Occultation of
Saturn by the Moon on 16th April 2002
Introduction
An occultation is when one
celestial body passes in front of another. The most well known
of these are solar eclipses when the Moon passes in front of the
Sun, but occasionally the Moon passes in front of a planet. Like
eclipses they are quite rare though often come in batches when
they do occur. One such event took place on 16 Apr 2002 when the
Moon passed in front of Saturn.
The Event
From London the event was
due to start at 10:00pm BST and with the sky looking reassuringly
clear I set up the telescope on the patio with a view to the west...
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The view to the west from
Hurst Green in anticipation of the event; 16 Apr 2002, 20:48BST
(19:48UT).
Saturn lies about half a degree
above and to the left of the Moon, a distance that the Moon's
own orbital motion will cover in just over an hour. Earthshine,
Earthlight shining on the Moon's dark side, is clearly visible.
Venus shines brightly low
in the fork of the nearest tree, while Mars can be seen just
to the left of a branch of the same tree, about 3/5 of the way
between Venus and the Moon.
The bright star below and
to the left of the Moon is Aldebaran, the brightest star in Taurus.
Minolta Dimage 7, 13.4mm,
1s @ f/3.5, ISO400
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Some further pictures taken
with my camera near or at maximum zoom show the gradual progress
of the Moon towards Saturn. Each picture is aligned relative to
Saturn so the Moon's movement along its orbit is apparent...
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| 20:51BST
(19:51UT): T - 1h 9m |
21:13BST
(20:13UT): T - 47m |
21:30BST
(20:30UT): T - 30m |
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The crescent Moon is substantially
over-exposed so that the Earthshine can reveal the outline of
the dark side. At this magnification Saturn should be only 1
pixel across, but for some reason the distinctive oval shape
that the rings give the planet can be clearly seen, especially
in the 3rd picture. An exposure of 1s is too short to reveal
the Earth's rotation, and the star Aldebaran in another part
of the 1st picture (not visible here) shows no sign of trailing.
Possibly an artifact in the imaging process, but definitely a
mystery!
Minolta Dimage 7, 40.9mm (20:51)
/ 49.1mm (22:13 & 22:30), 2/3s (20:51) / 1s (22:13 &
22:30) @ f/3.5, ISO400
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After this, no more pictures,
while I prepared the telescope and checked that it was tracking
correctly with the added weight of the camera attached.
As the Moon and Saturn sank
lower in the west I became increasingly concerned that, despite
my best efforts to position the telescope in the best available
place, the trees beside the railway (visible in the top picture)
would interfere. I contemplated moving the whole setup next door,
but with time running out we decided to chance it and stay put.
At about 9.50 (10 minutes
to go) the Moon did indeed sink into the trees, but the lack of
leaves and size of the telescope mirror meant that the branches
did not seem to interfere too much. Just before 10.00 I began
taking pictures of Saturn and stepped up the pace as soon as the
first bite began to appear on the left hand side of Saturn's rings.
I have now cropped and compiled the frames into a sequence that
shows the event running at 8 times natural speed...
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Saturn disappears behind the
dark side of the Moon; 21 frames between 21:59:10 & 22:01:55
BST (20:59:10 - 21:01:55UT). 0.74s per frame.
Each frame is aligned on Saturn
so the orbital movement of the Moon carries it across from west
to east. The individual exposures are too short for the Earthshine
on the dark side of the Moon to show up. The pictures are rotated
from their original inverted view in the telescope to give a
more natural view in keeping with the pictures above.
Minolta Dimage
7, 49.1mm, afocal projection using 25mm eyepiece on 8.75inch
f/7.3 Newtonian telescope. 2/3s @ f/14, ISO800. Scale of 0.7
arc seconds per pixel.
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Because of Saturn's own
size the whole disappearance took about 1m 47s between 1st and
2nd contact (estimated at 21:59:57 and 22:01:44 BST respectively).
It spans 18 frames in the above sequence, taking 13s to display.
The Moon's orbital motion carries it west to east at the rate
of 0.5 arc seconds per second (= 1/2 degree per hour) so Saturn
should have been covered in about 1m 20s seconds. However, because
the occultation as seen from London was substantially off centre,
the Moon's southern limb approaches obliquely, so takes longer
to creep across Saturn.
I was delighted to obtain
these pictures, but even more remarkable was the reappearance
25 minutes later. I had fully expected this to be hidden by the
trees, but in the event the Moon had moved into a gap between
two trees. Although it was low by this time and atmosphere pretty
unsteady and murky, the event revealed itself in full.
My rough and ready alignment
of the telescope meant that its tracking had not followed the
(invisible) Saturn exactly so I wasn't quite sure where on the
Moon's limb to expect the reappearance. However, as soon as I
noticed a suspicious bump on the edge of the Moon I fired the
camera into life again!
I have similarly compiled
these into a sequence speeded up 8 times. Please be patient while
it downloads [813KB], because in order to show more of the moon
it is larger than the previous one...
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Saturn reappears; 18 frames
between 22:25:16 & 22:26:31 BST (21:55:16 - 21:26:31UT).
0.51s per frame.
The telescope drive had not
quite taken up the slack after my realigning it on the re-emerging
Saturn, so Saturn drifted towards the edge of the field of view.
As each frame is aligned on Saturn, the circular edge of the
field of view therefore appears to drift to the right. The Moon's
eastwards movement is of course real, at 3,200km per hour.
Minolta Dimage
7, 49.1mm, afocal projection using 25mm eyepiece on 8.75inch
f/7.3 Newtonian telescope. 1/3s @ f/14, ISO800. Scale of 0.7
arc second (= approx. 5,000km (on Saturn) / 1.2km (on Moon))
per pixel.
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I of course missed the the
initial reappearance (3rd contact) while hunting for it, but 14
frames cover the point up to where Saturn is completely uncovered
by the Moon (4th contact), estimated to be at 22:26:13 BST. This
is an interval of 57s, taking 7s to display.
Although the Moon is brighter
than Saturn, and I reduced the exposure to compensate, the difference
in brightness is a lot less than had the Moon been nearer full,
when it would have been impossible to have had both objects correctly
exposed. As it is, the result is quite pleasing for it also shows
the Moon's craters well highlighted by the long shadows cast by
the low angle of the sun just after lunar dawn.
I also intended that shortening
the exposure would help reduce the effects of atmospheric turbulence
as the Moon was now at only about 10 degrees altitude. However,
the fuzziness of the results indicate that this was not entirely
successful.
Nevertheless, a successful
sighting of an unusual event, well captured for all to enjoy!
External Links...
The final event in the series,
on 14 May 2002 at 06:44UT, will be impossible to see from the
UK as it takes place in daylight, near the eastern horizon and
just 21 degrees below the Sun.
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