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This is still a somewhat contentious question among theorists, but essentially the density waves that create the spiral pattern in spiral galaxies are only stable in the outer parts of a galaxy, beyond a certain radius called in “inner Lindblad resonance”. In the inner parts of the galaxy, different mechanics operate.
Simulations suggest that in these inner parts, density waves still occur, but they tend to cause material to pile up into a linear feature (i.e. a bar) rather than a spiral pattern. However, the timescale for the bar to appear is quite long, and so young spiral galaxies may not have had time to grow a bar yet, even if their spiral structure is well developed. They may well start acquiring a bar in the future.
Moreover, when spiral galaxies collide with one another, or have close gravitational encounters, this tends to mess up the delicate spiral structure and bar formation processes and cause them to ultimately become elliptical galaxies.
Edwin Hubble believed that elliptical galaxies turned into spiral galaxies over time, but we now believe that the opposite is true.
Happy New Year, everybody.
My Pi Gazing meteor cameras monitor sky clarity in Cambridge by taking one-minute stacks (with a Watec 902H2 Ultimate) every four minutes and counting how many stars they can see. In good conditions, the images go down to mag ~6.
The bar charts below show the number of days in each month when my automated system thought it was clear.
Amber means there were at least 7 images with 800 stars visible (mirky conditions for 30 minutes). Green means there were at least 15 images with 1000 stars visible (good conditions for an hour). I invented these thresholds mostly at random.
I’m sure these figures are somewhat dependent on when I last cleaned the cobwebs off the cameras.
Interesting, the autumn doesn’t appear so bad here, though most of the clear skies have been at 4am.
Ron,
I would add to what David said, that the original citations are submitted to Council by the nominating individuals, usually without the knowledge of the award recipient and without any official input from the BAA. While many citations may be very well written, there is no guarantee that the original citation is accurate and does a good job of summarising a member’s lifetime work. Nor is there any guarantee that the recipient themselves is happy with the way it presents their work.
I think, precisely because an award can celebrate many years of an individual’s work, it’s natural that the BAA may want to tidy up the citation and check it with the award recipient before it appears in print. It’s not at all about being mean and saving space!
Best wishes,
Dominic
Hi Robin,
Yes – that would be really nice, though I fear it’s unlikely to happen for a little while.
At the moment the website operations group is focused on preparing for the launch of the new-look BAA website, due in a few weeks’ time. And after the launch, I suspect there will be a crazily busy period of fixing glitches and helping our many contributors to find their way around the new content management system.
So I fear any non-urgent website issues are likely to end up on a to-do list that we won’t have a chance to look at before the spring!
Best wishes,
Dominic
I absolutely agree that in the past the BAA has published surprisingly little (consistently) about the recipients of awards. I think it would be great if they were announced with a news story on the front page of the website, and a matching article in the Journal.
As Jeremy says, your paper subscription to the BAA Journal actually already includes access to the electronic version (in PDF format) which you can download here: https://britastro.org/journal
So, feel free to try reading it on your tablet and see how you get on with it.
For the record, I switched to an electronic subscription a few years ago when I was living abroad and was anxious not to accumulate too much “stuff”. But Jeremy’s point (1) is absolutely right: I read far less of the Journal now than I used to.
In case you’re wondering, the December Journal isn’t available electronically yet, as it normally appears online shortly after the paper copies arrive.
As requested, I’ve deleted the comment which didn’t belong in this thread, and have copied that content into a new thread.
Karl – as others commented, I’m sure your post will be of interest many people, but it didn’t belong in this particular thread!
While I’m here…. I was curious about Paul’s comment above that he’d not heard whether the WHT was opening up currently. Am I right in thinking the WHT is (still) offline all this year for WEAVE commissioning? But presumably that’s now reached a point where they’re doing on-sky calibration work most nights? (Assuming the engineers are actually able to travel to La Palma at the moment!)
Hey folks,
I’m afraid the university / academic sector is taking an extremely cautious approach to Covid right now. If you think the IoP is going over-the-top in demanding pre-registration, you should compare it to the situation in Cambridge. Here, there’s no prospect of public events happening on-site before Christmas – today is, in fact, the first day in 18 months we’ve been allowed visitors of any kind in our building.
I know a few organisations have tried polling their membership (as Robin suggests), but generally the polls only show that there’s a huge spectrum of different opinions. So, I’m not sure how useful such a poll would be, especially since the BAA is constrained by restrictions dictated by venues anyway.
I hope the BAA manages to have a face-to-face Christmas meeting (for those who are comfortable with it), but I fear the BAA will find it tricky to find venues for meetings for a little while to come.
Cheers,
Dominic
Apologies it took me so long to get onto this.
It’s now fixed…. or at least, considerably improved. The duplicate results are gone, but the result counter may not be entirely accurate.
Among other fixes I rolled out this evening, the plate-solution indicator now appears red (“pending”) when images are still in the queue for the plate solver.
There’s also now a much better interface for searching for images of particular objects, either by type <https://britastro.org/observations/object_search.php> or by catalogue reference <https://britastro.org/observations/object_catalogues.php>.
Solar system objects can be sorted by current brightness and/or position, but that’s based on orbital elements and absolute magnitudes I hoover up from various online sources, so beware that accuracy is not always guaranteed – especially for comets. Indeed, I have deliberately hidden my magnitude estimates for comets as the BAA Comet Section webpages have much better and more up-to-date information available.
Hi Tor,
Currently the BAA only allows members to upload images that are public to all. We don’t offer storage of private data, and I suspect it’s unlikely the BAA ever will.
The difference between public and private collections is simply whether the grouping of images is publicly visible or not. If you want to make a grouping “My favourite images of Jupiter”, for example, you can choose whether your member album page allows other people to see that grouping. In any case, the individual images will still appear in your album.
The “Add image” button at the top is used to upload images to your album, and not to add images to collections.
To add an image to a collection, you should navigate to the image in question, and scroll down to the top of the comments area, below the image. There you should find some controls to add (or remove) the image to/from your collection(s).
Hope that helps,
Dominic
In the past, I think people did indeed tend to sweep the question of where the impactor went under the carpet.
In the past 10-20 years, it’s become possible to do full hydrodynamical simulations, and that’s where things start getting awkward. You need a pretty big impactor to knock a Moon-sized chunk out of the Earth. And in the vast majority of orientations, most of the impactor ends up in the Moon, with relatively little in the Earth. That totally mucks up the Moon’s composition.
You can get it right, with precisely the right orientation. From memory, a low-velocity and fairly oblique impact. But, the skeptics would ask, what are the chances nature would manage to play exactly the right snooker shot?
In the new era of robotic exploration of the Moon, I suspect we’ll see a lot more debate about this. The Apollo missions returned lots of rock from a small number of sites, and some people question how representative they actually are.
The giant-impact hypothesis was widely popularised as a direct result of the Apollo samples, most notably by Hartmann & Davis in 1975.
It seems very likely that the Moon has a fairly uniform composition all the way down, because its physical density is compatible with that. And its lack of magnetic field also suggests there isn’t any metallic core at the Moon’s centre.
That is quite suggestive that the Moon is indeed a big chunk of Earth mantle which got knocked off at some point. It’s hard to think of another way to form the Moon with such a similar composition to the Earth’s mantle, but systematically missing all the siderophillic metals which the Earth has in its core.
The theory goes that the impacting body would have been entirely melted / vapourised in the collision, and got thoroughly mixed with the (now molten) Earth’s mantle before the mixture divided into the Earth and Moon. That bit of the theory is certainly the weakest link, and people still run computer simulations and argue about how possible it is. It’s hard to make it work – but not impossible, with a finely-tuned oblique collision.
But as things stand, the debate is basically over the details of the impact geometry. Nobody has yet presented any credible alternatives to the broad outline of the Hartmann & Davis model, despite it seeming to require extreme fine-tuning to give the right result.
I think Andy has already answered the question about the Earth’s orbit. Yes, the Earth’s orbit will have changed. But it found a new orbit. In the solar system’s early history, that would have been commonplace.
John,
PDF password protection is indeed (usually) rather easy to get around. Simply by opening the PDF in any third-party viewer other than the official Adobe Reader, you can usually circumvent all the restrictions, including printing (plus editing, page extraction, etc).
Taking a quick look at a handful of files, I believe the BAA has used various different security settings when preparing Journal PDFs at different times. All the ones I’ve looked at do allow printing, but block other actions. I could well believe that some might have restricted printing – presumably unintentionally.
It would be interesting to know exactly which PDF files are giving you trouble, though in the short term it will probably be much easier for you to circumvent than it will be for us to fix.
Cheers,
Dominic
I think this thread from 2018 got returned to the top of the list of forum topics when Robin edited the web link earlier today, but the content is as timely now as ever.
I would add that the “Objects” box on the image-upload form now allows everybody to tag their images with “Spectrum” and/or “Lightcurve”. You can see all the images that have already been tagged here:
https://britastro.org/observations/index.php?library=0&tagged_object=SPECTRUM
https://britastro.org/observations/index.php?library=0&tagged_object=LIGHTCURVEThis gives a similar but slightly different collection of images from Robin’s, and most importantly, everybody can tag their own images to add them to these galleries.
The object tagging system is a really useful way to make images searchable, so please do fill it in, even if it seems to be duplicating information already entered elsewhere. However, the text you enter does need to exactly match the name of an object which already exists in the database, so wherever possible you should enter the first few characters of an object’s name, and then use the drop-down list of suggestions to ensure your formatting of the name precisely matches the database.
I would add that Andy Lawrence’s book of the same title is also well worth a read: https://www.amazon.co.uk/Losing-Sky-Andy-Lawrence-ebook/dp/B08W9NR7DX
It’s the best in-depth technical account I’ve seen of what Starlink’s motivations are, how much of a problem it is, and why international regulation doesn’t work.
Like Alex, I’ve also been seeing these flares among the captures from my two meteor cameras. The most I’ve ever picked up is 3-5 satellites, but the Pi Gazing software deliberately masks out pixels that seem to be scintillating, which probably works quite well as an unintentional Starlink mask (after the first few members of the chain have gone past, at least).
One feature I added to Pi Gazing over my Easter break was the ability to automatically identify satellites by correlating their positions with orbital elements downloaded from CelesTrak. All of the hard orbital mechanics is calculated by Python’s SGP4 module. I also do the same with aircraft, using a separate Raspberry Pi which records the ADS-B positional squitters of any aircraft within 15 km. (In the long term, I’m wondering about using this dataset to improve my fitting of the radial lens distortion in my cameras).
Here’s one example, from April. Three satellites, about one minute apart, all positively identified as Starlink:
https://pigazing.dcford.org.uk/moving_obj.php?id=20210418_202953_90449fcaf6b4a614
https://pigazing.dcford.org.uk/moving_obj.php?id=20210418_202855_04b04a118e8cb850
https://pigazing.dcford.org.uk/moving_obj.php?id=20210418_202742_4ad72653031ba119There’s a full list of my satellite sightings here… https://pigazing.dcford.org.uk/search_satellites.php… though it’s not been updated since 2 May.
I think we need to draw the line somewhere. The object tagging need to be good enough to enable observers to search for images of their favourite objects, and BAA Section Directors need to be able to quickly find images that may be of interest to their sections. BAA members do often take images of quite faint / obscure comets and asteroids, and may want to compare them with others. But beyond that, the tagging system should be as simple as possible.
Already a significant fraction of users clearly find the tagging quite tricky to use, and I am typically cleaning up the tagging of several dozen images each week.
The BAA Sections haven’t historically tended to collect large numbers of images of planetary moons.
As it happens, although the asteroid database is indeed large, it was very easy for me to build. I downloaded a copy of Ted Bowell’s database of asteroids, and filtered on objects with more than 10 years of observations and more than 500 observations in Ted’s database. That yields 155,000 asteroids… actually a very manageable number to just stick in a computerised database.
As for comets, I just asked the Comet Section to give me a list. I got a list of 4,000 comets, and they’ve been happy to add new comets themselves as they appear.
The old BAA Member Pages (before December 2020) did allow observation dates to be set in the future. Indeed, that may even have happened rather often: from memory, if people forgot to set the observation date, the default was set to the current time with a granularity of 15 minutes, while the upload time was set precisely. All of that was faithfully migrated across to the new database, even though the user interface no longer allows new observations to be in the future.
For some images, it’s very important to record the observation date – for example, planetary images, variable nebulae, etc. But as you say, there are other types of observation (e.g. blink comparisons) where it’s more complicated. For some deep sky images, where the photons have been collected over many nights, it’s very complicated indeed.
I think it would be reasonable to set the “observation date” to the date of the most recent observation, but there’s no “BAA convention” on this. What’s most important is to provide the details in the explanatory text. I’m sorry that your explanation got missed in the migration – there were a small number of comments that had migration issues, mostly due to character encoding issues.
