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November 2002

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Solar Eclipse - December 4 2002

Southern Africa is getting set for another spectacular event in the astronomical calendar, the second total solar eclipse in under 18 months. If you are fortunate enough to be in the narrow strip, the path of totality running across Southern Africa, you will view a total solar eclipse for up to 80 seconds. Regions outside of this will only view a partial eclipse. 

The detailed map of the path of totality passes through Angola, Botswana, Zimbabwe, South Africa (the extreme northern parts of South Africa: Messina and the Kruger National Park) and Mocambique. 

The Johannesburg planetarium gives detailed information on the times of the eclipse over various towns and regions in southern Africa.

But what exactly is a solar eclipse?

Case Rijsdijk from the South African Astronomical Observatory explains.

 

Understanding Eclipses

by Case Rijsdijk
South African Astronomical Observatory

The darkening of the Sun during the day was for ancient people an omen, usually evil. In China people would beat drums and set off firecrackers, hoping to chase away the "evil dragon that has swallowed the Sun". Great riches and power were bestowed on those who were able to predict eclipses of the Sun and Moon. Failing to predict them accurately had dire consequences as two Chinese astronomers found out: they had their heads cut off!

The Sun (diameter 1 400 000 km) is very much larger than the Earth (diameter 13 000 km). The Sun is also far away from the Earth, about 150 million km. So in its orbit around the Sun the Earth casts a cone shaped shadow pointing away from the Sun. Fig. 1 below shows the Earth in four different positions in its orbit around the Sun with the shadow pointing away from the Sun.


The Earth orbits the Sun in a plane called the ecliptic, and the Moon's orbit around the Earth is inclined at 5o to this plane, Fig. 2





For a lunar eclipse to occur the Moon must be full. In addition the plane of the Moon's orbit must be oriented so that when the Moon is full, the Sun, Earth and Moon are more or less in a line. As shown in Fig. 2 the Moon usually passes above or below the Earth's shadow. The Moon crosses the plane of the Earth's orbit when the Moon is far from full and there is no eclipse.





Fig. 3 shows what happens when the orientation of the Moon's orbit enables the Moon to pass into the Earth's shadow. A lunar eclipse or an eclipse of the Moon occurs when the Moon passes into the Earth's shadow as shown below in Fig. 4. A lunar eclipse is visible from everywhere on the dark side of the Earth. During a lunar eclipse the Moon is never absolutely dark.



Even when the Moon is totally eclipsed, it is still illuminated by sunlight which filters through the Earth's atmosphere around the base of the shadow and is refracted and scattered into the shadow and onto the Moon. Red predominates for the same reason that the setting Sun is red.

Astronauts on the Moon would see a bright red ring around the Earth as they saw all the sunrises and sunsets happening simultaneously around the world! The Moon can take on a range of colours from dark brown and red to bright orange and yellow. The exact appearance depends on how much dust and clouds are present in the Earth's atmosphere.

It is also possible that the Sun, Earth and Moon alignment is not exact. The Moon can then pass through a part of the umbra (region of total shadow) and then there is a partial eclipse. In other cases the Moon does not pass through the umbra at all, just going through the penumbra (a region of partial shadow). Such an eclipse is called a penumbral eclipse. Finally it is possible for the Moon to just pass through a part of the penumbra and then there is a partial penumbral eclipse. These last two types are often difficult to see as the Moon hardly appears to darken.
Solar Eclipse (or eclipse of the Sun)

A solar eclipse occurs on one of the rare occasions when the Moon comes exactly between the Earth and the Sun as shown in Fig. 5. This happens when the Moon is new and the orbit is oriented so that the Sun, Moon and Earth are in line at the time of new Moon.

If the Moon is close enough to the Earth at this time, the Moon's shadow can then reach the Earth, blocking direct sunlight for anyone inside the patch P in Fig. 6, covered by shadow.


As the Earth rotates the spot P will trace out a narrow path across the Earth's surface. This path is called the path of totality and everywhere within the path an observer will see a total eclipse of the Sun, Fig. 7A, where the Sun is totally obscured by the Moon. In areas close to the path of totality observers will see a partial eclipse of the Sun and it will be partially obscured by the Moon, Fig. 7B. As observers move further and further from the path of totality, they will see progressively less of the Sun obscured by the Moon. This means that the total eclipse cannot be seen by everyone on the daylight side of the Earth, but only by those in the path of totality.



The Sun's corona is a tenuous envelope of ionized gas, glowing partly from sunlight being scattered off free electrons and partly from the recombination of energetic ions. During a total eclipse it is possible to see the Sun's corona around the Moon's silhouetted disk. Even though the corona extends a great distance from the Sun, it is normally too faint to be seen in the bright daylight sky. The shape and size of the corona varies through an 11 year sunspot cycle. Eclipses are important for studying the Sun's corona.

Because the Moon's orbit around the Earth is not circular, but elliptical (egg-shaped), it is possible that the Moon's shadow, the umbra, does not reach the Earth and then there is an annular eclipse. Here a ring of the Sun can be seen around the Moon when observed from a place directly in line with the Sun and Moon, Fig. 7C. Of course anyone not directly in line would see a partial annular eclipse. The word annular comes from the Latin anulus meaning ring.

The diagrams above show the appearance of the Sun at different times during total, partial and annular eclipses:

7A the Moon passing in front of the Sun as seen from within the umbra,
7B the Moon passing in front of the Sun as seen from within some part of the penumbra,
7C the Moon passing in front of the Sun as seen during an annular eclipse.

Some Future Lunar Eclipses

Date Time (SAST)
(Mid-eclipse)

May 16 2003 05:40
Nov 09 2003 03:18
May 04 2004 22:30
Mar 04 2007 01:21
Feb 21 2008 05:26

 
LINKS: 

SAAO

HartRAO

Johannesburg Planetarium

 




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