by Knut Jørgen Røed Ødegaard
How do eclipses occur and why are they so rare? Here's the
answer.
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The Moon circles around the Earth using approximately 4 weeks. So
approximately every 4 weeks the Moon will be 'new' and
be positioned between the Sun and the Earth. The orbit
of the Moon has an angle of about 5 degrees compared
to the line between the Sun and the Earth, so usually
the Moon passes either under or over the solar
disk. If it passes in front of the solar disk which
can be partial, annular or total.
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An eclipse can be partial, annular or total.
- If the Moon only obscures a part of the Sun the eclipse is
partial. These eclipses are the most common and occur over large
areas.
- If the Moon passes directly in front of the solar disk and also
is relatively close to the Earth in its elliptical orbit the eclipse
is total. The Moon covers the whole solar disk. It will become quite
dark, even in the middle of the day. The brightest stars and planets
will be visible, street lights will often turn on. But most
importantly: the amazing corona - the outer ultrahot atmosphere of
the Sun - is visible. This is an incredibly impressing sight which
at all times has frightened and fascinated people. Solar eclipses
are only total during a few minutes and only over narrow bands on
the Earth. Outside the zone of totality the eclipse is partial. On a
given place on the Earth more than 100 years may pass between each
total solar eclipse.
- If the Moon passes directly in front of the solar disk, but is
far from the Earth, the eclipse is annular. A thin circle of the Sun
will shine around the Moon. Annular eclipses are rare.
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On Island the eclipse will be annular. This is how the
eclipse may be in Reykjavik if the weather is
good. On Island the Sun will be vary low on the
horizon under the eclipse.
Illustration: Instittute of Theoretical Astrophysics,
University of Oslo
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THE SOLAR CYCLE
The periodicity and repetition of eclipses are controlled by the Saros
cycle. This has a length of about 6'585,3 days (18 years 11 days 8
hours). It has been known since ancient times. The nodes are the
points where the Moon crosses the Earth's orbit.
The cycle is related to three periodicities of the lunar orbit:
- The synodic month: The time from new moon to new moon. 29,53059 days =
29 days 12 hours 44 min.
- The draconic month: The time from node to node. 27,21222 days = 27
days 5 hours 6 min.
- The anomalistic month: The time from perigee (the closest point to the
Earth) to perigee. 27,5455 days = 27 days 13 hours 19 min.
A Saros equals 223 synodic months. This equals 242 draconic months ans
239 anomalistic months (accurate to a couple of hours).
Two eclipses separated with a Saros cycle have very similar
geometrical qualities. They occur in the same lunar node at close to
the same distance from the Earth and at the same time of the
year. Because the Saros cycle doesn't correspond to an integer number
of days, the following eclipses are visible from different
parts of the planet. The extra third part of a day's shift
means the Earth has to rotate an extra 8 hours or 120 longitudinal
degrees for each cycle. For solar eclipses this means that the
eclipses move 120 degrees west. Therefore a Saros cycle will return to
approximately the same geographical position after 3 Saros cycles (54
years and 34 days).
A Saros cycle doesn't last forever because the periods of the Moon
doesn't exactly correspond. Most importantly the Moon's node moves
approximately 0,5 degrees east each cycle. A typical Saros cycle for a
solar eclipse starts when the new moon is about 18 degrees east of a
node. If the first eclipse occurs at the Moon's descending node, the
entire lunar shadow passes 3500 km underneath the Earth and a
partial eclipse is visible from the South Pole area. Next time the
entire lunar shadow passes 300 km closer to the Earth and a
somewhat larger partial eclipse will occur. After 10 or 11 Saros
cycles (about 200 years), the first central eclipse will occur near
the South Pole. During the next 950 years a central eclipse will occur
in each Saros cycle, but during each cycle it will move on average 300
km north. Half way through the period long-lasting eclipses will occur
near the equator. The last central eclipse will occur near the North
Pole. The last 10 eclipses will be successively smaller, partial
eclipses.
Eventually the Saros series will end approximately 13 centuries after
it began on the opposite pole. A typical series can consist of 70-80
eclipses, of them about 50 central (total or annular). If a Saros
series begins near the ascending node, the eclipses will move in the
opposite direction - from north to south.
Since there are 2 to 5 solar eclipses each year, there are about 40
different Saros cycles running at the same time. In the last half of
the 20th century there were for example 41 single series of which 26
produced eclipses. As old series end, new will begin.
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The partial phase of the total solar eclipse in Finland in 1990 seen
through a thinlayer of clouds.
Foto: Oddleiv Skilbrei
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