UPDATE: On the night of Sunday the 20th / Monday the 21st of January, 2019 a total eclipse of Moon will occur that can be observed from both North and South America, Europe and western Africa. Here you can see if it will be visible in your area and what the schedule will be for each phase of the eclipse.
This Friday, July 27, just after sunset, we’ll be able to see a “Blood Moon” appear over the horizon. While this may seem like the beginning of a horror novel, we don’t expect any vampires or monsters, but rather the most spectacular astronomical ephemeris of 2018; it’ll also be one of the best lunar eclipses of recent years. Totality will last for 103 minutes, making it the longest eclipse of the 21st century.
Eclipses of the Moon are by themselves the most outstanding phenomena of the sky. Over a few hours you can see how the shadow of the Earth bites into the lunar disc, until it’s coloured red when entering the phase of totality.
To see the eclipse, it’s only necessary to find a horizon as clear of clouds as possible in the direction where the eclipse is going to take place, bring a chair or a beach lounger to observe it comfortably and, if you can, get away from the cities and light pollution. So, from the countryside or the mountains you’ll be able to enjoy the event against a backdrop of stars.
With the naked eye the sight of a Blood Moon is already worthwhile, but using binoculars will make it even more striking. The pair that lie almost forgotten at the bottom of a drawer will work just fine; and it doesn’t matter if you think they’re bad binoculars—the simplest ones will serve you perfectly.
Why isn’t there a lunar eclipse every month?
Lunar eclipses are not rare phenomena. Every year there are at least two, which can be seen throughout the hemisphere where it’s night-time at the occurrence of the event. So, at least once every two years, it is possible to see an eclipse of the Moon from any point on the planet without having to travel to observe it. However, it’s much harder to see a solar eclipse. Although they also occur twice a year, they can only be seen in a narrow strip of shadow a few kilometres wide; therefore, to experience a total solar eclipse without changing location, it’ll be necessary to wait an average of 400 years: either you’ll have to be very lucky, or you’ll need to travel to see one.
However, if we look at the classic diagram of a lunar eclipse, in which the Moon passes through the zone of the Earth’s shadow every 28 days, something doesn’t seem to fit… shouldn’t there be an eclipse every month?
The reason they don’t occur more often is that the Moon orbits the Earth with a certain inclination. The necessary condition for the eclipse to be fulfilled, that the centre of the Moon, Earth and Sun be on the same line, or, in other words, the three bodies be at the same “height,” only occurs twice a year. During the rest of the full moons, the satellite passes over or under the shadow of the Earth.
Why doesn’t the Moon disappear as it passes through the Earth’s shadow?
We might also wonder why the Moon doesn’t disappear completely during the eclipse. If it passes through the shadow of the Earth (where it doesn’t receive sunlight) and has no light of its own, how is it possible to see it coloured red? The explanation is that the Earth is not a completely opaque body since one of its outermost layers is transparent: the atmosphere. The light that crosses that thin layer is refracted or bent, so that it reaches the Moon and illuminates it slightly.
This makes the red colour of lunar eclipses variable. The more polluted the atmosphere is, the more light will be blocked and the less the Moon will be visible during the eclipse. Depending on how dark the eclipse is, the Danjon scale measures its intensity.
It is difficult to predict how dark or clear an eclipse is going to be. Volcanic eruptions, which release a large amount of ash and dust into the atmosphere, also have a significant influence, so no two eclipses are the same.
Why does the Moon appear reddish during a total eclipse?
As we said, the Moon remains a little illuminated during the phase of totality because at that moment a part of the Sun’s light passes through the Earth’s atmosphere, as if it were flowing through a slit. Our satellite looks red because that light, while travelling through the atmosphere, undergoes a phenomenon known as Rayleigh scattering, which also helped physicist John Tyndall explain why the sky is blue and sunsets are red.
We can test this with a simple experiment at home using a flashlight and a glass of water in which we dilute a little milk. If we try to see the bulb of the flashlight directly through the water, the light will appear reddened, while if we point the beam of light perpendicular, the colour will be bluish-grey.
The red colour of the Moon during the totality phase of an eclipse is actually the reflection of all the sunsets and sunrises of planet Earth at the same time. Nowadays we know this thanks to Rayleigh and Tyndall, but in the past the explanations were undoubtedly more mythological and supernatural, especially because of the colour red, the colour of blood. It seems natural that in the past, the name “Blood Moon” was given to a total lunar eclipse, and that spectacular name has managed to reach modern times.
A change of perspective
We can get used to seeing eclipses of our Moon—they are easy to observe phenomena that occur a couple of times a year and which, when they happen, are visible across half the planet. But in the future we might be able to enjoy an even more spectacular phenomenon: a lunar eclipse on the Moon; or rather, an eclipse of the Sun viewed from the Moon.
From the lunar surface we would see how a great “bite” devours the Sun, little by little, and then, just at the moment of totality, we would see an intense red ring forming around the Earth and perhaps even some light from the large cities of the Earth in which it was night at that moment. Meanwhile, the entire horizon around us would be tinged red, to match the colour of the light reaching the satellite. Due to the reflectivity of the lunar landscape, it would be a breathtakingly picturesque moment.
To enjoy this in all its magnitude, we will have to return to the Moon. Without any doubt, in the future this will be a tourist attraction offered by the interplanetary travel agencies. However, the more impatient among us need not employ their imaginations because, thanks to the robotic eyes of the space probe Kayuga—from the Japanese Space Agency (JAXA)—we were already able to see this phenomenon back in 2009, as an aperitif.
Whether we coincide with an eclipse on the Earth or on the Moon, we should mark this one and the following ones in our calendars. An eclipse of the Moon never disappoints.