Orwell Astronomical Society (Ipswich)
Solar Annular Eclipse, 22 August 1998
In August 1998, I travelled to Sumatra, Indonesia with a small group of geologists and astronomers to observe an annular solar eclipse. During my time there I was also lucky enough to visit Bosscha observatory near Bandung in West Java.
The eclipse started on 22 August 1998 at about 6.10am, just before sunrise. This meant that I had to make a very early start from my hotel at Prapat on the north eastern shore of Lake Toba. I left at 2.30am and joined nine other observers and a local tour guide in a minibus for the three hour drive to the observing site. It was cloudy when we left but it soon started to drizzle and, in fact, for some two-thirds of the trip, the sky was completely overcast. Fortunately as we approached the observing site, conditions began to improve and some stars became visible and, by the time of our arrival, the sky was almost completely clear.
Our guide had chosen the observing site in advance but, unfortunately, obviously hadn't dealt with eclipse chasers before, as it was simply too small and had a poor view of the sky. We therefore continued along the road searching for a better site. By chance we came to a narrow, winding road leading up a hill to a microwave transmitter station. Ideal we thought, there must be a good view of the sky from the vicinity of the transmitter. As we ascended, the sky began to brighten as dawn approached. After a hair-raising drive we arrived at the transmitter station only to find the easterly view blocked by trees. Following a brief discussion, we headed back downhill as rapidly as possible to continue searching for a site. By now the Sun had risen and the partial eclipse started, although we couldn't see it behind the tree-lined hills. We soon came across an excellent observing site at the side of the road, about five kilometres south-west of Tarutung, at approximately 2° 00' N, 98° 54' E. We hurriedly set up our equipment just as the partially eclipsed Sun rose over trees on a distant ridge.
As we were only two degrees north of the equator the Sun rose almost vertically so it soon cleared the ridge. During the eclipse, the motion of the Moon carried it straight down across the solar disc. It soon became clear that the Moon appeared smaller than the Sun and, half way through the partial phase, the crescent Sun started to extend more than half way round the Moon. (This was quite different to the total eclipse we saw in February 1998 where the solar crescent shrank as totality approached.) For several minutes before second contact we saw Bailey's beads at the cusps of the crescent Sun. Unlike a total eclipse, however, they were not dazzling points of light seen against the corona but were just detached parts of the photosphere visible through the dense filters we were using. Second contact arrived and the Moon became completely enveloped by the Sun at around 7.15am. We were slightly north of the centre line so the Moon at mid-eclipse was not quite central on the disc of the Sun. Although this eclipse covered just over 97% of the Sun, the sky and landscape were still brightly lit by the remaining 3%, again strikingly different from a total eclipse.
I don't think anyone in the party timed the annular phase but it was predicted to last for just over two minutes. But time appeared to rush past as the Moon slid across the Sun's disc. By third contact, the Sun was above 15° altitude in a clear patch of sky. Our observations had been successful, and only slightly troubled by some high cloud. Conversely, the hill-top where the microwave station was sited was completely covered in cloud at this time so it was lucky we hadn't stopped there!
There were only ten people in our group but there was a broad range of eclipse experience. Mike and Wendy Maunder were the most experienced observers and Mike observed and imaged the eclipse with a specially made refractor on a driven equatorial mount. He also used a wide field camera to take a sequence of shots of the partial and annular phases on a single frame. Several observers used telephoto lenses between 400 and 1000 mm focal length, binoculars for visual observing and eclipse glasses made from Mylar or fully darkened black and white negative film. Of course all the cameras and binoculars were used with either Mylar or metal on glass high density filters for safe viewing throughout the eclipse.
Eclipse reports are co-ordinated by Jay Pasachoff of Williams College in the USA and, since we returned from Indonesia, a number of reports have appeared on his internet pages of other groups who observed the eclipse. It would appear that only a few groups travelled to see this eclipse. Our group was furthest west and hence the first to experience 2nd contact and annularity. The other groups were in Malaysia, north of Singapore. Jay Pasachoff and his group were in Mersing, Malaysia and had relatively good weather conditions. A Japanese group was on Pulau Dayang, a small island near Mersing. An IOTA expedition was near Kota Tinggi on the southern edge of the eclipse path; the group was interested in measuring the position of the eclipse path to test for changes in the Sun's diameter over time but, unfortunately, suffered cloudy skies.
In 1994, out of the blue, I received a letter from the director of Bosscha Observatory in Java. It was a request for a copy of an article that I had written on telescope-making, which wasn't available through their library. I sent the article and thought no more of it. Four years later, having booked the trip to the Sumatra annular eclipse, I discovered that a visit to Bosscha Observatory was on the itinerary!
After a night's rest in a hotel in Bandung, West Java (where we experienced a minor earthquake!), we headed off to the observatory, situated on a hill in Lembang just a few miles north, at 6° 57' S 107° 34' E. The host for our visit, Dr Bambang Hidayat, was extremely hospitable and made us feel like visiting royalty. The director of the observatory, many of his staff and students welcomed us with tea and cakes on the lawn outside the director's house. Local and national press interviewed our tour leader, Dr Peter Cattermole, and an article appeared in the newspapers the following day. Dr Hidayat arranged for us to be shown around the observatory and its several telescopes.
The observatory was founded in 1923 using money donated by K A R Bosscha, a German who had made his fortune from tea plantations in Java. The date of foundation puts the observatory just slightly younger than Cape Observatory, South Africa, the oldest observatory in the southern hemisphere. The observatory buildings are surrounded by tropical plants and flowers as well as fearsome-looking yellow and black spiders, their webs draped between the trees and bushes. Not a place to walk around in the dark...! The largest instrument is a double refractor completed in 1928 with each telescope having a 60 cm Carl Zeiss lens of 11 m focal length. Mounted together in a single tube they are held in an English-style yoke mount. As the observatory is only 7°S of the equator, the mount appears to English eyes to be at a very odd angle, with the polar axis almost horizontal! The telescopes are used for double star work and high resolution photography of clusters. There are a number of buildings with run-off roofs housing smaller instruments. One of these is a 45 cm Cassegrain built in Japan and given to the observatory as part of an Indonesian-Japan project. It is used for photometry of close binaries and spectroscopy. The last of the three instruments we were shown was the 51 cm Schmidt camera which was inaugurated on 28 May 1960. The corrector and the 70 cm primary mirror, made at Yerkes Observatory in Chicago, combine to give an f/3.5 instrument that can cover a five degree field on 11 cm square glass plates. It is used for studies of the structure of the Milky way. One student to whom I spoke was using the camera to measure the rotation curve of our galaxy by taking images using a 6° objective prism giving 312 Å/mm dispersion. The spectra of the stars can be used to work out their velocities around the galactic centre.
The observatory is close to Bandung, with a population of 2½ million, which inevitably means that light pollution is becoming a problem. However, the astronomers carefully define their observing programs, enabling them to make good use of the average 200 clear nights per year. The observing programs include studies of double stars and star clusters.
See Fred Espenak's eclipse web site eclipse.gsfc.nasa.gov/eclipse.html.