Look up

Look up

Be prepared for the shortest total lunar eclipse of the century

The moon moves out of totality as seen from Sydney during the December 2011 eclipse. Neerav Bhatt/flickr, CC BY-NC-SA

A total lunar eclipse will be visible from across all Australia this Saturday, April 4. But it will be a quick one. Rather than passing deep into the Earth’s shadow, the moon is skimming close to the shadow’s edge, as seen in this animation.

The period of totality, when the moon is fully enclosed in the Earth’s umbral shadow, will last just five minutes or so. This makes it the shortest total lunar eclipse of the 21st century.

In fact, this eclipse has the shortest period of totality for almost 500 years. Back in 1529, on October 17, there was an eclipse where totality lasted for just 1 minute and 42 seconds.

The moon skims the edge of the Earth’s umbral shadow. Note: this graphic is oriented for the southern hemisphere. Museum Victoria

How to see it?

The great thing about lunar eclipses is that no special equipment is required to view them. Just look for the moon, which will be visible in the north-eastern sky, weather permitting.

From Western Australia, the eclipse begins shortly after sunset with the moon low to the eastern horizon. However, by the time of totality the moon will have risen to a great vantage point.

For the rest of Australia, the eclipse occurs an hour or two after sunset.

The whole event will last for three-and-a-half hours – it’s only the moment of totality that will be very short.

The moon disappears into shadow during the total lunar eclipse of April 2014. Phil Hart, CC BY-SA

Watching the moon slowly enter into the Earth’s shadow is always an amazing sight. And it’s during this partial phase of the eclipse that the shadow appears lovely and dark – seen in sharp contrast to the part of the moon which is still in sunlight.

Because totality is very brief and the moon sits so close to the shadow’s edge, this eclipse will not give us a deep red moon. Instead, we will likely see a variation in tone across the face of the moon.

The southern region of the moon – the topmost part as viewed from Australia – travels deeper through the shadow and should take on a reddish-orange glow. Whereas the northern part (or the bottom of the moon) that skims the shadow’s edge, will remain fairly bright.

The moment the moon enters totality during the October 2014 lunar eclipse. A similar variation in colour across the moon is expected for this upcoming eclipse. Martin George

Where the shadows lie

What’s interesting to note about this eclipse, is that it’s a great test for eclipse modellers. It’s not surprising, that there is no sharp edge to the Earth’s shadow and therefore the timings of this eclipse are highly dependent on the model used to estimate the shadow’s size.

This was first recorded in the early 18th century when the French astronomer and mathematician Philippe de La Hire realised that to match the timings of a lunar eclipse, it was necessary to increase the predicted radius of the Earth’s shadow by 2.4%.

In fact, observations of lunar eclipses over the next 200 years showed that the size of the Earth’s shadow varied slightly from one eclipse to another.

By the late 19th century a standard value of 2% was adopted as the most effective increase to be made to the Earth’s shadow size to produce the best predictions for an eclipse.

The ‘fuzziness’ of the Earth’s shadow is due to a number of factors such as the sun’s apparent size and the transparency of the Earth’s atmosphere. Eclipse models also have to take into account the fact that the Earth isn’t completely round, but flattened slightly towards the poles. And even the Earth’s axial tilt can add a small variation in the size of the Earth’s shadow depending on what season it is on Earth.

But for most lunar eclipses, these variations have a small effect on the eclipse timings, amounting to differences of the order of 20 seconds or so. But because this eclipse occurs so close to the edge of the Earth’s shadow the model used is much more critical.

Timey wimey

For this article, I’ve sourced the eclipse timings from NASA’s eclipse website maintained by Fred Espanek, a retired astrophysicist from NASA’s Goddard Space Flight Center. His Earth shadow model is the smallest and gives the shortest period of totality at 4 minutes and 38 seconds.

But it’s not surprising that other reputable eclipse models are found to give different results. Australian David Herald is the author of the fantastic Occult program that predicts all types of occultation events (such as the Saturn occultations we saw last year), and it also predicts eclipses. The latest version of his program (Occult4), measures totality for this eclipse as lasting 7 minutes and 14 seconds.

While the longest measurement of totality is sourced from the U. S. Naval Observatory. Their lunar eclipse computer gives a prediction of 12 minutes and 18 seconds.

But what will we see?

What this highlights for me, is the beauty of nature. We can do our best to predict what will happen, but sometimes we just don’t know for certain until we see the event itself. And it’s one of the lovely things about lunar eclipses that each one is unique.

With this eclipse, it is the path through the Earth’s shadow which makes it particularly different. But what can also affect eclipses is how dusty the Earth’s atmosphere is at the time. The dustier the atmosphere the more light is blocked.

That’s when the moon can turn a really deep red, like it did 2011 when the atmosphere was full of ash from the eruption of Chile’s Puyehue-Cordon Cualle volcano.

Even nonlocal weather conditions can affect the appearance of a lunar eclipse. Storm systems and poor weather in the part of the world where the sun ‘disappears’ as seen from the moon, will make the atmosphere less transparent. In particular, if it’s a very deep eclipse and the moon passes right through the centre of the Earth’s shadow, such weather systems can help to deepen the Earth’s shadow so that the eclipsed moon becomes almost impossible to see.

So why not try your hand at timing this eclipse event and see which model gives the best prediction for you?

And while you are watching the eclipse, the star to the right of the moon is called Spica, the brightest star in the constellation of Virgo. While in the east Saturn will be visible at the head of Scorpius and over in the north-west Jupiter can be found shining brightly.

But if the weather doesn’t cooperate in your local area, you can also follow the eclipse via live streaming by Sydney Observatory, Slooh or the Virtual Telescope.

Most importantly, this lunar eclipse is well worth a look because it will be a while before we get the chance to see another. The next lunar eclipse to be seen from Australia is a partial one on August 7, 2017 and only 25% of the Moon’s diameter will be in shadow. The next total lunar eclipse won’t occur until January 31, 2018.

_Many thanks to Martin George, Curator of Astronomy and Assistant Director, Queen Victoria Museum, Launceston, for very useful discussions that improved this article.

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