It鈥檚 2 pm on 26 February 1998. On the Dutch Antillean island of Aruba, 31
kilometres off the coast of Venezuela, night is falling in the middle of the
day. It is eerily quiet except for the frenzied chattering of terrified birds.
In this Hitchcockian world, 100 000 people are holding their breath as overhead
an intricate cosmic dance nears its dazzling climax.
A total solar eclipse, when the Moon passes directly in front of the Sun, is
an awe-inspiring sight. Wherever one happens, it attracts legions of sightseers
from across the world, many of whom become hopelessly addicted and dedicate the
rest of their lives to chasing the 鈥渉igh鈥� of an eclipse. An eclipse also
attracts legions of scientists.
Jay Pasachoff, chair of the International Astronomical Union鈥檚 working group
on eclipses, is on Aruba for his 26th eclipse. His 20-strong team of researchers
and undergraduates from Williams College in Williamstown, Massachusetts, has set
up banks of telescopes, cameras and computers on the flat roof of 鈥淭he Mill鈥�
condominiums. The equipment and personnel, screened from the hot northeasterly
trade wind by makeshift wooden windbreaks, are being filmed by TV crews on an
island in the grip of eclipse fever.
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Pasachoff鈥檚 team picked Aruba because the cactus-strewn desert island is
famed for its clear skies and boasts a mere 43 centimetres of rain per year. But
though it has been clear in the afternoon every day for the past week, the sky
is almost completely covered by cloud with only minutes to go before totality.
This is nail-biting time and several of the scientists are mouthing silent
prayers
Pasachoff鈥檚 team has set up three major experiments and two minor ones. Most
are designed to study the 鈥渃orona鈥�, the tenuous outer atmosphere of the Sun. The
corona shines only as brightly as the full Moon鈥攁 million times fainter
than the Sun鈥攁nd is usually overwhelmed by the glare of the solar disc, or
鈥減hotosphere鈥�. It is visible to the naked eye only during the few minutes of a
total eclipse.
The big puzzle of the corona is why it is so hot. 鈥淚f you move away from a
stove, it gets cooler, but bizarrely, if you move away from the Sun it gets
hotter,鈥� says Pasachoff. 鈥淭he surface of the Sun is at 6000 掳C whereas the
corona is at 2 million 掳C. Why?鈥�
Astronomers have some general ideas but need to fill in the details. For
instance, they know that the Sun dumps energy into the corona via some kind of
ripple or wave in the solar magnetic field. Evidence for this comes from the
behaviour of the corona, which emits its strongest X-rays鈥攁n indication of
extremely hot gas鈥� immediately above 鈥渟unspots鈥�, regions of the
photosphere characterised by extra-strong magnetic fields. 鈥淵ou don鈥檛 have to
transfer a lot of heat,鈥� says Pasachoff. 鈥淭he corona may have an extremely high
temperature but it is so rarefied it would be considered a good vacuum on
贰补谤迟丑.鈥�
One idea is that heat gets into the corona via loops of coronal gas confined
by tight bundles of magnetic field. Each 鈥渃oronal loop鈥� is like an upturned 鈥淯鈥�,
with the tips of the 鈥淯鈥� anchored in the photosphere. 鈥淚t鈥檚 possible that
convective motion in the Sun is wiggling the base of the loops,鈥� says Pasachoff.
鈥淓nergy is then transferred via the wiggles to the corona.鈥� If this is correct,
coronal loops should be wiggling with a period of about a second. One of the
Williams College experiments is designed to take a series of images鈥�10 per
second鈥攖hrough a special filter that only admits light from the coronal
gas. 鈥淲e hope to discover whether coronal loops are really wiggling,鈥� says
Pasachoff.
A second Williams College experiment will map the temperature of the corona
and a third, its brightness. The plan is to compare this with a brightness map
obtained at the same time by the Solar Heliospheric Observatory, a European
Space Agency and NASA satellite hanging in space 1.5 million kilometres sunward
of the Earth.
SOHO observes the corona by making an artificial eclipse with a small opaque
disc onboard the satellite. But unwanted light from the photosphere and the
lowest, brightest part of the corona bends around the disc and gets into the
instrument. 鈥淣ature鈥檚 eclipse is far superior to anything we can create,鈥� says
Pasachoff. By comparing the image of the corona as seen from the ground with the
SOHO image, scientists will be able to quantify the stray-light problem and
compensate for it. 鈥淭he result will be a significant improvement in a
$300-million satellite,鈥� says Pasachoff.
The SOHO 鈥渙ccultation disc鈥� is also too big. This was necessary to minimise
stray light but it means the disc blots out both the inner corona and the
chromosphere, the reddish atmospheric layer immediately above the photosphere.
During a natural eclipse, however, the whole of the Sun鈥檚 atmosphere is
visible.
A total eclipse has other advantages too. Although the Williams College
expedition had to airlift 2 tonnes of equipment to Aruba and is occupying half a
dozen $400-a-night suites at The Mill, the cost is still less than a
tenth of a per cent of SOHO. And eclipse expeditions can test this year鈥檚
theories and use this year鈥檚 technology, rather than technology kept frozen for
years until a satellite鈥檚 launch. According to Pasachoff, there is no instrument
in space capable of seeing the rapid variations of coronal loops.
Pasachoff鈥檚 team is being guided by a SOHO image, downloaded from the
satellite last night, which details the regions of the corona that are most
active and so merit the closest scrutiny during the total eclipse. The only
problem is interpretation. 鈥淚 only hope we鈥檝e got the image the right way up!鈥�
says Pasachoff.
It seems the astronomers鈥� prayers have been answered. By some miracle, the
clouds have parted to reveal the dying Sun. In the eerie twilight, planets are
gradually coming into view. Close in, Mercury and Jupiter are burning red and
white. Farther away from the Sun are Venus and Mars.
The shadow of the Moon has already passed over the Gal谩pagos islands
and central America. It is now racing across the sea to Aruba鈥攁 wall of
darkness moving at 2400 kilometres per hour. Someone is counting the seconds to
totality. 鈥淭en, nine, eight . . . 鈥� The Sun, which has been sliding
remorselessly behind the Moon for an hour and a half, is now shining at only one
place on the lunar circumference. This is the most spectacular phase of the
total eclipse鈥攖he aptly named 鈥渄iamond ring鈥�.
All around people are whooping and cheering and clapping. Later, Aruban TV
will report that an American man chose this very moment to get down on his knees
and give his girlfriend a diamond engagement ring. (She said yes.)
But after a few seconds the diamond ring is suddenly gone. And so too is the
solar disc. It鈥檚 as if someone has covered the Sun in a factor-million sunblock.
Where once there was a blazing orb, now there is a monstrous black hole hanging
in the sky. Totality has begun.
Cars are tooting everywhere. Bats are swooping low overhead. Fireworks are
bursting all around the sky, their rattle and boom adding to the overall
cacophony. All across the island streetlights have come on automatically. In the
south of Aruba the refinery at Seroe Colorado is suddenly ablaze with fierce
white light.
Nobody can tear their eyes from the gaping black hole where the Sun used to
be. Since the Moon is invisible before, during and after an eclipse, ancient
people thought a monster had eaten the Sun. They used to gather together in town
squares, banging and shouting and screaming, to frighten away the monster. You
can see their point.
People have discarded the solar filters they have been using to view the
partial phases of the eclipse. As their eyes adjust, there are 鈥渙ohs鈥� and 鈥渁ahs鈥�
and ecstatic cheers. The black disc of the Moon is breathing white fire. A
reporter for Aruban TV puts it perfectly. 鈥淚t鈥檚 like someone shot a hole in the
Sun and left only the rays,鈥� she says.
The white fire surrounding the Moon is the corona. Tongues of flame extend to
the east and to the west. And on the Sun鈥檚 upper limb is a 鈥減rominence鈥濃�攁
spectacular loop of fire.
Behind the windbreaks, scientists are working frantically. Few members of
Pasachoff鈥檚 team get the chance to do more than glance at the corona as they
operate their equipment. All are terrified of fumbling or ruining an exposure,
desperate not to waste this fleeting opportunity.
But totality is already coming to an end. After only three minutes, a
brilliant light appears on the dark limb of the Moon. There is the second
diamond ring. And then the eclipse is over. The shadow, travelling faster than
Concorde, has gone racing onward to Montserrat, Antigua and Guadeloupe and its
final nemesis 1000 kilometres off the coast of Morocco.
People are standing about, spent. 鈥淎 total eclipse is like sex,鈥� someone
says. 鈥淎fterwards, you just go, `Ahhh鈥�!鈥�
Many people on the island will be relieved it鈥檚 all over. There were rumours
predicting a giant tidal wave at totality and 鈥渄angerous rays鈥�. There was even
speculation among the locals that the rays might 鈥渃ure鈥� 840 passengers on a gay
cruise anchored in the port of Oranjestad.
The Williams College team is besieged by camera crews seeking an instant
reaction. Students are groping for words to add to the spectacle everyone
witnessed. 鈥淚t was one of the best and clearest eclipses I have ever seen,鈥� says
Pasachoff.
It emerges, however, that there has been a disaster. As totality started
someone on the roof inadvertently tripped over a power cable, pulling it free.
The electricity supply was cut to the motor driving one of the telescopes
tracking the Sun. The experiment to map the temperature of the corona obtained
no data.
One of the William College students is devastated. 鈥淭hat was my undergraduate
thesis,鈥� he says. Pasachoff is putting on a brave face. 鈥淲e lost only one of our
five experiments,鈥� he says. 鈥淚 can live with an 80 per cent success rate.鈥�
People are beginning to disperse. Equipment is already being dismantled and
carried off the roof. The eclipse party is over鈥攁t least for this year.
Next year, on 11 August, the party moves to Europe, including Britain, for what
promises to be the most watched total eclipse in history
(see map, p 27).
The lesson from the eclipse in Aruba is clear. It鈥檚 totality that takes your
breath away. People who are slightly off the track and expect a 95 per cent or
even 99 per cent eclipse to be a spectacle will be bitterly disappointed.
Granted, the light in Aruba grew gradually dimmer as the eclipse progressed, but
the effect was hardly different from a cloud passing in front of the Sun. 鈥淭he
partial phases are almost nothing,鈥� says Pasachoff. 鈥淚t鈥檚 like going to stand in
the square outside the opera. But you didn鈥檛 hear the fat lady sing. You weren鈥檛
inside. And inside for an eclipse is the total eclipse.鈥�
Pasachoff urges anyone off to the side of the path of totality to do
everything they can to get there. 鈥淚t鈥檚 only about 100 miles wide but you must
be there because the things you see are so fantastic,鈥� he says. Then he breaks
open a can of Mexican beer. It鈥檚 labelled 鈥淐orona鈥�. On a day like this, what
else would it be called?
During a total eclipse, the Moon moves in front of the Sun, casting a shadow
on the Earth. The shadow, moving at about 2400 kilometres per hour, sweeps out a
270-kilometre wide track across the Earth鈥檚 surface.
The duration of a total eclipse depends on where the Earth is in its orbit
around the Sun, and where the Moon is in its orbit around the Earth. Usually,
places along the track of a total eclipse experience less than 5 minutes of
totality. The maximum possible duration, when everything is lined up favourably,
is 7 minutes and 31 seconds, though this is rare. Totality during the solar
eclipse of 25 June 2150 will last 7 minutes 14 seconds, longer than any total
eclipse since the 9th century.
Although there is a total eclipse somewhere on the planet every year and a
half, the chance of seeing one at a given spot in a given year is very small.
The gap between eclipses at the same place is about 350 years.
We are extremely lucky to be treated to total eclipses. They happen because
of a celestial coincidence: although the Sun鈥檚 diameter is 400 times greater
than the Moon鈥檚, it is also 400 times farther away, so it appears the same size
as the Moon in the sky. This has not always been true. Because of tidal
interactions, the Moon is gradually receding from the Earth. So billions of
years ago, the Moon would have seemed larger in the sky, and would have blotted
out more than just the solar disc. In the distant future, the Moon will seem
smaller, and the outer part of the Sun will always be visible during an
eclipse.
We are fortunate in our location in space as well as time. There are hundreds
of moons in the Solar System, and yet the requirement for a total
eclipse鈥攖hat one of the moons seems the same size as the Sun in the
sky鈥攊s very rare. There could be a few moons circling the outermost
planets that would fit the bill. But out there, the Sun looks like a tiny,
unremarkable star, and an eclipse would be very dull compared with the full
coronal splendour seen on Earth.