Get set for a once-in-a-lifetime astronomical event on Saturday morning. It's worth getting up early for.

About 4.30am, before sunrise, a shadow will start to creep across the moon. Bite by bite, the white orb will be consumed in shadow – and appear to turn a dark, blood red.

The blood moon, seen over Melbourne in 2014. Credit:Darrian Traynor

And glimmering nearby, above and a little to the left, will be a yellowish-red dot. Mars, the closest it has been since 2003.

The last time these two events, a total lunar eclipse and the opposition of Mars, lined up was in the year 792. It won't occur again for hundreds of years, says Sydney Observatory's curator Dr Andrew Jacob.