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The Sky in Motion
Astronomer Will Gater reveals the best way to observe and image transient and evolving celestial phenomena, and how you can help scientists in the process.
For most of us our interest in astronomy is, and hopefully will continue to be, a lifelong passion. In 10, 20, even 30 years from now we'll look up to the night sky and in the stars, galaxies and nebulae that fill our view we'll see old friends, unchanged over all that time. The truth is, of course, that the stars and galaxies we see are moving through space, and nebulae are evolving — they're just changes that are unfolding on an extraordinarily long cosmic timescale.
But that's not to say that we humans can't perceive any alteration or movement in the night sky. Quite the opposite. One could argue that the heart of amateur astronomy — and indeed one of the key elements of astronomy as a field of scientific study — is a rich and deep tradition of observing the changing night sky, from the appearance of comets to the monitoring of variable stars and the searches for supernovae in distant galaxies.
In the following pages we're going to explore some other transient and evolving celestial phenomena that you can observe and photograph with relatively simple equipment — the kind of kit that many amateurs have access to — so that you can see for yourself that the sky, and indeed the cosmos all around us, really is in motion.
Watch an Asteroid Whizz By
While the planets might be the quintessential 'wandering stars', drifting against the night sky's sparkling backdrop over weeks and months, there are other objects within the Solar System whose movement across the heavens is far more dramatic — so much so that their motion against the stars can be discerned over hours and minutes, rather than many days. Near-Earth asteroids are small, typically irregularly shaped bodies, whose orbits bring them relatively close to our planet at times. If a near-Earth asteroid is big and bright enough it can be a thrilling object to catch sight of in a telescope eyepiece, or capture on camera, as it makes a close approach. ESA maintains a database at http://neo.ssa.esa.int/web/guest/close-approaches that you can examine to see when any large, relatively, bright objects are next passing by — and of course the BBC Sky at Night magazine Sky Guide will usually contain news of upcoming notable near-Earth asteroid passes.
Capturing a Near-Earth Asteroid Pass on Camera
Watching a near-Earth asteroid slowly wander across a star field at the eyepiece can be tremendously exciting, but it's the sort of target that really requires a medium- to large-aperture instrument to be seen well. On the other hand, even a modest astrophotography setup can capture brighter near-Earth asteroids — here we explore how.
Step 1 — Equipment Small refractors or Newtonians combined with a CCD camera or DSLR are well-suited to imaging bright near-Earth asteroids; we've even had success using just a DSLR and a 135mm telephoto lens. You'll also need a mount that can track the sky accurately for a few minutes at least.
Step 2 — Track and focus Set up your imaging kit. If you're using an equatorial mount get the polar alignment (and thus the mount's tracking) as accurate as you can, as this will help both image quality and processing later on. Next, focus on a bright star — ideally with the help of a Bahtinov mask.
Step 3 — Locate and image capture Use Stellarium (stellarium.org) and its Solar System Editor plug-in to find a near-Earth asteroid's location. Slew to the coordinates, take brief test exposures, then cross-reference the star field with Stellarium. When you've confirmed the near-Earth asteroid is in frame, check it's not moving out of shot. Capture a series of exposures.
Step 4 — Stack or animate You should now have a set of images (typically taken over several tens of minutes) that shows the near-Earth asteroid moving between frames. You can now process and stack these together with your chosen image processing software to show the asteroid's path, or collect and save the frames as an animated GIF.
Marvel at a Lunar Sunrise
As astronomers we're familiar with the Moon's phases, caused by its movement around the Earth and the changes in illumination that come from the varying geometry of the Earth, Moon and Sun relative to one another. Prior to full Moon, the boundary demarcating night and day on the lunar globe, and the line that gives the phase its 'shape' — called the terminator — is the swathe of terrain where the Sun is rising over the lunar landscape. At this point in the lunar cycle the phase is waxing (growing), as the terminator travels across the disc. After full Moon the terminator moves westwards from the eastern limb once again, but is now where the Sun is setting, with the phase waning (shrinking).
This night-by-night movement of the terminator, and consequently the daily change in the lunar phase, is large and easily visible to the naked eye. But you can also observe and image subtle variations in the Moon's phase over the course of just one night. Watching the Sun rise or set over a chain of mountains or a large crater rim is a captivating observing experience; it is quite something to see the lighting change, and shadows lengthen or shorten. It's evidence of the Moon's orbital motion, happening right in front of your eyes.
The UK winter months, when the Moon is high for hours in a dark sky, are an ideal time to attempt the observation. Our favourite targets to see this phenomenon on are the large craters Copernicus and Plato — the latter especially, for the shadows from its rim that creep across its smooth floor — the lunar Alps and the Sinus Iridum.
A high frame rate camera and a modest amateur telescope can capture the changes easily. If you are able record an AVI video every 20-30 minutes or so for several hours, you can create dramatic animations of the changing illumination. This requires each processed image produced from the raw AVI videos to be brought into software — such as Photoshop or GIMP — as a separate layer. Multiple layers within a single picture can then be saved as an animated GIF file.
How You Can Help The Professionals
Taking pictures or making observations of some of the phenomena we've covered in this article can be an exciting experience in itself, but it's also possible that your records could help professional astronomers with their research. For example, if asteroid imaging is your thing, the scientists working on the OSIRIS-REx mission — which will return samples from the surface of the asteroid 101955 Bennu in 2023 — run a project called Target Asteroids! (https://www.asteroidmission.org/get-involved/target-asteroids) It uses data captured by amateurs to help learn more about certain asteroids. Alternatively, if you've been lucky enough to capture a picture or timelapse of the Northern Lights on holiday, the Aurorasaurus citizen-science project (http://aurorasaurus.org) is collecting images of a poorly-understood auroral phenomenon dubbed, rather unusually, 'Steve' — if your snaps show the unusual filamentary feature they could be useful to researchers.
And of course many national astronomical societies and organisations gather reports and observations of transient and changing astronomical phenomenon sometimes for publication and analysis in their journals.
So whether it's through a citizen-science project or a more traditional endeavour, like meteor counting, planetary imaging or variable star observing, there are many ways that we amateurs can make a meaningful contribution.
The Spectacular Seething Sun
We needn't look lightyears out into space to find evidence of the dynamic and ever-changing nature of the cosmos we live in. In fact you'll find it on our celestial doorstep in the form of our star, the Sun. This seething ball of plasma is constantly changing. Its churning 'surface' — the photosphere — is occasionally pockmarked by dark, transitory, blemishes known as sunspots, while above huge tendrils of plasma, called prominences, rise and waver as they are corralled by the star's magnetic fields.
To observe these features safely however you'll need specialist equipment. To study the photosphere, for example, a telescope needs to be fitted with a certified solar filter and any finder scopes should be removed too. With careful and correct use and installation — conforming to the manufacturer's instructions — certified solar filters can provide superb views of evolving sunspots and large sunspot groups.
There are also specialist dedicated solar telescopes available which, as well as filtering the Sun's light so it is safe to view, show only certain specific wavelengths of the Sun's radiation. One type of dedicated solar telescopes shows what's known as the 'hydrogen-alpha' band in the Sun's spectrum. These solar scopes reveal a layer in the Sun's atmosphere known as the chromosphere and in doing so open a window onto one of the most dynamic regions of our star.
While an ordinary certified solar filter will show the solar photosphere as a smooth whitish or yellowish disc, perhaps marked by sunspots or speckled bright patches known as faculae, a hydrogen-alpha solar telescope will show the Sun's chromosphere as a bright, scarlet-red globe shrouded in a mass of plasma 'fibres'.
A hydrogen-alpha solar telescope will also often reveal the prominences leaping off the limb of the Sun, and these can change in literally a matter of minutes, meaning they are a wonderful target for high-resolution imaging where spectacular animations can be made of their evolution. Sketching can be a great way to record the changes in these features too.
The powerful magnetic fields associated with sunspots also have an effect in the chromosphere. There they manifest themselves as bright 'active regions' where loops of plasma twist and turn around the dark sunspots. Like prominences these too can change and evolve over short periods. Sometimes they may even exhibit very bright, fleeting, beads or filaments of light. These are thrilling events for solar observers and imagers, and are known as solar flares.
Create a Timelapse of the Turning Sky
One of the most obvious signs that we live on a rock spinning in space is the motion of the stars across the sky during the course of a night. This movement is a result of Earth rotating on its axis, and you don't need a hugely advanced setup to capture it on camera; a DSLR, wide kit lens and static tripod are ideal for tackling a classic star trail shot. Leave the shutter open for 30-60 seconds and the rotation of the Earth will blur the stars into short arcs. If you want to take things a step further, try creating a timelapse of the sky — and perhaps the Milky Way too — moving. You can use the same kit as for a star trail shot, but you'll need to approach the way you capture the images in a slightly different way. For timelapses you don't actually want the stars to trail. What you need are for them to be points of light so that when you come to animate the shots it looks almost as if the sky is a static picture that's drifting over a landscape. This may mean that you have to keep the exposure length short, increase the ISO and open your lens's aperture right up to compensate. When you've found the right settings, set the camera taking exposures continuously, say for 30 minutes for a short timelapse. You'll typically capture hundreds of photos doing this, so make sure your camera's memory card and your computer are up to the task! The images can then be processed as a group in image processing software and then imported into a video editor to be animated into a smooth video. There are numerous ways of achieving the latter — for example in iMovie you'd do it by setting the 'duration' of each still image to 0.1 seconds. This technique can also be used to make timelapses of other dynamic astronomical phenomena, such as aurorae and noctilucent clouds.
The Skies In Motion This Month
A particularly fine chance to watch the motion of the heavens is on offer in the UK this month when, in the early hours of the morning on 6 November, the gibbous Moon will occult (slip in front of) the bright star Aldebaran in Taurus. As the Moon journeys across the background stars of Taurus, Aldebaran will disappear behind the brightly lit western limb of the Moon, emerging 40-60 minutes later from behind the unlit eastern limb. Occultations are great events for video astronomy, so if you have a digital camera that can shoot video try capturing Aldebaran suddenly popping into view as it reappears from behind the Moon. The exact moment of Aldebaran's reappearance (and disappearance) will depend on where in the UK you're observing from, so consult a planetarium programme, such as Stellarium (http://www.stellarium.org), for location-specific times.
ABOUT THE WRITER
Will Gater is an astronomy journalist, author and presenter. Follow him on Twitter at @willgater or visit willgater.com
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