In 1952 Jung published a book called "Synchronizität als ein Prinzip akausaler Zusammenhänge" ("Synchronicity: an Acausal Connecting Principle"). The concept of synchronicity goes beyond purely causal explanations of the world - which is still the domain of our natural sciences. Jung argues that incidents which occur synchronously (i.e. at the same time) do not necessarily need to be causally related. There may, however, be a meaningful connection between them.
Anthony Stevens describes an experience Jung had. In a dream he met a figure with the wings of a kingfisher. Jung wanted to draw the figure in order to remember the image. While drawing, he found in his garden the dead body of a kingfisher. These birds are extremely rare in the area around Zürich. This extraordinary situation coincided with strong inner emotions.
You are probably familiar with situations which cause you to think: "This cannot be a co-incidence!" Maybe you have finished reading a book which communicates unusual ideas. All of a sudden people in your environment talk to you about these ideas, there are reports on TV, and on the internet you keep coming across similar concepts. Such incidents occur simultaneously, but obviously one does not cause the other. They seem to be connected in a different way.
Brigitte Hamann, a German astrologer, sums up this phenomenon in her article "Gedanken Über Astrologie, Synchronizität und Prognose" (thoughts on astrology, synchronicity and prediction):
A certain incident occurs to a certain person at a certain time in such a way that it gains a special meaning to him in that it reveals important meaningful connections in this persons life. Any other observer of the same incident would consider it a random occurance without any meaning whatsoever. For him, there is no synchronistic connection to the event, and therefore it means nothing to him.
Astrology is based on the principle of synchronicity. The "influence of the stars" does not exist in a causal sense. There is no causal influence at all. Astrology "works" - if this is the right word - in the way inscribed on the tabula smaragdina:
What is below
is like what is above.
And what is above is like what is below,
so that the miracle of the One may be accomplished.
One could say that the universal is reflected in the specific. It should therefore be possible to draw conclusions regarding earthly events from planetary constellations.
Liz Greene: The positions of the heavens at a particular moment in time, by reflecting the qualities of that moment, also reflect the qualities of anything born at that moment. [...] One does not cause the other; they are synchronous, and mirror each other.
This is no doubt an extended notion of synchronicity, because it does not simply refer to an individual and his relationship with his direct environment. In fact, it sees everything in the universe as being interconnected in a meaningful way. This attitude of assuming meaningful connections between phenomena which occur simultaneously is common to astrology and Jung's synchronicity.