The planets Uranus, Neptune and Pluto. They have extremely long orbital periods: Uranus 84 years, Neptune almost 164, and Pluto 248 years. Saturn, which has an orbital period of 29.5 years, is also sometimes considered to be a slow-moving planet. Because these planets are so far from the sun, they take longer than the Inner Planets to complete their orbits. Consequently, the slow-moving planets stay in the same sign for years.
They are also called Generational Planets because everyone born within the same period of years will have these outer planets in the same sign.
- Richard Tarnas: The Uranus-Neptune Cycle (Excerpt/ afterword from Prometheus the Awakener; gaiamind.com)