Tidal Forces from Planetary Alignments

Every 20 years or so there is some fuss about planetary alignments causing a catastrophe because of the increased tidal force upon the earth. If we assume that the average water-displacement tide due to the moon is 1.0 meter (it varies from this in specific places due to the topography and the delay of the current), we can compare the strengths of the planetary tides with this to see that their effect is negligible.

If the sun or planets lie on the same line as the earth-moon direction, their tides will add to that of the moon, while at right angles their tides will subtract, and at a somewhat lesser angle (55 degrees from the moon) they do not alter the tidal amplitude in the earth-moon line. [A micron is one thousandth of a millimeter.]

                       weakest tide       strongest tide
Moon                    800 mm              1180 mm
Sun                     430 mm               480 mm
Mercury                 0.2 microns          0.5 microns
Venus                    42 microns           67 microns
Mars                    0.5 microns          1.1 microns
Jupiter                   5 microns            7 microns
Saturn                  0.2 microns          0.2 microns
Uranus                  0.0 microns          0.0 microns
Neptune                 0.0 microns          0.0 microns
Pluto                   0.0 microns          0.0 microns

largest asteroid (Ceres)                      16 mm
passing the earth at the
same distance as the moon

Now compare adding the all the maximum planet tides to the strongest tide produced by the sun and the moon.

(76 microns) / (1180 + 480 mm) =     1 part in  22000 

In other words the rare change in tidal stretching of the earth due to the planets lining up is miniscule compared to the frequent tides caused by the sun and moon alone.