees to the north so you will not be able to pick up Polaris. Hoping to provide many IYA Galileo Moments, you've prepared an ambitious plan, a gaggle of celestial targets for the entire evening, including some planets at dusk.

You want to hit the ground running. You want to be very close to the North Celestial Pole when you pop off the lens caps, as close as you can get without visually seeing it.

But where's NCP? It's above the Earth's rotational axis, of course. How can you find that? You can use your compass. But of course the Earth's magnetic pole and true North Pole are different. Geographically, they're 1600 kilometres apart. For people in Toronto, the difference, in an angular measurement, is about 10 degrees. i.e. your compass bearing will be 10 degrees different than the NCP.

Before leaving for your destination, surf over to National Resources Canada Geomagnetism web page (http://geomag.nrcan.gc.ca/index-eng.php) and look for the Magnetic Declination Calculator. If you're very close to a major city or town, you could select it. For greater accuracy, enter the site latitude and longitude (which you'll need anyway). This will produce a measure in degrees and minutes. For example, for the City Observing Session in High Park for Monday 23 February 2009, the magnetic declination was 10° 37' west. Yes, it varies with the date and (by small amounts) time! How do you interpret this?

You're outside looking at where the compass is pointing. But the declination value means the compass is pointing further west than it should. True north a.k.a. grid north is back a bit. Assuming you're facing north, you need to look 10° 37' further to the east to know where the planet's North Pole and NCP is.

If your compass has a compensating dial, like a protractor compass, you can pre-set this declination for your observing session (or hiking trip).  

Off you go. Now you can set your tripod mount quite close to NCP in a new location in the day time.