The rare planetary alignment of four planets will be visible to Australia's pre-dawn skies on Friday, the Sydney Observatory has announced.

In a rare moment that only happens every 50 to 100 years, Jupiter lines up with Venus, Mercury and Mars in the pre-dawn sky - will be visible in Australia's eastern sky tomorrow. The next planetary alignment is estimated in 2056.

The alignment of the four planets, which happens only once every 50 to 100 years, will occur just before sunrise, Sydney Observatory said.

"Every couple of years you get two or three planets that come close together, but to have four is particularly rare," the observatory's education officer, Geoffrey Wyatt, said.

The last time Australians saw such a show was in 1910, and the next occasion would not be until 2056, Mr Wyatt said.

During the alignment, from 5am (AEST) on Friday, the four planets will be visible just above the horizon in the east.

According to an emailed note of the observatory, the planet Venus, will be the brightest. Just above and to the left will be Jupiter, while Mercury will be to the right of Jupiter and slightly fainter. Mr Wyatt, as a scientist, do not wish to comment on how this planet movements will have any effect on human behaviour as predicted by astrological commentators.

Sydney Observatory is hosting a public breakfast viewing with astronomers.

Tickets are $21 for adults and $14 for children, and bookings are essential. To book, see sydneyobservatory.com.au or phone (02) 9921 3485.