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Eclipse Encounter
This year, Zambians will, once again, be able to witness a number of
eclipses, both solar and lunar.
The first, a partial solar eclipse, will take place on 19 April.
This is the 65th eclipse of the Saros series 119, an old series
which produced its last central eclipse (annular) on 18 March 1950.
The series will continue to produce partial eclipses of decreasing
magnitude until the final event in June 2112.
The recurrence of
eclipses is governed by the Saros cycle, a period of approximately
6,585.3 days. The Saros arises from a harmony between three of the
Moon's orbital periods; the
Synodic Month
(new moon to new moon), the Draconic Month (node to node) and the
Anomalistic Month. One
Saros is equal to 223 synodic months. However, 242 draconic months
and 239 anomalistic months are also equal to this same period, to
within a few hours. Eclipses separated by one Saros cycle share
roughly the same geometries. They occur at the same node with the
Moon at almost the same distance from Earth and at the same time of
year. But because the Saros period is not equal to a whole number of
days, subsequent eclipses are visible from different parts of the
earth. Thus, a Saros series returns to more or less the same
geographic region every 54 years and 34 days.
This solar eclipse will be visible from Zambia. In Lusaka the
eclipse will begin at 4.03 pm, the maximum eclipse will be at 4.52
pm and will end at 5.37 pm.
On Tuesday 4 May, we will experience the first of two total lunar eclipses in 2004. This eclipse occurs 1.3 days before perigee so the moon will appear rather large.
The Moon's path will be through the southern part of the Earth's umbral shadow and although the eclipse is not central, the total phase will last 1 hour and 16 minutes. At the time of greatest eclipse (10.30 pm) the southern sections of the moon will appear much brighter than the northern parts which will lie in deeper shadow. Its appearance will most likely change dramatically through the eclipse. This, of course, is not possible to predict and serious observers will be estimating the Danjon value at different times during totality.
The Danjon value is a five point scale used for evaluating the appearance and brightness of the moon during lunar eclipses and was proposed by the French astronomer, A Danjon.
The Moon's appearance during a total lunar eclipse can vary hugely from one eclipse to the next, for the geometry of the Moon's path through the umbra plays an important role. But what is also important is the effect of the Earth's atmosphere on eclipses. The physical mass of the Earth blocks off all direct sunlight from the umbra, but its atmosphere refracts some of the sun's rays into the shadow. The material in the atmosphere (clouds, mist, dust, organic debris, volcanic ash) filters the sunlight. As an example large volcanic eruptions are often followed by very dark, red eclipses. Extensive cloud cover can also darken the eclipse.
Danjon values are stated as 'L' values and are defined as follows:
L = 0 Very
dark eclipse. Moon almost invisible, especially at mid-totality.
L = 1 Dark
Eclipse, gray or brownish in coloration. Details distinguishable
only with difficulty.
L = 2 Deep
red or rust-coloured eclipse. Very dark central shadow, while outer
edge of umbra is relatively bright.
L = 3
Brick-red eclipse. Umbral shadow usually has a bright or yellow rim.
L = 4 Very
bright copper-red or orange eclipse. Umbral shadow has a bluish,
very bright rim.
Serious observers will also be timing
craters in order to determine the atmospheric enlargement of
Earth's shadow. In 1702, Pierre de La Hire made an interesting
observation about Earth's umbra - to accurately predict the duration
of a lunar eclipse, he found it necessary to increase the radius of
the shadow one arc-minute larger than was necessary by geometric
considerations. Although the effect is related to Earth's
atmosphere, it is not fully understood since the shadow enlargement
varies from one eclipse to the next. The enlargement can be measured
through careful timings of lunar craters as they enter and exit the
umbra. The idea is to record the moment when the most abrupt
gradient at the umbra's edge crosses the apparent centre of the
crater and predictions of umbral immersion and emersion times for
twenty well-defined craters have been generated. It goes without
saying that you should be thoroughly familiar with these features
before an eclipse in order to prevent confusion and
misidentification.
On 4 May, the
partial eclipse begins at 8.48 pm, total eclipse beings at 9.52 pm,
maximum eclipse at 10.30 pm and ends at 11.09 pm with the partial
eclipse ending at 12 minutes past midnight (all local times). The
next lunar eclipse which will be visible in Zambia will be on 28
October this year.
Under a very dark
sky is the best place to view a lunar eclipse, preferably without
city lights in the immediate area. Unfortunately, with 4 May falling
on a Tuesday, many people will not be able to get out of the towns.
But if you are able to take a couple of days of work, both Mukambi
Safari Lodge in the Kafue National Park and The Houseboat Company in
Sinazongwe are offering special deals for the night. For more
information, contact their booking office on 01 223-608 or email
Sources:
http://sunearth.gsfc.nasa.gov/eclipse
Mr Eclipse
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April 2004