I just learned that Mercury tends to retrograde in signs of the same element within a given year. The four elements are Fire, Earth, Air and Water.

Fire signs are Aries, Leo, Sagittarius.
Earth signs are Taurus, Virgo, Capricorn.
Air signs are Gemini, Libra, Aquarius.
Water signs are Cancer, Scorpio, Pisces.

Mercury retrograded in Pisces in March; it will retrograde in Cancer in July (techically, beginning at 1° Leo), and in Scorpio in October this year. All water signs.

In 2005, its three periods of retrograde occurred in Aries, Leo and Sagittarius. Fire signs. In 2007, it will once again retrograde in the Water signs of Pisces, Cancer and Scorpio, but in 2008, it will be in the Air signs of Aquarius, Gemini and Libra.

This suggests that taken collectively, Mercury retrogrades can highlight whole themes for not just an individual year but a cyclical period of several years.

The proper astrological explanation can be found in Erin Sullivan’s article; I’ll quote a short passage here:

The retrograde cycles occur in consecutive zodiacal order as the calendar year progresses, yet the overall cycle indicates a subtle subcycle of a gradual precession of Mercury backwards through the elements. This is because Mercury stations-retrograde in each of the signs at an earlier degree each time, gradually dropping back into the previous element. In doing so, it traverses the three signs of each element — the trigons — each station connecting a loose grand trine over a full cycle of three stations of Mercury in the course of thirteen months.

The use of this cycle alerts us to the unconscious rhythms of reflection, and attunes us to our inner clock by dint of knowing where concentration and introspection are most needed. When to consider and reconsider ideas, projects, relationships and lifestyle according to one’s own unique timing is the greater purpose of Mercury retrograde.

However, as the signs express the emotive and intuitive qualities of the Water element differently, Mercury retrograde in each of the Water signs will highlight different aspects of the overall theme. More on the Water signs soon.

Sullivan, Erin. Elements of Mercury Retrograde. Accessed 21 April 2006.