The moon will rise from the horizon tinged a deep, rusty red as it is totally eclipsed by the Earth. About half an hour later, Mars will rise in the same place, sparkling brighter than usual because, in an astronomical coincidence, it is closer to us than at any time during the past 15 years.
Both the moon and Mars will be visible to the naked eye (although binoculars might add to the occasion). The total lunar eclipse is one of 85 this century, but tonight's will be the longest, lasting an hour and 43 minutes.
Just how spectacular may depend on weather conditions – astronomers are hoping for clear conditions without too much haze, which could obscure the moon and Mars until they rise higher in the sky.
In a lunar eclipse, the Earth, sun and moon are almost exactly aligned, with the Earth in the middle. The moon travels to a similar position every month at full moon, but the tilt of the lunar orbit means that it normally passes above or below the terrestrial shadow and no eclipse takes place.
The full moon will move into the shadow of the Earth on Friday evening before it rises, causing it to appear dramatically dimmed, lit faintly by sunlight that has filtered through the Earth's atmosphere.
The atmosphere scatters blue light more strongly, meaning the light that reaches the lunar surface is predominantly red in colour. This is the same reason sunsets appear pink or red.