How do you find an imaginary point when it's moving?

The imaginary point

As we all know, the Earth rotates on its axis. This is why we see the sun rising and setting, as well as the stars moving through the night.

But this gives astronomers a challenge. If you have an equatorial mount, it has to be parallel to the Earth's rotation. The more accurate the alignment, the better the mount will perform. Once aligned, you only have to move one axis has to follow a star accurately. The other one can stay still.

If the mount is poorly aligned, you'll have to adjust the declination axis occasionally. Of course, this isn't a problem for visual observing.

But for photography, a small polar error will ruin your photos. How can you get an accurate alignment? Geometry!

I use a computer program called SharpCap to help with my polar aligning. This needs a computer and a camera on the scope. A PoleMaster does pretty much the same thing.

SharpCap is very clever. It takes a photo of the polar area, you rotate the RA axis and it takes a second photo. This gives the computer all the information it needs!

Have a look at this rather busy image. It's two photos of the polar area laid on top of each other. I've marked a few asterisms so you can see how the photos are rotated.

SharpCap is able to read the stars, and so it knows where the actual pole is in the first photo. I've circled that in green.

Once it has the the second photo, SharpCap constructs perpendicular bisectors of the lines between pairs of stars (see the image). These bisectors all meet at the point of rotation. This is where your RA axis is actually pointed. I've marked that point with a red circle.

To get your axis to be pointing at the Celestial Pole, SharpCap talks you through moving the red circle to the green circle, using just the altitude and azimuth bolts on your mount. Clever!

Now you're accurately polar aligned, you can go ahead and get those long exposure images you were after. I've been up to 20 minutes, but you can go longer if you need to. The stars won't drift... yet.

... but the point is moving

You can use a computer program to help you with an accurate polar alignment on an equatorial mount. Basically, you can polar align your mount by looking at the stars and using a bit of geometry. All you need is a camera that connects to a computer on your scope.

There area a number of different programs you can use, and even hardware attachments that do similar things. It's a fast and accurate way of setting up your equipment. The alternative, called drift alignment, is probably more accurate, but takes a lot longer.

Astronomers, and particularly astrophotographers, have been using geometrical (or more properly "astrometrical") procedures like this for decades.

A crucial part of all these programs is that the computer can see the stars around the pole. Once it has an image, the computer can determine where in that picture the Celestial Pole is.

But in the long term, there's a problem. The Earth is wobbling, and the poles are shifting slightly.

The Earth is probably wobbling for a number of reasons, but we do know that a significant contribution is a change in distribution of ice and water around the planet. Climatge change is unlocking vast amounts of ice from glaciers and the polar regions. This liquid water flows into oceans and subtly changing the Earth's centre of gravity.

Astrometrical studies are based on very precise measurements of star movement - just like SharpCap. These measuements have shown that the axis of rotation moves pretty irregularly, but seems to be moving towards Britain. It's moving about a metre in six years or so. Not fast, but measurable.

Seriously? Is it though?

It's not significant enough to see yet, but in future the makers of SharpCap and similar aids might have to alter their software to track the actual pole in your alignment photo. That depends, of course, on how much ice melts in the future.

So, yes, we can go on like we have with our polar alignment. But if climate change goes on to the point where the poles are shifting significantly, I somehow think that polar alignment for astronomers isn't going to be the biggest problem humanity will be facing!

Image: JPL/NASA