Background:
One of the prime mission objectives of Voyager I was to get a good look at
Saturn's largest moon Titan. It is the 9th largest object orbiting the Sun,
outranking the planet Mercury in size. It also has a thick atmosphere composed
mainly of nitrogen which has a pressure at the surface 50% greater than the
Earth's and when Voyager I arrived there in November 1980 it found the surface
hidden by a thick orange haze. This has since been attributed to molecules of
hydrogen cyanide.
During Voyager I's flypast of the moon, a radio occultation experiment was
carried out whereby a signal was transmitted towards Earth, passing through
Titan's atmosphere en-route. Analysis of the signal revealed details about the
structure and depth of the atmosphere, calculated to be about 200km.
From the Earth, Titan presents a disk less than 1 arc-second
across and so appears starlike in most ground based telescopes. This would
suggest that any detailed study of the moon is beyond the reach of amateurs.
However, just as NASA scientists carried out the radio occultation experiment
with Voyager I, if a bright star was to pass behind Titan, then its light would
pass through the moon's atmosphere on its way to Earth, creating a natural
occultation experiment. This idea is nothing new, both the rings of
Uranus and Neptune were discovered by observing the light of a star flickering
before and after passing behind the planets. Astronomers also use the
occultation of stars by asteroids to determine the size and shape of
the minor planet. A total eclipse of the
Sun is in fact an occultation of the Sun by
the Moon.
It had been known for a few years in advance that the star 28 Sagittarii
(28Sgr) was to pass behind both the rings of Saturn and the planet itself on
July 3rd 1989. This event however would not be visible to observers in Europe.
Then 3 months beforehand it was discovered that the star would also pass behind
Titan. I learnt of this 3 days before the event and also discovered that this
could be seen from parts of Africa and Asia and from Europe, including the UK
(weather permitting!).
The central track of the shadow was expected to pass from the Caspian Sea to a
point just south of the Canary Islands. Observers in Britain could expect to
see some part of Titan's northern hemisphere occult 28Sgr.
Observations:
The occultation was monitored using a method
outlined in Astronomy magazine for the ring occultation
visble in North America. This involved recording the observed changes in
brightness of the star and moon by announcing them on a tape recorder whilst
also recording the time.
The event was observed through a 220mm, f/5 Newtonian
reflector at 180x magnification, which showed the scene below:
Saturn and its brighter moons, 1989 July 3.
When the time came to begin, the sky was completely clear with average
seeing. Initially the star and Titan appeared as two separate objects
but eventually merged into one point of light of about
magnitude 5.7. After a wait which saw a few false starts due to
flickering caused by the seeing conditions,
a definite decrease began in the star and moon's brightness. The changes in
brightness were watched and recorded for 38 seconds when the changes stopped.
The pair had levelled out at about magnitude
8.2. Just over two minutes later they began to brighten again, or so I thought.
They revereted to their magnitude 8.2
where they remained for almost another two minutes. The brighteneing then began
again. This continued for another 56 seconds after which the brightness changes
appeared to have stopped, it was difficult to tell. Later the two objects would
appear separately again and the event was completely over.
The light curve below shows the brightness changes as recorded:
The occultation light-curve.
Analysis
The event began at 22:39:17UT at this site.
The decrease in brightness, the immersion phase, took 38 seconds. 4 minutes and
4 seconds later the emersion phase began as the brightness increased again for
56 seconds.
A small increase in brightness occurred at 22:42:02UT which lasted for about 5
seconds and almost went unnoticed. This time was at around mid-event.
A rough calculation, based on these results gave me an estimated size for that
part of Titan which had occulted the star. It turned out to be just under
2500km, which was surprising since Titan has been measured to have a diameter
of 2560km. This suggested that I was close to the centre of the shadow's track.
This was well north of that expected. This was further confirmed by the brief
brightening at mid-occultation. This "central flash" as it became known, was
only visible to those close to the central track and was due to the moon's
atmosphere "focusing" the starlight around the moon by refraction.
Since I was close to the centre, then the 500km I calculated for the amount of
atmosphere encountered should be close to the figure for the depth of the
atmosphere itself up to the point where it affected the light from 28Sgr.
The outcome of these results and others were presented in the April 1994
British Astronomical Association Journal.
One useful result of this is that Titan's position is now better known which
means that the forthcoming Cassini mission to Saturn and Titan can be targeted
more accurately.
The observation of this event proved to be very worthwhile.
Kevin Deakes - The Andromeda Observatory 1997.