You'll need to set your alarm clock if you want to see this century's longest lunar eclipse.

If you can brave the early morning winter chill, you'll be rewarded with a spectacular red moon that fades into the western horizon as dawn approaches.

Weather permitting, the eclipse will be visible right across Australia at the same time, starting around 4:24am AEST (and equivalent local times) on Saturday.

And to top it all off, the Moon will be joined by Mars, which will be at its brightest in 15 years, said Tanya Hill, an astronomer at the Melbourne Planetarium.

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"You'll have a red moon and a red planet. It's a lovely coincidence," Dr Hill said.

A total lunar eclipse occurs when the Earth's shadow moves across the Moon, blocking out light from the Sun.

As the shadow glides across the surface of the Moon, it takes on a distinctive red glow — an effect caused by the bending of light in Earth's atmosphere.

The period of totality during this eclipse, when Earth's shadow is directly across the Moon and it is at its reddest, will last 1 hour, 42 minutes and 57 seconds.

"It's quite a long eclipse. You're only about three minutes short of maximum," Dr Hill said.

When to watch the eclipse

The eclipse will be visible at the same time all over the world, except in North America.

"It's visible everywhere where it's night," Dr Hill said.

What you'll see will depend on your time zone — and what time the Sun rises in your location.

July 28 Partial eclipse begins Total eclipse begins Maximum eclipse Total eclipse ends Partial eclipse ends Adelaide 3:54am 5:00am 5:52am 6:43am Brisbane 4:24am 5:30am 6:22am - (after moonset) - Canberra 4:24am 5:30am 6:22am - (after moonset) - Darwin 3:54am 5:00am 5:52am 6:43am - Hobart 4:24am 5:30am 6:22am 7:13am - Melbourne 4:24am 5:30am 6:22am 7:13am - Perth 2:24am 3:30am 4:22am 5:13am 6:19am Sydney 4:24am 5:30am 6:22am - (after moonset) -

What it will look like where you live

If you live in the eastern states, you'll see the fully eclipsed Moon sitting right on the western horizon before it slips out of sight in the dawn twilight.

"That will be a really nice effect, to see this low-hanging Moon totally eclipsed and lovely and red in the western sky as we're getting up in the morning," Dr Hill said.

Just how much of the total eclipse you'll see depends upon how far north you live.

"The Moon sets earlier as you move north," she explained.

Unfortunately, if you live in New South Wales, the ACT or Queensland you will miss out on the final minutes of the total eclipse (so technically it's not the longest totally eclipsed moon you'll actually see this century).

If you live in Western Australia you'll not only see the entire total eclipse, you'll also see the Moon come out of the eclipse before it slips below the horizon.

During totality, the Moon will be set against a dark sky and appear redder than in the pre-dawn twilight in the eastern states.

The Moon always looks bigger when it is just above the horizon. ( ABC News: Andrew O'Connor )

Whether you're in the east or west, the Moon will look bigger than usual as it sits on the horizon thanks to an optical illusion.

"It's our brains tricking us into making us think the Moon has become bigger," Dr Hill said.

And while telescopes and binoculars will give you a close-up view of the shadow and craters, you don't need any special equipment to see it.

"Every lunar eclipse is a pretty amazing phenomenon, so go and have a look at it," Dr Hill said.

Hello Mars!

Eclipses occur during the full Moon phase, when the Moon is directly opposite the Sun.

Mars will also be at opposition at the end of this month.

"That means both objects are opposite the Sun in the sky, so that's why they're both rising as the Sun sets and setting as the Sun rises," Dr Hill said.

When Mars is at opposition it is also closest to the Earth, so it will be super-bright.

"It's one of the most favourable oppositions since 2003," Dr Hill said.

Mars will be just above and to the left of the Moon during the eclipse. ( Stellarium/Genelle Weule )

Together, the Red Planet and Moon will put on a show as they journey across the sky with each other.

"The wonderful thing is [Mars is] going to be just up to the left of the Moon," Dr Hill said.

"They'll look quite amazing [together] because the Moon being so close will actually dim Mars a little bit, but when the Moon is eclipsed and goes into shadow, you'll really be able to see Mars shining."

Why is this eclipse so long?

This total lunar eclipse will be the longest of the 21st century — even if you can't see all of it. There won't be anything longer for more than 100 years.

The length of an eclipse depends on where the Moon passes through the Earth's shadow.

"It might just dip into the shadow, whereas this time it's going right through the centre of the Earth's shadow," Dr Hill said.

But there are some other factors that make this a longer eclipse.

"The Moon is also at a point in its orbit we call apogee, which is when it's furthest away from the Earth," Dr Hill said.

This means the Moon is at its smallest and moving more slowly in its orbit — a phenomenon known as the Kepler effect.

The eclipse is also occurring close to when the Earth is furthest away from the Sun — a point in our orbit known as aphelion, which occurs in the southern hemisphere's winter. This affects the width of the Earth's umbral shadow.

"The umbra increases, even though the width of the total shadow decreases," Dr Hill said.

The umbral shadow greater when the Moon is furthest away from the Sun. ( Supplied: Museums Victoria )

So a greater shadow and a smaller, slower Moon makes for a longer eclipse.

While this eclipse is long, they can be longer — up to 1 hour and 47 minutes.

The longest eclipse of the 20th century fell on July 16, 2000. It lasted 1 hour, 46 minutes and 24 seconds and also occurred when the Moon was at its smallest and the Earth was near aphelion.

The next eclipse to last that long will occur on June 9, 2123, but it will not be visible from Australia.

We'll have to wait until June 19, 2141.