All the astrological heavenly bodies periodically go retrograde except for the sun and the moon. If you were to observe a planet in its orbit across the night sky this week, you would see Mercury seeming to slow down, stop (station), and start going in a backward direction (retrograde). Of course, Mercury is not literally going backward in its orbit, but it appears to us to be doing so. Mercury appears to go backward a few times a year for about 3 weeks at a time, give or take.
The 'shadow' period of a retrograde planet encompasses the retrograde period and then some. The shadow period is the length of time it takes for a planet to get back to the original place in the sky it was before it started going backward.
Observe the example illustration below. Each sign is made up of 30 degrees. In this example, on September 4th, Mercury reaches 7 and 1/2 degrees of Libra. On September 24th, it reaches about 22 and 1/2 degrees of Libra. It then begins to move backwards through Libra. On October 15th, it will find itself all the way back at 7 and 1/2 degrees of Libra. By October 31st, it will make it out of its shadow period, when it reaches 22 and 1/2 degrees of Libra.
Why You Care
It's nothing to fear, but it is a period when a little extra care goes a long way. When a planet goes retrograde, and when it is in its shadow period before it's even begun moving backwards, the earthly things that planet symbolizes often start to behave in unusual ways more frequently than normal. Since Mercury rules communication, it rules all objects and methods that are used in communication, including fax machines, computers, email, and objects used in transit for short trips such as cars, buses, bikes, etc. So the stereotypical but not-so-funny-at-the-time snafus begin to happen: parking tickets, undelivered/misdelivered emails, computer programs not working correctly, any electronic equipment malfunctioning or breaking - you get the picture.
We seem more vulnerable to miscommunication during this period, especially if you assume the other person understands you or don't think or see things through to their outcome without clarifying and double checking. Appointments missed or meeting places miscommunicated, arguments/hurt feelings created by lack of explanation or assumptions.
It is important at this time to communicate thoroughly and clearly, but the reason goes deeper than wanting to avoid troublesome circumstances. Any time a planet retrogrades, it is a time when reflection and review is needed. Frequently, during a Mercury retrograde period, we often find that some error or some miscommunication was set up a long time ago, and only now comes to light during the period of retrogradation. It's the Universe giving us a chance to clean up messes that have gone unnoticed for too long, so things come back so we can do that.
But sometimes it seems like a non-stop comedy of errors. Good luck, people.