The Spectacle of a Blood Moon and Mars: the total lunar eclipse of July 27th/28th 2018.

If you live in Western Europe, moonrise on Friday July 27th will be particularly unusual. For the inhabitants of Madrid, Paris and Stockholm, the Moon will rise just about at the same moment that the Moon is totally eclipsed. To the west of this line – for example, London, Dublin or Oslo – totality will be well-advanced when the Moon rises. In Rome, the eclipse will start as the Moon rises above the eastern horizon and the Earth’s shadow will advance across its disk as the Moon gets higher in the sky.

In South America, Argentina, Chile and most of Brazil will see the eclipse ending as the Moon rises. North America, Central America and north-western South America will miss out, as the Moon will not have risen before the eclipse ends. All of Africa, apart from the west and north-west will see the full eclipse, as will all of Asia out to central China, while Japan will see the eclipse starting as the Moon sets, with the best view in the southern islands.

Solar and lunar eclipses are completely different animals. In an eclipse of the Sun, the Moon passes between the Sun and the Earth and its shadow falls on our planet. As the Moon is small, its shadow just barely reaches the Earth’s surface and only a tiny corridor sees a total eclipse: I was in Nebraska last year to observe the eclipse from the Agate Fossil Beds: it was absolutely stunning, but so brief… most total eclipses of the Sun last for less than three minutes. In contrast, a total lunar eclipse can last for nearly an hour and a half and can be seen over approximately half of the entire Earth’s surface. In a lunar eclipse, the Moon enters the Earth’s shadow, which is much larger than the disk of the Moon. However, unlike the Sun, the Moon does not disappear in the shadow: the Earth’s atmosphere refracts light around the edge of the disk that lights up the lunar surface. In effect, during a lunar eclipse, the surface of the Moon is lit with the light of all the sunrises and sunsets that our planet is experiencing at that moment. Of course, we know that sunrise and sunset are normally red, because red light penetrates our atmosphere more easily, while blue light is dispersed and scattered (that is why the sky is blue). So, the colour that we see the Moon during a total eclipse is really the sum total of the colour of a million sunrises and sunsets around the Earth.

Many people will remember the sunsets at the time of the Pinatubo eruption in the Philippines in the early 1990s: the day that the ash cloud reached Europe, the deep red colour at sunset reached the zenith; this unearthly red colour at sunrise and at sunset lasted for weeks. In contrast, there are days when the atmosphere is particularly clean and clear and the sunset shows just warm yellows. So, when there is a lunar eclipse, the colour that the Moon’s disk shows when eclipsed depends on how clean the Earth’s atmosphere is: if it is very dusty from a big eruption, it can go a very deep red, or even dirty brown – just occasionally it has disappeared, or almost disappeared – on other occasions, if the atmosphere is particularly clean, the eclipsed Moon will be bright, coppery orange, almost yellow.

It is this red colour to the disk of the Moon that leads to lunar eclipses being described as “blood Moon’s”. This total lunar eclipse also has another feature that will add to the spectacle: Mars will be just 6 degrees south of the Moon during the eclipse, with both in the constellation of Capricorn. It turns out that, by an extraordinary coincidence, the eclipse of the Moon will take place on the same night that Mars is at opposition – its closest point to Earth, when it is largest and brightest. At the time of writing, a dust storm is raging on the surface of Mars. One of the results of this dust storm is that Mars appears, at least to my eyes, to be unusually bright and more strongly red than normal (a few nights ago I compared Mars and Jupiter when they were at the same altitude in the sky – they should have been almost the same brightness, with Mars just fractionally the more brilliant of the two but, Mars was clearly significantly brighter and a surprisingly intense orange-red in colour, particularly in contrast with the warm yellow of Jupiter): this is not unexpected as the dust makes Mars more reflective and changes its colour. As the Earth’s atmosphere is quite clean at the moment, we can expect the Moon to be quite bright and the colours quite intense: the contrast with the nearby Mars will be interesting.

Observing a totally eclipsed Moon as it rises will be a considerable challenge. Even though the eclipsed Moon looks bright, in reality, in most eclipses, its brightness drops by a factor of anything between one thousand and ten thousand compared to an un-eclipsed Full Moon. If, in addition, the sky is still bright, seeing the eclipsed Moon on the horizon is next to impossible. For observers in Madrid, the Moon will rise, a few minutes before sunset, almost totally eclipsed, with the light of the setting Sun adding to the difficulty of observation.

Unlike a total eclipse of the Sun, in which the moment seems frozen – the solar corona shines with a constant, ghostly, pearly light as if sculpted in ice – a lunar eclipse is dynamic. Rarely, if ever, do we see changes in the appearance of the Sun during totality but, a lunar eclipse is dynamic – the colours change constantly.

In a total lunar eclipse, we see the effects of both the Earth’s atmosphere and the colours within the Earth’s shadow. A Moon low on the horizon will be dimmed and reddened by the atmosphere and can show curious colour changes due to atmospheric effects. The Earth’s shadow shows different bands of colours as the Moon penetrates it: the central part of the shadow is the deepest orange-red; outer parts can be strongly yellow; while there is also, often, a bluish rim to the shadow. Normally, before and after totality, the shadow appears grey due to contrast with the brilliance of the un-eclipsed part of the disk which drowns-out the much fainter colours. As the Moon crosses the shadow, the yellow band on the edge of the Moon will appear to rotate around the limb and you will see the edge of the disk brighten slowly as it approaches the edge of the shadow. These changes are slow – they happen over a few minutes, or even tens of minutes – but a total lunar eclipse really is a dynamic, living entity with constant changes, so you should watch it right through.

Umbraphiles – the name given to that singular breed that chases total eclipses of the Sun around the world, which I suppose now has to apply to me given that I have gone to three total eclipses now – often treat lunar eclipses with a certain disdain, as not being a patch on a real eclipse but, a total lunar eclipse is also a very beautiful and colourful spectacle and a great deal easier and cheaper to observe! The slowness of the eclipse also makes it much easier to set up spectacular shots, photographing the eclipsed Moon and buildings, monuments, or the landscape. On this occasion, it will be interesting to photograph the eclipsed Moon and Mars together although, in western Europe, Mars will still be very low in the sky when the total eclipse ends (although that will allow foreground objects to be included more easily in the composition).

Below is an annotated version of the eclipse circumstances chart produced by Fred Espanek. As you can see from the chart, the Moon passes close to the centre of the Earth’s shadow, but not quite centrally: it will be a little north of the centre, but still very deep in the shadow.

LunarEclipse-20180727LEMatrixLb_tezel

The slowness of the eclipse also makes it much easier to set up spectacular shots, photographing the eclipsed Moon and buildings, monuments, or the landscape. However, as you can see from this amazing montage of images of lunar eclipses taken by the brilliant Turkish astrophotographer, Tunç Tenzel, in each lunar eclipse the colours and tones and hues are different.

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