If you’ve heard about it, but not really understood it, maybe this will help.
NASA has a list of how many Lunar Eclipse Tetrads occur in each century for 5,000 years. See the far right column @ http://eclipse.gsfc.nasa.gov/LEcat5/LEcatalog.html#cattab
If you click on the century, you can scroll down to find the four Ts in a row. Note: There was no year zero; 1 BC was followed by 1 CE. This makes calculating the number of years between a BC and a CE event awkward, so astronomers use 0000 for 1 BC and negative numbers for earlier years. Thus, all astronomy year numbers for the Common Era are the same, but for BC, they are all one less. E.g., Jesus was born in 2 BC, which is -0001 in astronomy years.
The path graphics I use can be accessed by clicking on the red underlined number to the left of the date of the eclipse at the NASA site.
I prefer to see the center of the path in the center of the graphic, so I photoshop them for clarity only. I don’t change any of the data; I just make it easier for non-astronomy geeks to interpret.
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