Thursday, December 1, 2016

Emperor Nero's "planetarium" - reality or a myth?

A report on Nero exhibitions in Germany this year made me aware of the claim that the octogonal dining room - coenatio principalis - of his palace Domus Aurea in Rome had either one rotating dome or several nested ones in order to display celestial motions 1),2),3). Or - since there is no archaeological evidence for such mechanisms - that there was a fixed dome with moving lanterns instead 4).

The only contemporary source seems to be one line in a text by Gaius Suetonius Tranquillus - and Ref. 5 says that scholars "have long debated whether this planetarium-like aspect of the room was a marvel of Roman engineering or simply a figment of Suetonius' often whimsical imagination." Is there a consensus by now about what clever astronomy display system was or wasn't installed in the Domus Aurea? And what about an even better 'planetarium' in Domitian's Domus Augustana Ref. 1 mentions?

1) Dewdney, Acquainted with the night
2) von Stuckrad, Das Ringen um die Astrologie
3) Merola, Rome's Domus Aurea
4) Goesl, Modern Projection Planetariums as Media of Iterative Reinvention
5) VROMA, Photographs of Domus Aurea

Monday, November 7, 2016

Why you should embrace the 14 November "supermoon" - and why not

On November 14 a slightly unusual sky event awaits us: a perigee full moon (i.e. a near-coincidence of full moon and the closest point of the Moon in its elliptical orbit around the Earth) which is the closest between 1948 and 2034 - though beating those full moons only by a few hundred kilometers i.e. a fraction of one percent of the distance. Here's what that means and what it doesn't. In a nutshell:
  • The full moon of November 14 is the largest in the sky of the year, and it is possible to notice with the unaided eye that its angular diameter and especially area are larger that at other times, by up to 14 and 30 percent, respectively, relative to full moons at apogee i.e. when farthest from the Earth.

  • Typically 3 or 4 subsequent full moons each year occur pretty close to perigee (as this diagram clarifies) and thus look indistinguishable to the eye; all of them are colloquially known as 'supermoons' these days, a decidedly non-astronomical term reluctantly picked up in astronomy outreach in recent years.

  • While the November 2016 full moon holds a proximity record for several decades in both directions, it is totally indistinguishable for the eye from the perigee moons of any other year (and there are many 'close calls' much nearer than the above-mentioned record years: for example next year already).
So: enjoy the show but be aware that something looking exactly the same happens several times each year. Now for the hard numbers (disclaimer: I'm taking the 1948/2034 claim reported in many places for granted and haven't checked it year by year; also distances and times listed and calculated in different places deviate by tiny amounts) ...
  • On 2016 November 14 full moon occurs at 13:54 UTC, when the distance between the centers of Moon and Earth is 356,520 km.

  • On 1948 January 26 full moon occured at 7:12 UTC, when said distance was 356,490 km: in 2016 it thus stays only 0.008% farther away.
  • On 2034 November 25 full moon occurs at 22:34 UTC, when the distance will be 356,446 km: in 2016 it stays 0.02% farther away, still not a difference 'to write home about.'

  • In 2015 at the closest full moon (which coincided with a total eclipse) it stayed 0.1% farther away than in 2016 and looked exactly the same when out of eclipse.
  • In 2017 at the closest full moon the distance will be 356,605 km or only 0.02% farther away than in 2016 (i.e. by same factor by which 2034 will be closer).

  • In October 2016 the full moon was only 0.55% farther away than it will be on 14 November 2016.
  • In December 2016 the full moon will be 0.82% farther away than on 14 November 2016; even that won't be evident in any way: thus three supermoons in a row.
In contrast in April 2016 the full moon was 13.9% more distant than it will be in November when it will thus have a 29.7% larger disk area and brightness. Claims vary how evident this is to casual but attentive observers and all too often "experts" just write it off - but those who actually look, including this blogger, can find it quite evident. So don't let the Moon orbit ellipticity visibility deniers distract you from giving it a try!

Finally some more math, inspired by a message received after posting a draft of this analysis: remember that all numbers above refer to the distance between the centers of Earth and Moon – while most observers (minus the residents of ISS and Tiangong-2) reside on the surface of the former which rotates quite rapidly, namely once per day. We are some 6370 km from the center and typically at an angle to the line connecting the centers of both. Which means a lot.

Take 14 November 2016: For the center of the Earth the distance to the Moon changes little during the day, beginning at 356,472 km at 0:00 UTC, reaching the minimum (the excitement is all about) at about 11:30 UTC and rising again to 356,788 km at 24:00 UTC – the distance shrinks by ~230 km and rises again by ~280 km during that (UTC) day.

But now go to Hawaii, well placed for actually seeing the Moon at perigee: At 0:00 UTC it is 361,133 km away, at local(!) perigee at 10:00 UTC 350,175 km and at 24:00 UTC 361,995 km – the distance first shrinks by 11,000 km, then increases again by 12,000 km! This diurnal effect just dwarfs the tiny differences between the various “supermoons” over the years and centuries where we are talking of a few dozen kilometers.

Oh, and at which latitude you sit also makes a difference. For example in Tahiti at 17°S the 14 November Moon culminates (58° high) at 9:35 UTC and is 351,043 km away. In the same longitude it culminates in the zenith at 12°N and 350,141 km distant – but at 50°S it culminates only at 26° and 353,669 km distant, i.e. 3500 km farther away at the same time. Once again a difference two orders of magnitude more than the differences between the various supermoons …

So to conclude: forget the “largest full moon in decades” meme – it’s mathematically correct but dwarfed in magnitude by effects of your place on the planet and the time on perihelion day. But embrace the fact that a few full moons each(!) year are significantly closer than the others. They are not exactly “super” but perhaps a bit ‘superior’ to the others and can be a bit more impressive than other full moons. That’s all, folks …

Main sources: basic information on ‘supermoons’ (date in 1948 off), distances of many full moons and many calculations performed on JPL’s HORIZONS. A selection of stories on the 2016 supermoons (often in denial of the visibility of the perigee effect and sometimes with funny astronomical misunderstandings): here, here, here, here, here, here, here, here, here, here, here, here, here, here, here and here. Stories - by this blogger - on the actual visibility of the perigee effect: here, here, here, here (bottom) and here.

Sunday, August 7, 2016

The Perseids of 2016 ... beyond the hype

It's one of those years again when we are getting not only the usual bubbly stories about a summer sky full of "shooting stars" (with often highly misleading illustrations in the form of composite images with numerous meteors in one frame or completely made-up artist's views sometimes resembling the end of the world) from mainstream media and certain astronomy website alike but this year, we are told, the Perseids will be super-awesome, much stronger than usual, the best in a decade, unlike any before. Wow ...

Reality check: page 12 of the 2016 Meteor Shower Calendar of the International Meteor Organization. The key sentence there: "Results from Mikhail Maslov and Esko Lyytinen indicate that we will cross a part of the stream which was shifted closer to the Earth’s orbit by Jupiter in 2016. As a consequence, the background ZHR may reach a level of 150–160." Which would be 1/5 to 1/4 more meteors at the peak than an typical year with a ZHR (zenith hourly rate) of 120 or so - this is not a dramatic increase, on a par with some recent years and well below e.g. the 1993 Perseids show which reached a maximum ZHR over 400.

Maslov's current calculation can be found in more detail here: He sees the - somewhat - higher than usual peak at 12:40 UTC on August 12 which for Europe means that the nights Aug. 11/12 and 12/13 should be comparable. Taking into account the radiant altitude and the bright Moon - which sets only after midnight, meteor party planners beware! - one can hope for actual maxmimum hourly rates under otherwise excellent conditions in the 70s, i.e. on average one meteor per minute: see the first and third diagram at the bottom of this Dutch website, blue = what you would see.

There is, however, a minority view based on a NASA model mentioned in this presentation from 2015 which sees a somewhat higher maximum ZHR (around 200) half a day earlier (around 0:30 UTC on August 12): should that happen European observers would be in a sweet spot an see twice as many meteors per hour in the wee hours of August 12 (middle diagram on the Dutch page). It is this vague possibility that much of the (extra) hype this year is based upon, but be warned that NASA's model has had a worse track record that what goes into the IMO Calendar.

So far I've seen only this one article strongly arguing against the PER 2016 hype and pointing out (some of) its problems, though it still uses exaggerated ZHR numbers and doesn't discuss the competing models. Some further information pages and articles of widely varying quality and in several languages about the 2016 Perseids can be found here, here, here, here, here, here, here, here, here, here, here, here, here, here, here, here and here - but from the preceding paragraphs you now know what to believe and where to be skeptical. And what really happens can be followed here, with a few hours delay while visual reports from experienced observers are being ingested.

Monday, June 20, 2016

When the full moon meets the summer solstice

It's one of these days when you would want to grab your 'fellow' journalists, shake them and shout: look at the numbers, st*pid! Countless web stories tell us that today (20 June 2016) full moon and summer solstice fall on the same day and that this is super-rare and thus newsworthy. Alas a simple check of the actual times tells you that full moon was at 11:03 UTC and solstice will be at 22:34 UTC - which is already on 21 June in all of Asia and most of Europe. So much for the coinciding dates which are true mostly for the Americas but simply do not exist for the majority of humanity.

So one has to ignore absolute dates and just go after the time difference to figure out - if one so desires - whether the 11 1/2 hours time difference today is a rarity. A table full moon times and a solstice & equinox calculator allow for a quick check: In 1910 there was a 12 1/2 hour difference (June 22/20 vs. 7 UTC), in 1929 a 6 hour difference (June 22/4 vs. 21/22 UTC), in 1948 a 40 minute difference (June 21/12 UTC), in 1967 a 2 1/2 hour difference (June 22/5 vs. 3 UTC), in 1986 an 11 hour difference (June 22/4 vs. 21/16 UTC) and in 1997 a 13 hour difference (June 20/19 vs. 21/8 UTC). Oh, and there was 2005 with a 22 hour difference (June 22/4 vs. 21/6 UTC): In Chicago e.g. solstice was at 1:46 a.m. CDT and full moon at 11:15 p.m. CDT - on the same day, 21 June (though in subsequent nights).

So this year's half-day difference isn't so rare at all: We had comparably close pairs of full moons and equinoxes in 1997 and 1986 and much closer pairs in 1967 and especially 1948. It is particularly obnoxious that the 1997 case - a mere one Metonic cycle ago - is flatly ignored in the "reporting" today. The reason, though, is obvious and casts a sharp light on how media mechanisms work: since full moon was 5 hours before midnight UTC while solstice was 8 hours after midnight UTC the pair appeared on two different dates also in most of the U.S. and so wasn't "important" (and the scanty 2005 case was overlooked, too). In contrast to the current 'sensation'. Sigh ...

Friday, June 17, 2016

Professional astronomer lets the crowd fund her science

A few hours ago a bold astronomical crowd-funding campaign succeeded when over USD 100,000 were pledged to support an intense photometry campaign for a single star. This raises public involvement in professional science to a new level but it raises some questions, too. Normally a professional astronomer who believes to have found a promising observing project applies for telescope time at a professional observatory which costs nothing - but getting enough time (or time at all) is not guaranteed. Alternatively a dedicated instrument could be built for which funding would be obtained via a grant from a funding organization if the latter deems it a worthy project - or, as it is possible these days, observing time could be bought from a private telescope network. Again the usual approach would be to try to get a grant to obtain the needed cash.

But in this case the astronomer didn't expect to be lucky either way - and instead asked the public at large to fund the telescope time buy. That this worked out so well in the end was due to the enormous hype that had been building (or built deliberately) around the star in question, which is the famous KIC 8462852, of course, with its erratic dips in brightness discovered by the Kepler satellite (and citizen scientists looking at its lightcurves). Its behavior is not fully explained, but some comet debris clouds are the likely culprit - and yet this star has been firmly associated the potential 'alien megastructures' in the public mind. Without these wild speculations - not exactly supported by the scientists in question but not actively discouraged either and rehashed in the media again and again - and also an added layer of drama about historical data and a long-term brightness trend or lack thereof the crowd-funding would have hardly raised a dime.

So there, the pay-per-view telescope network will soon monitor KIC 8462852 with high cadence and enough photometric precision to catch further dimmings (some of which were so strong that no Kepler would have been needed to detect them) - if any occur in the bought time interval, of course. In the best of all worlds, the dimmings (for which amateurs with their own telescopes are on the look-out as well) will return in time and display some property not seen in the Kepler data which will lead to a viable explanation. Equally likely is that nothing happens, the money is gone and a null result remains which wouldn't constrain modelmaking much. KIC 8462852 as 'star of mystery' for the public at large is a unique case in the history of astronomy so far: whether such a let's-all-fund-my-science-pet-project approach could - and should - be applied to other astronomical problems is anything but clear. The outcome and aftermath of the observing run will certainly shape opinions eventually: both amongst astronomers and the public asked to pay.

Monday, June 13, 2016

Involuntary data journalism ...

This doesn't happen - fortunately! - too often: while preparing a "simple" popular article about a paper making headlines already some real scientific detective work ensued that led to fundamental new insights and a revision of some conclusions of said paper ... The topic were the new light pollution atlas and the associated claims - pimped further by the media - that most of this planet was so bright by now that no sensible sky observations were possible anymore, esp. for Europeans and Americans.

Obviously nonsense, so what went wrong? In a first step the unfortunate mix of three different measures for the night sky brightness in the paper - absolute full, absolute artificial increment and relative increment - had to be cleared up which was trivial compared to mastering the formulae to convert between the three different absolute methods in use. That done the paper's key contents could be condensed into this master table which was then - crucially - amended with my own SQM measurements in two Dark Sky Places in Germany and on Rhodes in the past two years.

Since I had been present during these measurements I knew what actual sky appearance they meant - and that finally connected the numbers in my table and in the paper's main table and graphics with the real sky. It turned out that the paper's authors had been way too demanding in what a non-light polluted sky had to be like (and they had also been a bit too conservative re. the visibility of the Milky Way). Using my own experience and their - calibrated better than ever - numbers the article could finally be written after several hours of quite exciting "data journalism" and practical math. You're welcome!

Sunday, May 1, 2016

The weasel that wasn't

Just another quick - and sad - notice about the state of science reporting these days. Each and every major science news website in the Anglo-Saxon world is reporting that "a weasel" had attacked an electrical system of the Large Hadron collider, causing a shutdown. But there was no weasel: as everyone can read on page 10 of the Daily Summary for April 29 of the LHC machine status, the culprit was a fouine, and you have just to go to the respective French Wikipedia page to find out that this refers to one species of carnivore, known - use the sidebar with the 'Autres langues', always a good idea to translate animal names precisely - as the beech marten in English or the Steinmarder in German.

It's so simple: martens are the genus Martes while weasels and some close relatives form the genus Mustela. Both belong to the same subfamily Mustelinae and are thus related (although recent genetic research seems to remove the Martes and others from that subfamily) - but they are neither the same nor is one a subset of the other. (In German confusion might arise as the genus Martes is known as 'Echte Marder' while the family Mustelidae that includes the martens, weasels and much more is called 'Marder', so Wiesel are Marder but no Echter Marder can be a Wiesel. In English the term 'weasel' usually refers to one Mustela species while the Mustelidae may be called "weasel family".) A little digging would also unveil that the beech marten is the only Mustelid known to bite into cables (the reason for which is a subject of interesting research all by itself) - weasels don't do that.

With one "weasel" article after the other appearing on the web (by one author copying from another without checking the simple facts one may conclude, a chain of errors going back to the early incomplete stories) I got almost angry: how were we to trust these sources on reporting correctly on the LHC's complicated science when they can't even name the critter correctly that bit into it? Eventually my trust in journalism was partly restored, though, by the German Press Agency DPA which in its article - widely distributed among German newspapers - named the animal precisely and correctly as a Steinmarder. Still not convinced that this is an important issue? Then check out this bizarre article from 2011 ...

Tuesday, April 19, 2016

Fermi vs. the Wave: battle of the press releases

It all began with this paper that was made public just hours after the announcement of the first detection of a gravitational wave on Feb. 11: the Fermi satellite had seen "a weak transient source above 50 keV, 0.4 s after the GW event was detected, with a false alarm probability of 0.0022" - which no one had predicted to accompany a merger of two black holes. A flood of papers followed within days ("Eine Flut von Papers zur ersten Gravitationswelle") trying to make astrophysical sense of the coincident signals. And there was also a paper on the non-detection of a gamma signal by the Integral spacecraft at the same time which its authors said killed the Fermi observation (in an updated version of their paper the Fermi observers immediately rejected that). So far all this debate had taken place in academic circles and on ArXiv only, though noted by a handful of science writers who actually follow scientific developments first- or close-second-hand.

One of the theory papers trying to explain the allegedly related gamma signal, however, was eventually hailed by the Center for Astrophysics in a Feb. 23 press release which led to several more media reports. But then came a counter strike by the ESA PR department with a press release on the Integral non-detection on March 30 once said paper had been accepted by the journal it had been submitted to. Way down in the text (under the 'fold' actually) the release stated that "if this [Fermi-reported] gamma-ray flare had had a cosmic origin, either linked to the LIGO gravitational wave source or to any other astrophysical phenomenon in the Universe, it should have been detected by Integral as well. The absence of any such detection by both instruments on Integral suggests that the measurement from Fermi could be unrelated to the gravitational wave detection." The arguments in the Fermi paper trying to fit the Integral negative as well were not discussed.

And now Strike Three in what has become a rare transatlantic battle of press releases: On April 18 NASA suddenly came around with a press release on the Fermi paper, now over two months old and apparently still not accepted by its journal. "Gamma-rays arising from a black hole merger would be a landmark finding," the text read, and the first author is quoted: "This is a tantalizing discovery with a low chance of being a false alarm, but before we can start rewriting the textbooks we’ll need to see more bursts associated with gravitational waves from black hole mergers." The non-detection of the signal by Integral and the - pretty adamant - claims by its observers that the Fermi result cannot be right (emphasized in discussions with this blogger, I may add), are not mentioned at all! And so the I'm-only-reading-press-releases faction of science churnalists is left in utter confusion - while everyone else is waiting for more concurrent LIGO and Fermi / Integral / etc. observations that should eventually settle the issue.

Tuesday, April 12, 2016

Communicating Rosetta

It may seem odd to 'review' an issue of a magazine, but the March 2016 "Rosetta special" of Communicating Astronomy with the Public forms almost a book with its 48 pages and interconnected points of view of most of the authors. With the sole exception of the very last article, all contributors were either insiders of the outreach machinery of the Rosetta mission or journalists 'embedded' in some way: this provides a unique behind-the-scenes view of a major science communications and public relations effort, and the broad range of ideas brought to fruition during Rosetta's key years from its 'waking up' in January 2014 has never before been presented in such a comprehensive fashion. The Rosetta outreach community really tried everything they or their contractors could think of to get the public at large excited about this risky flagship science mission which space and science nerds had been salivating about anyway, eager for nuts and bolts information and actual results, of course.

The most exotic venture discussed has also led to the most entertaining article of the Rosetta special in which the ESA-led making of the science fiction short "Ambition" is described as a super-secret operation eventually taking almost everyone by surprise. Here, for once, the actual struggle to succeed is coming to life while many of the other contributions to the magazine issue come over rather self-congratulatory, largely ignoring frustrations along the way and controversies erupting, some of which are even public knowledge. Or they are missing irony, e.g. when the introductory article talks about bringing a 'real-time' experience of the mission to the world (when the opposite was often the case, in stark contrast to the raw drama of Giotto's two comet encounters; best practice and/or failures from previous missions are missing in general). The same can be said when the 'Ambition' piece makes no mention of the fact that the key message of the movie was that comets brought water to the Earth when soon Rosetta's first major scientific discovery would be that at least this comet clearly didn't.

What the contributors - with exception of the one truly independent voice at the end - also gloss over is the major controversy over who's got to see which images from Rosetta's cameras when (that even left the ESA DG frustrated) and how it was semi-resolved with a moderately free sharing of NAVCAM imagery. Or how the unique chance was missed to illustrate the Philae (touch-and-go) landing at Agilkia on that very day with the complete ROLIS descent sequence which was available within hours but officially published only ten months(!) later. Absent is also Philae's biggest unscripted PR success when a MUPUS scientist suddenly revealed all the drama on Twitter - for once the space nerd community was served, too. But most sorely missing in the 48 pages are actual metrics beyond social media likes and anecdotes of how the unprecedented broad Rosetta communications effort really shaped the public's view of ESA: how many more in Europe and outside do now know about that space agency's very existence and/or appreciate its science activities? Were such numbers never researched (e.g. by not too complicated phone polling) or ...?