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Hey Jim, about that 4.5 degree to the West for magnetic declination. I wonder if thats why when i took my compass out lastnight and set Polaris in its sights, North was pointing off a little towards the West.

If I was bolting my pier down, would I follow what the compass seemed to indicate was North, or Just go by Polaris?

Id be lying If i said i wasnt just a tad bit stumped by the allowing for magnetic declination part.
I think its starting to make sense. I keep seeing Deans compass on his mount with the white line a little bit away from what his compass said was north.

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As you already know, Polaris is pretty close to the axis of the earth's rotation. However, magnetic north points to a location other than the axis of rotation.

Here's a little experiment to help you visualize it: Take and orange and draw a dot about 1cm from the stem at the top. That dot represents magnetic north, while the stem location is true north. Now imagine you are an observer at various locations on the orange. (yeah, you'd have to be really small!) At some locations, the true north and magnetic north locations would line up perfectly and appear the same. At other locations, there would be a larger difference between where your compass pointed and where true north actually is.

Bottom line, I'd go by Polaris because 1) you know true north is 4.5 degrees from magnetic north at your location, and 2) you don't know if your compass is accurate, so your bracket could possibly be off by more than 4.5 degrees.

Nice work on the observatory, by the way. I'm enjoying watching it come together.

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Edited by bdjeep (05/19/08 02:40 PM)

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If I was bolting my pier down, would I follow what the compass seemed to indicate was North, or Just go by Polaris?

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Absolutely, definitely, positively, 100% go by Polaris. Ignore that bloody compass! It might be inaccurate, your local magnetic declination may or may not match the computer model due to local conditions, and it's not so easy to sight accurately along that short needle, anyway. Polaris sits within a degree of the rotational axis.

Yes, you can succeed with a compass, but I'd double-check that against Polaris, anyway.

Your compass senses the Earth's magnetic field, and so points toward the north pole of the magnetic field, not toward the rotational north pole. And it's rotational north that you want to use to align your pier, obviously.

The north pole of the Earth's magnetic field isn't anywhere *near* the rotational axis. It's somewhere in the ocean off northern Canada and right now is sliding around fast enough to see (it's movement is irregular, but it has moved as much as 25 miles in a single year, which is 15 feet an hour, or 3 inches every minute). A hundred years ago it was at latitude 70 degrees, 20 degrees from the rotational pole! Even now, it's at about 80 degrees.

But it's more complex than that, because the magnetic field is not a nice, smooth, spherically-symmetrical thing. The north magnetic pole is at about 110 degrees west longitude rignt now, or essentially due north of my observatory. So a naive model would indicate that my magnetic declination should be close to zero. But instead, it's about 10 degrees -- and something in my environment adds about two degrees to that (assuming my compass is accurate).

Anyway, you're an astronomer, so you shouldn't be afraid go go outside at night and look at Polaris!

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Terry (astrotrf)

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Thanks, alot, yall.

Ive been out digging my hole for my pier. It started out being 2x2'...and its gotten progressively larger since.

We plan on putting a 2x2' box inside the hole, backfilling it with all the dirt thats came out of it, and filling the box up with concrete to just below the floor.

Im about 3ft. down and probaly about 28x28" squared.
Im hoping to fill it in real hard around the box, and throw another massive pile of rebar into the box, like I did the dob slab.

Im hoping we can get it filled, and started on my floor this weekend but weve got major ear reconstruction surgery scheduled for Tuesday in Cinncinatti, tommorrow is gastrointestinal and immunology in Cinn., Friday is dermatology in Cinn., and somewhere in between we have to eat, sleep, and be merry too.

We may have to stay all next week in the hospital after the surgery, we dont know yet, so im getting anxious to do all i can now.

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Absolutely, definitely, positively, 100% go by Polaris. Ignore that bloody compass!

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So youre saying ignore the compass and just use Polaris?

Yeah, how much faith can you put in a $4.00 compass from walmart anyways?

*shh...Ive got this plan to bribe that nice guy in Nicholasville when the time comes to help me get my alignment right.
In my mind it all works out perfectly and hes stunned by my perfect alignment and wants me to drive his smoking RX-8 as a reward....then we all eat cake and take turns riding the magic pony that just happens to pop up with gold bars in his saddlebags.

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Sleep is the Cousin of Death

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Thanks alot, Walt!

Naw, i havent come up with a name yet. Perhaps I should ask the boys, they love to be involved in everything we do. 'Squidward's observatory' might be their first choice im guessing. Anything 'Spongebob' related would suit them fine im sure.

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Back filled soil will never be as stable as undisturbed soil, square up the top and be done with it.

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I dont think I have another 60 bags of 80lb concrete in me left!


If I do that, which seems to make the most sense, wouldnt I need to let the in-the-hole concrete set before putting my box on top of it, to make sure it doesnt sink?

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Sleep is the Cousin of Death

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