Another week down, and nothing to show for it here – seriously, you’re not missing anything on my end. You want me to write about, like, what I had for dinner? I could feature some images from my friends’ trip to Costa Rica, except they’ve been so busy since they got back I’ve only seen the photos of them crashing a whitewater raft…
So today, in recognition of the winter solstice and thus the longest night of the year, we have Space. Back around this time of year in 2006, I had a borrowed Canon 10D, used to take these photos, and one night (or early one morning, if the EXIF clock was accurate,) I set it up on a tripod and locked the shutter open for a 906-second exposure, otherwise known as fifteen minutes – okay, fine, 15.1 minutes. Better? The trails that you see come of course from the rotation of the Earth, and almost hidden there in the branches over to the left is Polaris, the North Star, so named because it sits almost exactly straight up from the geographic north pole, dead along the axis of the planet’s rotation. Therefore, it’s in the same position all night long, every night, while all other stars in the sky shift position throughout the night and throughout the seasons.
And no, it’s not that bright blotch – it’s actually to the right of the blotch, the one single spot of light in the image (not counting the sensor artifacts, the ‘noise.’) In fact, I’m not exactly sure what that bright blotch is. Let’s have a closer look:
This is full resolution, and Polaris is the more-obvious center of the arcs. A lot of people think Polaris is the brightest star in the sky, but it’s not – that would be Sirius, which is dimmer than some of the neighboring planets anyway (chances are if you see an especially bright ‘star’ it’s Venus or Jupiter.) And no, Sirius is not that close to Polaris, and there are no particularly bright stars that close to Polaris, so what the hell is that blotch? I honestly don’t know. Since this was about 6 AM, I might have caught sunlight reflecting from a satellite, such as an Iridium flare.
Seeing things at this resolution, you can not only notice the noise, credited to the long exposure, but also that focus wasn’t bang on, which isn’t surprising to be honest. The sky did not look anywhere near this bright as I was framing the shot, and even spotting Polaris in the viewfinder can be tricky – autofocus is naturally not an option. So the best one can do in such situations is to pick a bright star and adjust focus manually until it looks the brightest and sharpest. By the way, you can’t simply crank the focus ring all the way out thinking this will automatically be ‘infinity,’ because most modern lenses have some additional play in their focus travel and you will only ‘pass’ infinity and be defocusing again. If the lens has a specific infinity pointer, that will get you a lot closer, but even a tiny bit off will result in, well, this. And framing foreground elements will have similar issues, because you might see them well enough with the naked eye, but the viewfinder is notably darker.
A couple of bits of trivia, while I’m here. Polaris is not exactly along the line of Earth’s rotation, and actually describes a tiny circle over 24 hours (which you will only capture around this time of year from well up north during the Arctic night – in populated areas you have to contend with daylight of course.) Polaris is used to align alt-azimuth telescopes, especially those with tracking motors that counteract the Earth’s rotation to allow for long exposures without these kind of star trails, but they have to be aligned with the true axis of rotation, the celestial north pole, which sits a little ways away from Polaris, as illustrated by this plot from Stellarium. Anything in the foreground, like the trees that I captured in the above photo, will be blurred into arcs themselves by the counter-motion of the telescope. And while we all know that Polaris was used to orient towards north for centuries, there is no real counterpart at the south pole of the Earth, no helpful star sitting right above the opposite end of the rotation axis. Well, that’s not exactly true; there are three which sit even closer, but they’re too dim to be spotted easily. Instead we have the Southern Cross, or Crux, four stars in a kite-shape that point in the right direction even while quite some distance from the celestial south pole. Alpha and Beta Centauri are two bright stars not far away, which can also be used to align south; Alpha Centauri is actually two stars in close proximity, part of a trio where the third is too dim to be seen without aid, and the three are our closest stellar neighbors, being a little over four light years away. While Beta Centauri, another triple system appearing similar in brightness to Alpha, is nearly a hundred times more distant.
A little before the above image was captured, if I remember right, I did an experiment on film that also made use of the apparent motion of the stars in the sky, that one being much more captivating. It’s a cool thing to try, but in a couple of weeks; we’re hitting full moon right now, and the light from that will seriously affect star trail photos.