By now, just about everybody must have heard of tonight’s total lunar eclipse. It is a particularly unusual one for many different reasons, so it is not one to miss. A whole number of phenomena coincide to make it unusual:
- The eclipse is the longest of this century – it is predicted to last for 1h42m57s.
- It happens with the Moon at apogee, when it is at its smallest and faintest.
- The Moon passes almost through the centre of the Earth’s shadow.
- It happens on the same night when Mars is at its closest point to the Earth – and thus largest and brightest – since 2003 and, the eclipse takes place close to Mars in the sky.
Point (4) is just a bonus, but is an extraordinary coincidence.
Mars reaches perihelion – its closest point to the Sun – around midday on September 16th, so we are quite some way from the closest possible approach between the Earth and Mars, which occurs when the encounter takes place in September, with the Earth still close to its furthest point from the Sun and Mars simultaneous at its closest to the Sun. That is not the case in 2018, so we do not quite hit the jackpot. Tonight, Mars will be 57.7 million kilometres away (that is, around 150 times the distance of the Moon). Even so, Mars will be 24”.3 across and the same apparent size as a 50km diameter crater on the Moon. That is pretty impressive.
The fact that opposition of Mars and a lunar eclipse occur on the same night is an incredible coincidence. Martian oppositions occur about every twenty-two months and there are about two lunar eclipses per year on average, which means that the chances of the two phenomena coinciding on the same night are about one in one hundred thousand. As both an opposition of Mars and an eclipse of the Moon have to happen in the point of the sky exactly opposite to the Sun, it is natural that, when they coincide, the Moon and Mars will be close together in the sky, although Mars is in the southernmost part of its orbit and thus swings well south of the Moon.
At the same time, Mars will be magnitude -2.8. The only object in the sky that can get that bright (apart, of course, from the Sun and the Moon) is Venus. Mars will be 30% brighter than the very brightest that Jupiter can get, it will be about four times brighter than Sirius and, if you compare Mars with Saturn, which will be just about 30 degrees west of the Moon and Mars, Saturn will appear very pale and dim in comparison.
A huge dust storm that hid the surface of the planet almost completely, has been raging on Mars for the last two months. Last images of the planet show that it is starting to clear, although the atmosphere of the planet is still very dusty. It will be interesting to compare the colour of the Moon and Mars during the eclipse. When the dust storm was at its height, to my eyes at least, the colour of Mars seemed far more intensely deep orange than usual: despite people calling it “the Red Planet”, I have always seen it as a warm orange; in recent weeks though, the amount of dust in the atmosphere has clearly deepened the colour of the planet visually. Last night, I had a good look at Mars as I walked around the village and clearly the colour, though still much redder than Jupiter, was going back to its more typical warm orange, instead of the recent orange-red.
The fact that Mars is nearby and at opposition is interesting for another reason. During an eclipse, the Moon, as is logical, gets a lot dimmer.
Tonight, it coincides that the Moon is at its very furthest from Earth – apogee. An apogee Full Moon is around magnitude -12.5, about 20% fainter than a normal Full Moon and dims typically by a factor of around one thousand to ten thousand during an eclipse: anything between about seven and ten magnitudes. That means that if tonight’s eclipse is a particularly dark one, it could mean that the eclipsed Moon is actually fainter than the planet Mars. Predicting how dark the eclipse will be though, is very tricky.
During an eclipse, the surface of the Moon is illuminated by the light passing through our atmosphere: literally, the colour we see is the light of a million sunrises and sunsets around our planet. At least three effects can influence the colour of an eclipse:
- When there is a lot of volcanic activity, the Earth’s atmosphere fills with dust and sunrises and sunsets get redder (do you remember the incredible red sunsets after the eruption of Pinatubo in the early 1990s?) An eclipse of the Moon will be redder and dimer because less light passes through the atmosphere.
- When solar activity is high, the Earth’s atmosphere expands outwards and tends to pass more light.
- The closer that the Moon gets to the centre of the Earth’s shadow, the darker and redder it will be.
Recent volcanic activity has been quite low, although Kilauea in Hawai’i has been very active – our atmosphere is not very dusty, so we would expect the eclipse to be fairly bright. In contrast, solar activity is very low: there have already been more days without sunspots in 2018 than there were in the whole of 2017. That means that the atmosphere should be retreating and passing less light through the stratosphere, suggesting a darker eclipse. Finally, the southern hemisphere of the Moon will pass exactly through the centre of the Earth’s shadow, which would make us expect a very dark eclipse, with deep, copper colours.
Depending on the brightness of the eclipse, getting a good image with the eclipsed Moon and Mars both correctly exposed may turn out to be easy… or totally impossible! Bear in mind too that the Moon’s brightness is spread over a large diameter in the sky, while Mars is a single, very bright point of light: it will be a fascinating and tricky challenge for photographs to register both, perfectly, at the same time.
So, why is this eclipse such a long one?
It is, again, a combination of factors. When the moon is at apogee – its furthest from Earth – it moves more slowly (and, because it is further from us, its apparent speed in the sky is also slower). Even though the shadow of the Earth is smaller at a greater distance from our planet, the Moon still takes longer to pass through it because the much slower speed has more influence than the smaller size of the shadow. What is more, obviously, the diameter of the shadow – the chord – is greater and when the Moon is moving more slowly, it takes longer to cross the longer chord.
At the same time, we say “predicted” duration of totality. In an eclipse of the Sun we can predict the duration with great accuracy: the Moon has a nice, well-defined edge. In contrast, the border of the Earth is less well defined due to the atmosphere and the cloud layer; in general, the Earth’s shadow appears slightly larger than expected – an estimate of this effect is included in the predicted duration, but the estimate of the duration of totality is just that… an estimate and could be off by as much as a minute or two.
So, this eclipse combines so many interesting and unusual factors: an apogee Moon, a central eclipse, an opposition of Mars, the tail-end of a major Martian dust storm and a solar minimum in one, great big, surprise package. Do not miss it, because you will never see one again that combines all these factors in one.
The featured image is an 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. What will tonight’s eclipse look like.