Vacuum in the manifold drops as engine demand increases. If you use ported vacuum you always have a vacuum signal. If you want to make adjustments it can be done using the advance pot on the distributor when you use ported vacuum.
The decreasing or waning phases are when the moon shrinks from the full moon down to the new moon (darkness). As the moon wanes during the 3rd and 4th quarters, this is a good time to prune plants, as the water table is diminishing and so less sap will flow out of the cut ends.
At the exact time of the New moon the moon is to close to the sun to see at all. Blue Moon - Today we consider the Blue Moon to be when a full moon occurs twice in one month. This is a relatively new concept for the term. Originaly a blue moon was considered to be when a full moon occured 4 times in a season. A season usualy has 3 full moons. The term has became popular when referring to any rare event. Black Moon - This is a term that is not used frequently and has no exact definition. Some sources use this term when there are 2 new moons in one month similar to the blue moon which is 2 full moons in a month.