What does this technical term, gamma, mean in solar eclipses?

by Sumit
Question by moscow bergen: What does this technical term, gamma, mean in solar eclipses?
I am an amateur star gazer especially obsessed by solar and lunar eclipses since my childhood, and just live, by chance, in a city within the band area from where the next annular eclipse in 20/May can be observed.

I have visited NASA eclipse website in order to check the latest information or enjoy reading articles about past or future eclipses.
I am wondering what the numbers expressed as "gamma" in their eclipse tables or charts mean. It is very hard for me to understand.

As for now I guess it perhaps expresses the relationship between the center of the earth and a hypothetical axis which imaginatively penetrates the center of the sun and that of the moon, or the relative positions of those three heavenly bodies, considering the facts such as:

1) the gamma of "one of the biggest total solar eclipses" that occurred in July 1991 being very small; minus 0.0041,

2) and the gamma of any of "pure" partial eclipses -without totality or annularity at any points on the earth-, possibly observed only from each of the polar regions, being relatively large; around 1.2 or so.


Then, my questions are:

1) What is gamma?

2) Is it possible for gamma to be exact zero?

3) If gamma is exact one, what does it mean?

4) What is the maximum gamma thinkable?

5) Gamma is sometimes negative number, while it is positive in other cases. Does this relate to whether the intersection of the sun and the moon is at the ascending node or at the descending node?


Is there anybody who explains these for me?


Best answer:
Answer by oklatonola"...Gamma is the parameter that gives the minimum distance (in Earth radii) of the shadow axis from the center of Earth during each eclipse. Gamma is positive or negative depending on whether the shadow axis passes north or south of Earth's center. Looking at any of the Saros catalogs (e.g., Saros 145) one can see how the value of gamma changes with each eclipse in a series. When gamma reaches its minimum (absolute) value, the series is then at its peak. In the case of Saros 145, the peak occurs with the eclipse of 2342 Mar 08 (gamma=0.008).

Since there are two to five solar eclipses every year, there are approximately forty different Saros series in progress at any one time. For instance, during the later half of the twentieth century, there are 41 individual series and 26 of them are producing central eclipses. As old series terminate, new ones are beginning and take their places. ..."

http://eclipse.gsfc.nasa.gov/SEsaros/SEsaros.html

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