To predict planetary alignment, astronomers use the angular speeds of travel around the sun of different planets. Alignment occurs when any of the planets is in a direct line with the Earth and the Sun.
The planets of the solar system are split into two groups. The planets between the Sun and the Earth are inferior planets. The planets that lay past the Earth are called the superior planets. When inferior planets align, the occurrence is described as a conjunction. When it involves superior planets, it's called opposition. Universe Today states that a complete alignment involves all of the planets, both inferior and superior, creating a single line.