A red moon will hang below a Red Planet in the night sky tonight, during a lunar eclipse that coincides with our closest approach to Mars in six years.

"This will be a great one for us. It's very dramatic," predicts Colin Haig, vice president of the Royal Astronomical Society of Canada of the lunar eclipse scheduled to peak at 3:45 a.m. ET Tuesday.

During a lunar eclipse, the Earth casts its shadow on the full moon as it passes between the moon and the sun.

This eclipse will be a relatively rare total eclipse, when the moon is completely cloaked in the shadow.

Lunar Eclipse

During a total lunar eclipse, like this one seen from Washington State in 2011, the Earth casts its shadow on the moon. That causes the moon to dim and turn red. (Mark Harrison/Seattle Times/AP)

"What makes this one special is the moon is going to end up in the darkest part of the Earth's shadow," said Haig.

That will cause it to dim and turn a dramatic red colour.

"Some people like to say it's sort of a blood-red moon," Haig said, "which has great emotional impact but it's not really blood colour —  it's more of a coppery colour."

That's because during a lunar eclipse, any light from the sun has to pass through the Earth's atmosphere in order to hit the moon. And because the Earth's atmosphere scatters blue light the most (the reason the sky looks blue during the day), most of the light that hits the moon and then is reflected back at us is red.

Lunar eclipses don't happen during every full moon because the moon's orbit around the Earth is tilted relative to the Earth's orbit around the sun.

While lunar eclipses happen several times a year, each one can only be viewed from certain places around the globe, and in most cases, only part of the Earth's shadow falls on part of the moon.

1 hour and 20 minutes

Another thing that makes tonight's eclipse special is it will be visible across the whole country. Canadians who live west of Kingston, Ont., will be able to see the entire event, which will unfold slowly over an hour and 20 minutes. For those who live east of Kingston, the moon will set while it is still orange, before the end of the eclipse.

Besides the lunar eclipse, there is another celestial event of note tonight – the planet Mars makes its closest approach to Earth in six years, just 96 million kilometres away. That means it will also be close to its biggest and brightest. appearing above and to the right of the moon by about nine degrees during the eclipse, Haig said.

The Red Planet will look like a reddish orange star, Haig said. But unlike a star, it won't twinkle.

The red moon will also contrast sharply with the bright blue star Spica in the constellation Virgo, which will be just below the moon and to the right.

All that may make for some nice photos.

Haig says you'll be able to capture the moon with just about any camera, but he recommends using a "modest" telephoto lens of at least 200 millimetres. He suggests taking a series of photos at intervals of about five minutes.

"You'll have a nice mosaic of the event as the moon works its way across the sky."

Tonight's lunar eclipse kicks off a tetrad — a group of four total lunar eclipses over the next two years. The next ones are:

  • Oct. 8, 2014.
  • April 4, 2015.
  • Sept. 28, 2015.

But Haig warns that tonight's eclipse will likely put on the best show for Canadians, as the next one won't be visible in eastern Canada.

April 15 Lunar Eclipse