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Lecture Handout Archive by Julene Packer-Louis
Eclipse Mechanics & Saros Series Meaning
Mechanics of Eclipses
I'll give another example with the autumn eclipses. Going back to December 4, 2002 we had a Total Solar Eclipse. Following that was the Total Solar Eclipse of November 23, 2003. The next year we had a Partial Solar Eclipse on October 14, 2004. Last year, we had an Annular Solar Eclipse October 3, 2005. Later this year we will have an Annular Eclipse on September 22, 2006.
Can you see the eclipse regression with each eclipse in its season occurring earlier than the previous year? Did you notice that each one is approximately 9 days earlier? Half of the 18.63 day difference per year goes to each season.
Types of Solar Eclipse & What Determines Them
During a Total Solar Eclipse the Sun becomes completely covered by the Moon. During a Partial Solar Eclipse, the Moon only covers part of the Sun's disk.
An Annular Eclipse is a type of Total Eclipse. The Moon does pass completely over the Sun and the degrees for this to occur are the same as for a Total Eclipse. However in an Annular Eclipse a ring of fire is left around the Moon as she passes in front of the Sun. Therefore the Sun is not completely covered even though the Moon passes entirely over the Sun from our plane of view. The reason has to do with the moon's apogee (her furthest distance from Earth). The Moon appears largest at perigee (when closest to earth) and smallest at apogee. So when the Moon is nearer her apogee point of orbit an annular eclipse can occur.
Lastly, we can have an Annular-Total Eclipse which means that over parts of the world a Total Eclipse will occur and an Annular will occur over others. This type of eclipse begins as one and ends as the other as it makes its eclipse path across the globe. This happened on April 8, 2005.
Types of Lunar Eclipses & What Determines Them
To understand Lunar Eclipses, we have to examine the shadow Earth casts on the side that is away from the Sun. The Earth's shadow is broken down into two categories: umbra and penumbra. The umbra is the direct portion shadow cast from head on. The penumbra is the angular portion of the shadow cast due to the curvature of Earth. A picture is worth 1,000 words in this case for sure!
A Total Eclipse occurs when the Moon moves completely into the umbra shadow (as in the picture above). The Moon disappears from view because no sun light is getting to her surface to reflect.
A Partial Eclipse occurs when the moon is partially in the umbra shadow and partially in the penumbra shadow. (The Moon would be on one of the horizontal lines in the above picture.) In this case only part of the moon disappears and we get those cases where it looks like someone took a bite out of the moon. The part of the moon that does not disappear becomes sort of strange in color because some shadow is cast and some light can still get through. It reminds me of how a dimmer switch can let only some of the wattage illuminate a light bulb.
Our current eclipse is an Appulse, also known as a penumbral eclipse. During an appulse the Moon moves into the penumbra shadow but never goes into the umbra portion. Hence we have the dimmer switch affect visually when we look at the moon.
The Saros Series - Mechanics
Notice also how the numbers of North and South in the Saros Series appendix table range from 1 - 19 (pp 337 - 351). This number 19 is significant. If you take an eclipse year of 346.62 days and multiply that by 19 you get a total of 6585.78 days. The synodic cycle of the Sun & Moon is 29.53 days. After 223 synodic months, this cycle also reaches that number of days. 223 synodic months of 29.53 days equals 6585.19 days. The two factors do not coincide at any time before that. This is the greater eclipse cycle that Babylonian astronomers noticed. Saros means 'repeats' and after 19 eclipse year cycles, the Saros Series starts over at 1 again. This equates to 18 calendar years and 10 or 11 days.
The Saros Series - Meaning
--"Inventiveness and flashes of genius are hallmark of this Saros Series. The individual will have intuitive leaps, insights, good ideas, visions or vivid dreams. The new found inspiration will pull the person away from his or her social life or relationship, thereby causing strain in the private life. This is a time when the person needs to be free, if only for a few weeks." [Brady, p318]
Eclipse Degrees for March 2006
Where do these fall in your chart? Bring your chart to the IAA Lecture Hall on Saturday March 18, 2006 at 1:00 p.m. EST and we'll discuss it!
© Julene Packer, 2006