Until quite recently it was a very hard one, too: The available predictions of where the tracks would move over our globe were notoriously unreliable, with uncertainties of many track widths. That meant that diehard observers had to spread out perpendicular to the track, even outside its predicted boundaries, and with luck some of them would see the star blink out. Most wouldn't see a thing. Once in a while observing campaigns like this were hugely successful, and the diameter and even shape of an asteroid could be determined with astonishing precision, just from the disappearance and reappearance timings of the star from different locations, but most such efforts failed, and very few observers were staying in this field for long. Imagine travelling the world for solar eclipses and much more often then not missing the zone of totality by a wide margin!
Fortunately things have changed in recent years, with excellent catalogs of star positions available (esp. from the Hipparcos mission) and progress in asteroid astrometry. Nowadays the orbit of the asteroid in question and the position of the target star are often known so well that the uncertainty of the occultation track on the globe is smaller than the track's width (at least if the asteroid is large enough). There are still cases in which the track is nowhere near the predictions, but in specific cases the chances are pretty high that you will actually see an occultation if you travel into the predicted track. Probably the most successful campaign took place in September 2002 when Tercidina covered a star and countless observers across Europe saw it disappear: The shape of the asteroid could be reconstructed from these so-called chords with high quality. And now another potentially excellent opportunity was looming: (308) Polyxo was to cover a 6th magnitude star, and it was supposed to be visible from Spain - and the island of Mallorca!
When I first learned about this possibility in early November from Steve Preston's Website, the irony in the track's path struck me immediately: While Mallorca is over 1200 km from my hometown in Germany, and I most certainly would not have driven that far if the track would have been somewhere on the European mainland, there was a good chance that cheap flights to this particular island would be available. For each summer millions of Germans go there, for unrestrained fun in the Sun and going on a binge along the coast. One of the leading holiday carriers had tickets available indeed, return for just 100 Euros! And, I soon learned, one of the most experienced occultation hunters in Germany, Eberhardt Bredner of the International Occultation Timing Association - European Section, was already making contact with a large observatory located in the middle of Mallorca, the Observatori Astronòmic de Mallorca (note the Catalan spelling; this language is more present than Spanish in the Balear islands today) or OAM.
But Bredner also warned me to wait with booking a flight unless there was independent confirmation of the track, and he pressed Jan Manek - who has been particularly successful in recent years - to provide his view. Manek did just that, and while he fell asleep at his computer (as he said on a mailing list), the resulting Update of November 7th agreed perfectly with Preston's work. For Bredner this was the signal to immediately book his Mallorca flight, even though the local logistics were far from clear, and a few days later I followed suit. Although our plans were well announced on several mailing lists, no other German seems to have been triggered into action, though we knew of a number of French who planned to travel on land to Northern Spain where the track would cross a few seconds earlier (it would probably also touch Sardinia, Sicily and Malta, but under less than ideal geometrical conditions). Meanwhile there were indications from the OAM that a few observers there would like to join our campaign.
When we had made our bookings, the longterm weather predictions for the morning of Nov. 16 - when the occulation would take place immediately before dawn - were good, but soon there was reason to worry: Torrential rains were hitting Mallorca and causing some damage, just days before the event. And even when Bredner and I (who had arrived on the same LTU flight from Düsseldorf to Palma) were driving in our rental car towards the OAM on the evening of the 15th, it rained several times. At the hard-to-find observatory we were met by none less than the director himself, Salvador Sánchez Martínez. In no time were we - and a growing number of other local astronomy enthusiasts - on our way to find a restaurant: None was working in the village of Costitx (pronounced »costitsh«) to which the OAM belongs and which is mighty proud of the latter, as this was a Monday, but in the close-by town of Inca we had success.
Communication may have been a bit difficult, with us speaking little to no Spanish (let alone Catalan) and our hosts not fluent in English, but astronomers tend to understand each other at a different level. Soon maps (and an astronomical video camera) were on the table and plans were made for the night. Bredner was the only one present who had experience with occultation observations (I had tried once a year ago but failed due to bad weather), and he stressed that we should not all be in the same place: You only had a chance to measure the size and shape of Polyxo when you spread out perpendicular to the central line of the occultation track. The latter was predicted to run through the North of the island, the OAM being a few dozen km to the South of it but securely within the track - if the predictions were true. I soon became clear that the OAM observers would try to work from their site while Bredner and I would go North, to position ourselves at smaller and different distances from the predicted central line. When it came to paying we learned that the OAM was inviting us - we were most thankful and felt very welcome already.
Back at the observatory we were greeted by nice clear skies, very dark in some areas and light polluted mainly towards the direction of Palma. The air was still rather moist and the transparency was not perfect (but rarely ever is on an island that small, I was told) though more than suitable for the task ahead. The star to be occulted, HIP 57629, had a visual magnitude of 6.2 after all, and it would be 35° high in the sky. Finding it would be extremely easy as it was just 2° from Beta Virginis - which itself would be easy enough to locate as Venus (15° high) and Jupiter (25° high) would practically point at it from below. But making useful observations of the occultation - which even at the central line would not exceed 4.5 seconds - still required some thought. Experienced observers can make analyzable timings by just watching a star visually through a telescope, but nowadays video is the way to go: The recordings can be analyzed frame by frame, and unequivocal timings good to 40 msec are rather easy.
Bredner was to use a Watec with a telephoto lens, and I had brought a Mintron with another lens; Bredner would record the video signal on a mobile VHS recorder, while I would feed it into a digital camcorder. No equipment immediately suitable for occultation work was ready at the OAM though: While this observatory has a fine reputation in minor body studies and is e.g. operating an amazing network of robotic asteroid-hunting telescopes (over the night's course, I got to see one computer-filled room after another), techniques for high-speed photometry of bright objects had to be improvised. Antonio García Lastra, the astronomer responsible for the technical equipment at OAM, and some colleagues worked hard all night to get several systems running, but eventually Bredner and I - who had been able to catch a few hours of sleep in a cosy room underneath one of the ten(!) domes of the OAM - could no longer support our hosts: We had to get to our own sites. Bredner took the rental car - and Salvador took care of me, volunteering as my chauffeur!
We left around 5 a.m. local time, 1 1/4 hours before the event: Setting up my equipment would take only minutes. Finding a good observing site was not so easy, however: While this agricultural region of Mallorca is only sparsely populated, there are still isolated cells of extreme light pollution, e.g. brightly lit road intersections or mysterious (to me, that is) industrial facilities in the middle of nowhere. At the North coast - closest to the predicted central line - several communities felt they had to heaviliy illuminate their empty promenades at 6 a.m., especially Alcúdia, but eventually we stopped at a deserted parking lot, near Ca'n Picafort. The target star was easy to find indeed, but getting a precise time signal onto the - now continuously running - video tape was more tricky. Bredner had provided me with a clever electronic box that received the longwave radio time signal DCF 77 from a transmitter in Germany (which is linked directly to the atomic clocks that represent Germany's contribution to the Universal Time Coordinated): It displayed both the time on an LCD display and had a red LED flashing with every new second.
The latter is most important, I was advised, and rightly so: LCD displays slow down when the temperature drops, LEDs do not - and even at our Mallorcan site, with the temperature still a few °C above freezing, the LCD was delayed to the LED by 0.22 seconds! This was evident from watching a video of both displays frame by frame, but getting a sharp view of them with the video camera already aligned with the target star had been less than easy. Especially since you had to point a ferrite antenna in the box (about which I didn't know at that time) in a certain direction or lose clear DCF 77 reception - but Salvador kindly helped me again, by holding the box in front of the camera while I illuminated it with a flashlight. I waited for a number of LED flashes, then focussed again on HIP 57629 and set the internal integration of the Mintron camera at x4. This means that it internally adds up every four frames (of 20 msec each), improving the limiting magnitude and the signal-to-noise ratio. With somewhat longer focal-length lens than the 45 mm f/2 I had, this would not have been necessary, but I just wanted to play it safe.
Without the internal integration the limiting magnitude would have been somewhere between 6th and 7th magnitude I had found out during crude tests on the Plejades back in Germany, but it depended strongly on the sky transparency (which the dark-adapted eye cannot judge too well). And the digital camcorder's LCD display into which the Mintron signal was fed was not dynamic enough to judge whether the target star was detected well enough with the integration off. At x4 the signal was good, even on the crappy display, and the time resolution was still 80 msec, decidedly better than what I could have achieved visually and with an audio tape recorder. Of course, I also wanted to see the occultation with my own eyes, and so I had brought 8 x 20 binoculars (incidentally all my equipment for this expedition, with the exception of a photo tripod, fit into a single small bag). With those I could see the target star well while leaning onto Salvador's car; the limiting magnitude was not nearly good enough to watch it naked-eye, since the light pollution of Ca'n Picafort was too strong.
From about 6:14 a.m. on I stared though the binoculars: The occultation was to take place around 6:16:30. Soon I wondered how many minutes had passed, but I didn't dare to take my view off the star. (Later I learned that experienced visual observers use a kind of metronome and count the seconds aloud.) And all of a sudden it happened: The star was gone! The feeling was indescribable, some mix of shock, sheer excitement and immense satisfaction: Seeing a star disappear behind a tiny asteroid (at 13.8 mag. way below detectability for the setup I used) was perhaps the last great challenge of observational amateur astronomy that had eluded me in 25+ years of activity, and now it was accomplished. But in an instant, it seemed, the star popped back into view: At that point I estimated that it was gone for at most 2 seconds. But analyzing the video I soon found out that it had actually been occulted for 3.44 seconds: It is a well-known fact that total solar eclipses feel to be far shorter - especially to novices - than they actually are, but I would not have expected that the same illusion would apply to this kind of minor eclipse ...
Back at the OAM - deserted by now - after 7 a.m. we tried to catch another few hours of sleep, but by 10 a.m. we were up again: In another ten hours we would fly home again and wanted to make the most of it. By now it was a bright (if cool) sunny day, and again the observatory staff surprised us with free breakfast in the great dining hall: Apart from the serious scientific work, the OAM is also doing much outreach work and it even running a new planetarium. There were also some gifts waiting for us, and the invitation to return - which I am actually considering on the occasion of the opposition of Mars in late 2005. First we went to the place from which Salvador and I had observed, to measure its precise coordinates with Bredner's GPS receiver, then we carried on to Artà: I had asked the OAMers for the most important megalithic structure in the area, and they had recommended the Talaiot de Ses Paisses. A truly impressive site, built 3000+ years ago - surely there are archaeoastronomers somewhere who see important orientations here, though there is no mention of that on location (but I know of such claims for neighboring Menorca).
Next, our fast and furious expedition went below surface, into the Coves del Drach, a cave system near the coastal town of Portochristo with Europe's largest underground lake - where they even give live concerts to visitors, from tiny light-carrying boats, and where you can travel a few meters on the lake in said boats. Back in Palma, there was still time for wandering around in the old town, dominated by the huge cathedral La Seu, and for a quick dash to the most infamous coastal strip at S'Arenal - where practically all the celebrated pubs had just recently closed for the year, and the main attraction was the lunar crescent hovering over the deserted beaches of Balneario a.k.a. Ballermann 6. So there you have it: a successful astronomical expedition with useful data, history, geology, culture as well as the negation of it at S'Arenal, all in just one day - per hours spent at the destination as well as per Euro, this was probably the most rewarding astronomy-inspired trip I have ever taken. Although I must admit that it went by so quickly that on the train home from Düsseldorf Airport it already felt like coming from a weird movie ...
Daniel Fischer - written on Nov. 17, 2004 (addenda updated until Nov. 23, 2004)
Addenda: Reports have come in of several successful observations on the Spanish mainland; all observations are listed here, and the shape of the asteroid is building up from the chords. Here are also Bredner's report from Mallorca - and a report by Durda on another stellar occultation by Polyxo observed in 2000 from an airplane; the shape of the asteroid could be approximated roughly then from this and groundbased observations.
For future events check these lists of occultations of bright stars by
large as well as
small asteroids as well as
the lists by Preston and
Manek and especially his
updates!