There are 4 kinds of solar eclipses, the rarest which is the Hybrid occurring about 5% of the time. And there are 3 kinds of Hybrids, the rarest which is the H3 occurring 5% of that 5% so it is the rarest of all solar eclipses. The H3 concludes as a total solar eclipse to produce that black sackcloth effect. It is even more rare when it is a long version according to NASA when it lasts for 1 minute 30 seconds or more. On Nov. 3, 2013 there is just such a long H3--the 4th since Christ. It won't happen again till 2172.

http://biblocality.com/forums/entry.php?918-Does-the-Bible-Give-Us-Any-Clue-as-to-When-Jesus-Returns