A full planetary alignment on one side of the sun, as calculated by John Savard of Quadibloc, will take place only once in over 1.6 million years due to the large orbits of outer planets. A full alignment with planets on either side of the sun is possible once in approximately every 516 years. read more
Think of a very simple alignment of two planets, Earth and Venus. This only occurs a little more than once a century. To add a third planet to this alignment would take many millennia. To have all eight planets aligned would require an infinite amount of time. Even so, the word for it is syzygy. read more
Universe Today estimates that all planets in Earth's solar system line up in the same quadrant about once every 200 years. NASA also adds that precise planetary alignment, where all planets align exactly, has probably never happened. read more