A lunar eclipse occurs when the Earth is positioned directly between the Sun and the Moon, thus blocking the light of the sun from reaching the Moon.

If you think about it, the Earth has a permanent shadow extending out into space on the opposite side from the Sun.  Obviously, as the Earth is much smaller than the Sun, this shadow will taper out to a point some distance away.

However, at the distance the Moon is from the Earth, it fits quite neatly into the shadow, with a little room to spare.

Astute observers might notice that the shadow cone from the Moon comes close to a point at about the distance the Moon is from the Earth - which is why a solar eclipse (when the Moon comes between the Earth and the Sun) lasts for such a few brief minutes.

A lunar eclipse lasts much longer - generally close to an hour of totality (with a maximum possible of about 1:45 when everything lines up perfectly).  Also unlike a solar eclipse, a lunar eclipse is visible from any part of the Earth from where the Moon may be seen.

Another point to contemplate is that a solar eclipse will frequently occur either 14 days before or after a lunar eclipse.  This event is no different, following a recent partial solar eclipse on November 25th.

Read on for the exact times of the eclipse and when future eclipses will occur.

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Next weekend's eclipse begins at 11:46pm AEDT (late Saturday evening) when the Moon first starts to enter the Earth's shadow cone; it will be completely inside the shadow from 1:06am until 1:57am. This period is generally referred to as 'totality' (hence a "total lunar eclipse").  Note that it possible for the Moon to pass above or below the shadow cone and thus have a partial lunar eclipse - in this case, the edge of the cone will be seen to pass across the upper or lower part of the Moon.

The eclipse will be completed at 3:17am when the last part of the Moon leaves the shadow.

Remember, a lunar eclipse is always safe to observe with the naked eye.  This is NOT TRUE of a solar eclipse, where appropriate eye protection should always be used.

A detailed plot of the relative positions of the Earth, its shadow and the Moon are available at NASA's eclipse web page for this specific event.  Readers wanting to check details of eclipses for the entire century should visit this NASA page.

The previous lunar eclipse (on June 16th this year) started at 4:23am and was still a few minutes from completion when the Moon set at around 7:40am.

This weekend's event will be in stark contrast to the lunar eclipse of 12 months ago (on December 21st) where Melbourne only saw the last 20 minutes of the Moon leaving the Earth's shadow at moonrise.

Clearly the upcoming event will be the best for comfortable viewing for quite some time, although probably a little too late for younger astronomers. 

This will be the last total lunar eclipse until April 2014 when a sequence of four total eclipses will occur at six-monthly intervals, all of which are favourable for Australian observers.