Last total lunar eclipse until 2029 is coming tomorrow

by Tanya Hill for The Conversation

Melbourne: On Tuesday March 3, the Moon will pass directly through Earth's shadow, creating a total lunar eclipse. Best of all, Australia and Aotearoa New Zealand are in a prime position to watch the show.

This time, the eclipse even occurs at a reasonable hour – no need to set an alarm and crawl out of bed at a ridiculous time.

On Tuesday evening, a dark shadow will begin to sweep across the bright and round full Moon. Once the Moon becomes fully immersed in shadow, it will take on a reddish glow.

Astronomers call this "totality". But with the ominous appearance of a red Moon hanging large in the sky, it's no wonder that throughout history and across cultures it was seen as a portent of evil. In today's world, the name "blood moon" has readily captured public imagination.

Watching a lunar eclipse is a reminder that we are part of a fascinating universe. No special equipment is needed and it's usually not hard to find the Moon in the sky. Tomorrow's eclipse will be our last chance to see a blood moon until 2029, when it will ring in the New Year in the early hours of January 1.

When can I see the lunar eclipse?

Lunar eclipses are leisurely events that take a few hours to unfold.

In this case, it'll take 75 minutes for the Moon to enter Earth's shadow – what's known as the partial eclipse phase. This will be followed by an hour of totality when the Moon turns red, and then another 75 minutes as the Moon emerges out of the shadow and returns to full brightness.

Since Earth's shadow is so large compared to the Moon, everyone on our planet's night side experiences a lunar eclipse at exactly the same time. To know when to see the totality, we only need to make adjustments for different time zones.

Across most of Western Australia the eclipse begins with the Moon below the horizon. As the Moon rises it will be partly in shadow, making it hard to spot, especially against the bright twilight sky (noting that the Moon is rising as the Sun is setting).

However, just give it some time and the eclipsed Moon will become easier to see as it climbs higher in the east and twilight gives way to night.

The eclipse will start later in the evening across the rest of Australia, with the Moon in the eastern sky. In New Zealand the eclipse will begin much later at 10:50pm local time. That will provide the best views, as the sky will be well and truly dark, and the Moon will be high in the north.

Why does the Moon turn red?

Against the bright Moon, Earth's shadow appears black at first. It's only when the Moon is fully immersed in shadow that the reddish "blood moon" glow becomes apparent.

How red the Moon appears depends entirely on the condition of Earth's atmosphere at the time. The dustier the atmosphere, the less light makes it through, turning the Moon a dark and deep red.

A clearer and more transparent atmosphere allows more sunlight to pass through, bathing the Moon in a bright orange glow.

Only red light makes it through the atmosphere because blue light (which has a shorter wavelength) is scattered away. Known as Rayleigh scattering, it's the same process that makes the sky blue. Blue light doesn't pass through the atmosphere towards the Moon, because it has been scattered across the entire sky. No matter where we look in the daytime sky, our eyes will happen upon one of those randomly scattered rays of blue light.

Celestial misalignment

Because the Moon's orbit is tilted very slightly relative to Earth's orbit around the Sun, the three objects don't always align perfectly for us to see a full lunar eclipse.

For the next six lunar eclipses, the Moon will only dip into Earth's shadow instead of being fully immersed in it.

In fact, during the three lunar eclipses of 2027, the Moon will only enter Earth's outer and much fainter penumbral shadow. Technically the Moon will dim slightly, but it'll be almost impossible to perceive this.

Even better to appreciate all the times when the celestial geometry works and we can be treated with such a marvellous, awe-inspiring evening gazing up at the Moon.

Let's hope the weather is clear across Australia and New Zealand on eclipse night, because we'll be waiting almost three years for our next blood moon.

Tanya Hill is a Senior Curator (Astronomy) at Museums Victoria Research Institute; The University of Melbourne.