Date: October 3, 2009
Title: Red Fireballs, Quiet Quakes and Heat Haze
Podcaster: Peter Grego, Alastair McBeath, Ian Morison, Robin Scagell.
Organization: Society for Popular Astronomy. www.popastro.com
Description: Contributions from several section directors and advisers from the Society for Popular Astronomy, which was set up in 1953 to help beginners to astronomy of all ages. We find out what you’d feel if you were at the epicentre of a moonquake, what meteor showers are coming up for 2009, what astronomers mean by seeing, and why red fireballs are apparently so common these days.
Bio: Peter is the Lunar Section Director of the SPA, Alastair is the Meteor Section Director, and Ian Morison is our Instrument Adviser. Robin Scagell is Vice President.
Today’s sponsor: This episode of “365 Days of Astronomy” is sponsored by Society of Popular Astronomy.
Transcript:
Hello, this is Robin Scagell of the Society for Popular Astronomy.
The SPA has been helping beginners to astronomy since it was founded in 1953, as the Junior Astronomical Society. Today we have members of all ages, not just juniors, mostly in the UK. As well as our magazine, Popular Astronomy, our News Circulars, and our regular meetings, which you can now view online, we have a number of observing sections and advisers which concentrate on particular aspects of the subject.
For this podcast we asked our section directors and advisers to record short items that interested them, and this is what they came up with. First up is our Lunar Section director, Peter Grego, who has some facts about the Moon that you might not know. He’s also looking forward to the society’s Moonweeks in 2009, which we’ve identified as the best weeks to observe the Moon from the UK.
Peter Grego: The Apollo astronauts used seismometers, or earthquake detectors, during their visits to the Moon, and they discovered that the Moon isn’t a totally dead place, geologically speaking. Small Moonquakes originating several kilometres below the surface take place, and these are thought to be triggered by the Earth’s gravitational pull. Now, we know that earthquakes on the Earth can be pretty devastating, but the Apollo seismometers found that the average moonquake you’d hardly feel it if you were standing over the epicentre, they are that gentle.
As you’re listening to this podcast the Moon is slowly moving away from us. Every year, the Moon is stealing some of the Earth’s energy of rotation, and it uses this energy to heave itself 3.8 cm higher in its orbit. So the Moon is slowly moving away from us. During this podcast, the Moon has moved a fraction of a micron more distant. I don’t think you’ll notice that, measuring it with any optical equipment from the Earth, although we can detect it by bouncing lasers off some reflectors left on the Moon by the Apollo astronauts: we can determine the Moon’s distance pretty precisely by timing the arrival and reflection of these laser beams.
To round off, I think the most amazing thing of all about the Moon is that even a small telescope will show stacks of features on its surface. You’ll see craters of all shapes and sizes, some with magnificent linear bright rays. You’ll see mountains, volcanic domes, vast lava plains, ridges, fault valleys, lava valleys and a whole lot more besides.
So, just point your telescope in the direction of the Moon and I’m sure you’ll enjoy the view. The SPA Autumn Moonwatch takes place between the 24th October and the 1st November, and the Schools’ Moonwatch between the 19th and 29th November, and you’ll find more details of how to observe the Moon and to get involved on www.popastro.com/moonwatch.
Robin: We’ve always worked on the basis that our members may not have telescopes, and one activity than needs nothing more than a keen pair of eyes, and an observer who doesn’t fall asleep easily, is meteor observing. Meteor Section director Alastair McBeath is looking forward to the meteors that are coming up for the rest of 2009.
Alastair: The last quarter of 2009 is a very promising one for meteor activity, with three major shower maxima falling very near new Moon, one each in October, November, and December, from the Orionids, Leonids, and Geminids respectively.
The Orionids peak around October the 21st, when Zenithal Hourly Rates from them will probably be about 25 to 30 this year. Orionid meteors radiate from the Club asterism in Orion, which lies between Taurus and Gemini then, so are usually observable only after midnight BST [British Summer Time — local time zones also apply].
Several maxima have been predicted for the Leonids on November the 17th and 18th, the strongest of which are due close to 21:40 to 22:00 hours on November the 17th. Rates then may reach a thousand to fifteen-hundred briefly. However, this is poorly timed for Britain, because the Leonid radiant, in Leo’s Head asterism, rises no sooner than 23 hours. As a result, places across Asia should be rather better-placed to witness this possible meteor storm.
Geminid activity should peak near 05 hours on December the 14th, with Zenithal Hourly Rates of roughly 120. Similarly excellent activity could persist for six to ten hours to either side of this time, and with a radiant just northwest of the bright star Castor, readily visible from 20 hours onwards, Geminid-watching could be continued virtually all night from Britain.
Good luck, and clear skies!
Robin: One term that’s often misused is seeing. You do hear people say, “The seeing looks good tonight,” when there’s a clear sky, but there’s a very specific meaning to the word, which our Instrument Adviser, Ian Morison, explains:
Ian: Often, if you perhaps use some binoculars, you can see distant objects shimmering in what we call a Heat Haze. We get the same basic effect when we look up through similar ripples high in the atmosphere, carried along by the jet stream. So our telescope looks through those, at the Moon and the planets, and it can affect what we call the seeing. If the seeing is bad, the Moon can appear to boil, and stars look like bloated blobs and sort of shimmer around. But on the other hand if the seeing is good, which sadly isn’t all that often here in the UK, then the Moon can look lovely and still, and you can actually see wonderful detail on the Moon and planets.
Now of course if you’ve got your telescope stored in your house, that may well be considerably warmer than it is in the outside air, and so when you take it outside, the air inside the telescope tube has to come into equilibrium, and you get little currents — tube currents they’re called — and they act just like the little ripples in the atmosphere and totally destroy the image quality.
So it’s a very good idea, if you think you’re going to observe one evening, to take your telescopes out into the outside an hour or two before if you can and let them come into thermal equilibrium, to stabilize with the outside air, and that will definitely help. It is true, in fact, that whereas refractors don’t need too long to thermally stabilize, telescopes like Schmidt-Cassegrains or Maksutovs tend to need rather more. So the bigger the telescope and the more complicated it is, usually the longer it has to actually settle down to give you the very best images.
Another factor which affects what we can see with our telescopes on any particular night is called the transparency of the atmosphere. If the air has come from the south it might be laden with dust and aerosols, then that will scatter light coming from the stars, making them appear less bright, but also reflecting more light from lights on the ground, so-called light pollution. On other occasions, particularly if the air has come from the north — it’s called polar maritime air — it’s actually very clear, there’s not very much dust in it, and then the transparency is often very much better. Ironically, that very clear air is often quite turbulent, so the transparency doesn’t always agree with the clarity of the images you get. Often, it’s the rather more hazy skies that give us the best views of the Moon and planets. But the clearer, transparent skies might be better for looking at distant galaxies. So basically, whatever the conditions, try and do what works best.
Robin: Finally, Alastair McBeath is seeing red — or at least, he keeps getting reports of red fireballs which are nothing of the sort. Here he is again to explain what’s going on.
Alastair: This year has seen an unwelcome upsurge in sightings of Sky Lanterns mistaken for very bright meteors, what we call fireballs.
Such lanterns are roughly metre-high, paper hot-air balloons, powered by a burning wick suspended below them, and released to go with the wind. They can look like literal balls of fire, often red, orange, or yellow in colour, sometimes with flickering flames showing, and occasionally seeming to drop burning material vertically below them. They remain visible typically for between tens of seconds to several minutes.
By contrast, natural meteors rarely last more than a few seconds, and follow a straight-line path through the sky, with any material shed from them following a very similar trajectory.
The best way to tell a Sky Lantern from a genuine meteoric fireball is to examine it with good, properly focused, firmly held binoculars. If the object vanished before you could get your binoculars into action, even when they were ready in your hand, chances are that was a meteor!
So that’s the end of this podcast from the SPA. Please visit our website, www.popastro.com, and sign up to our free Electronic News Bulletins for regular news stories that you might not otherwise hear about, and more information about meteor showers from Alastair. Happy stargazing!
End of podcast:
365 Days of Astronomy
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