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Today is a doomsayer's delight. They have predicted earthquakes, the Earth's poles shifting - even the end of the world.

The fuss is all because today the sun, moon and six of the planets are arranged like cosmic marbles in a planetary alignment of the sort that has been attracting predictions of doom for centuries.

Planetary alignments happen about once every 20 years.

And many of them, particularly when five or more planets are involved, have been accompanied by end-of-the-world predictions.

A book, The Jupiter Effect: The Planets as Triggers of Devastating Earthquakes, written by John Gribbin and Stephen Plagemann in the mid-70s, predicted severe earthquakes in early 1982 caused by the combined gravitational effects of the planets in line.

It did not happen.

Today's grouping includes the five bright planets: Mercury, Venus, Mars, Jupiter and Saturn.

But Richard Hall, astronomer at Wellington's Carter Observatory and Phoenix Astronomical Society president, says the event will be a disappointment for astronomers as well as doomsayers.

Astronomers will see little of the action because it all happens behind the sun.

"You've got the Earth, then the moon and then the sun," says Mr Hall.

"The rest of the planets are lined up behind the sun."

The alignment also happens in daylight hours.

Mr Hall says that, as with all planetary conjunctions, the gravitational and tidal effects on the Earth today will be negligible.

"The only thing that really has any effect is that you've got the sun and the moon together and you'll get big tides on that day.

"The effect of the planets sitting there is rather small."

Dr John O'Byrne, from Sydney University's school of physics, reckons that the "combined tidal force of the other planets is not much more than the effect of a 747 aircraft cruising overhead."