by Knut Jørgen Røed Ødegaard and Unni Fuskeland
How does eclipses arise, and why do they occur so rarely?
You will find the answer here.
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The Moon orbits the Earth in about 4 weeks. With an interval of 4 weeks it
will therefore be in the "new" phase and be situated between the Sun and the
Earth. The orbit of the Moon is inclined 5 degrees with respect to the line
between the Sun and the Earth, so the Moon usually passes above or below
the solar disc. If it passes in front of the solar disc, an eclipse will
arise. It can be either partial, annular or total.
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An eclipse can be either partial, annular or total.
- If the Moon only shades a part of the Sun, the eclipse is partial.
These eclipses are the most usual and frequents over large areas.
- If the Moon passes right in front of the solar disc and in addition is
rather close to the Earth in its eccentric orbit, the eclipse is total.
The Moon covers the whole of the solar disc.
It becomes pretty dark, even if it happens in the middle of the day.
The brightest stars and planets appears and the light in the streets
often turns on.
But most important: The beautiful corona - the outer super-hot
atmosphere of the Sun - appears.
This is a most impressive sight, and have fascinated
humans for a long time. Eclipses are total only in a few minutes
and only in small bands over the Earth. Outside of the zone of totality
the eclipse is partial. On a given place on Earth it can be
over 100 years between total eclipses.
- If the Moon passes right in front of the solar disc, but is a long way
away from the Earth, the eclipse is annular. A thin circle of
the Sun shines around the Moon.
The eclipse on May 31. 2003 is annular on Iceland, the Faroe
Island, Greenland and parts of Scotland. However, in these areas
there are normally a lot of clouds,
and the Sun is very low in the sky during the eclipse.
Annular eclipses are very rare.
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On Iceland the eclipse is annular. This is how it can be seen
from Reykjavik if the weather is fine. On Iceland the Sun is
very low in the sky during the eclipse.
Illustration: Institute of Theoretical Astrophysics, UiO
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THE SAROS CYCLE
The periodicity and repetition of eclipses is described by the Saros cycle.
This has a length of about 6585,3 days (18 years 11 days and 8 hours), and has
been known since the ancient times. A node is the points
where the orbit of the Moon crosses the orbit of the Earth.
The cycle is due to the union between three of the Moon's periods of orbit:
- Synodic month: The time interval between two full Moons.
29,53059 days = 29 days 12 hours 44 minutes
- Draconic month: The time interval between two nodes.
27,21222 days = 27 days 5 hours 6 minutes
- Anomalistic month: The time interval from perigee to perigee.
27,55455 days = 27 days 13 hours 19 minutes
A Saros cycle is equal to 233 synodic months. This equals 242 draconic months
and 239 anomalistic months (with a precision of a few hours).
Two eclipses that are separated with a Saros cycle have geometrical quantities
which are very much alike.
They occur in the same node of the Moon at nearly the
same distance from the Earth and at the same time of year.
Because the cycle is not a whole number of days, succeeding eclipses
are visible from different places on the Earth. The displacement of
1/3 day means that the Earth rotates 8 hours or 120 degrees of longitude extra
for every cycle. For eclipses this means that they are moved 120 degrees west.
Therefore, a Saros era returns to about the same geographic position
after 3 cycles (54 years and 34 days).
A Saros cycle does not last forever because the periods of months
do not fit accurately to each other.
Most important it is that the node of the Moon moves about 0,5 degrees east
for every cycle. A typical Saros cycle of an eclipse begins when the
new Moon is about 18 degrees east of a node. If the first eclipse
occurs when the node of the Moon is waning, the penumbra
of the Moon
passes 3500 km beneath the Earth and a partial eclipse is visible from
the South Pole area. Next time, the penumbra passes 300 km closer to Earth
and a larger partial eclipse arises. After 10 or 11 cycles (about 200 years),
the first central eclipse will occur near the South Pole. During
the next 950 years, a central eclipse will arise in every Saros cycle,
but it will move in average 300 km north every time. Halfway through the
period, long-term eclipses will occur near Equator. The last central
eclipse will occur near the North Pole. The next 10 eclipses will be
successive smaller, partial eclipses.
Finally, the Saros cycle will end about 13 centuries after it started, on
the opposite Pole. A typical cycle may consist of about 70-80 eclipses,
hence around 50 central ones (total or annular). If a Saros era begins
near a raising node, the eclipses will move in the opposite direction
- from north towards south.
Since there are from two to five eclipses every year, it is about 40
different Saros cycles at hand simultaneously. In the last half of the 20.
century, there were for instance 41 series and 26 of them was producing
eclipses. When old series end, new ones begin.
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The partial phase of the total eclipse in Finland in 1990 seen through
a thin cover of clouds.
Photo: Oddleiv Skilbrei
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CONTACT PERSON:
Knut Jørgen Røed Ødegaard
Project leader
Institute of Theoretical Astrophysics
P.O.Box 1029 Blindern
N-0315 Oslo
Norway
Phone: +47 22 85 75 22 (Institute of Theoretical Astrophysics)
+47 992 77 172 (mobile phone)
+47 613 11 359 (private, weekend)
Email: knutjo@astro.uio.no
