Total lunar eclipse: T minus 19 days

A partially eclipsed moon rising over L-Aħrax tal-Mellieħa last year. Photo: Josef Borg

A partially eclipsed moon rising over L-Aħrax tal-Mellieħa last year. Photo: Josef Borg

In just 19 days, Malta will bear witness to one of the more sought after astronomical spectacles visible to the naked eye. On Friday, July 27, the moon will enter the Earth’s shadow as it transits to its full phase, with the full moon disc slowly getting cast in shadow, acquiring a characteristic red hue upon reaching the full eclipse phase. The second and final total lunar eclipse for 2018, following that visible from the Americas in January, will be afoot.

Also termed ‘blood moon’ as a result of its reddish hue upon reaching total phase, a lunar eclipse is only possible when the Earth, moon and sun are exactly located in the same plane, with the Earth located in between the two. As the moon orbits around the Earth, it changes its inclination above or below this plane during its orbit, which is the reason why not every full moon phase is a total lunar eclipse.

When the moon happens to have zero inclination with respect to this plane during its full phase, a total lunar eclipse occurs. On the other hand, when its inclination at full phase is just above or below this plane, a partial lunar eclipse ensues as the moon only partially enters the Earth’s shadow.

The red colour of the moon during the eclipse has nothing to do with the colour of the moon’s surface itself. Indeed, the colouration of the lunar surface during the eclipse only arises as a result of the Earth’s atmosphere.

Since the moon would be completely cast in shadow, the only light falling onto it during the eclipse would be coming through the Earth’s atmosphere, with the light rays bent in the process and thus falling onto the moon’s surface. Since all other kinds of visible light are more successfully scattered by the Earth’s atmosphere than red light, only red light manages to make it through the Earth’s atmosphere.

This is the same reason why sunsets appear red, and why the rising and setting moon also appears redder in colour. For the total lunar eclipse visible from Malta on July 27, an event is being organised by the Institute of Space Sciences and Astronomy and the Department of Physics at the University of Malta, the Astronomical Society of Malta, Esplora and Heritage Malta, endorsed by the Valletta 18 Foundation.

The event will be held at Fort St Elmo, Valletta, on July 27, with the partial eclipse phase starting at 8.24pm  and the full eclipse beginning just over an hour later, at 9.30pm. The eclipse maximum will occur at 10.21pm.

Everyone is invited to attend. Those attending will be able to view the eclipsed moon through telescopes set up on site. In addition, those attending will be able to view Mars, Jupiter and Saturn through telescopes, and a number of activities intended for children will also be held.

A You Tube live feed of the eclipsed moon through the telescope will be transmitted for the entire duration of the lunar eclipse.

Josef Borg is a PhD student at the Institute of Space Sciences and Astronomy, University of Malta, and president of the Astronomical Society of Malta.

Did you know?

The moon’s crust has asymmetrical thickness. The moon’s crust on the near side of the moon is much thinner than that on the far side of the moon, which is explained by a theory explaining the moon’s formation from a collision between the Earth and a Mars-sized object. As the young moon formed after the collision, the side of the moon facing Earth was subjected to higher levels of heat from the Earth’s surface itself, still very hot after the collision. This resulted in material evaporating from the Earth-facing side and depositing on the cooler far side.

Eclipses are reversed when viewed from the moon. For someone on the lunar surface, a total lunar eclipse here on Earth would be a rather long solar eclipse, as the sun is hidden from view by the Earth. On the other hand, during a solar eclipse as seen from Earth, someone on the moon would be able to see the small shadow of the moon crossing a small region of the face of the Earth; the region in which viewers would be able to see the total solar eclipse on Earth.

Although we say that the moon is tidally locked, and thus we always see the same half of the moon from Earth, we actually see a bit more than that. Through a process called libration, viewers on Earth get to see around 59 per cent of the lunar surface, even though the same lunar surface appears to face towards the Earth. Although the moon’s rate of rotation around our planet is stable, its rate of revolution is not as stable, and the moon is seen to ‘rock’ in the east-west direction over several orbits, giving us a slight view over the edge of the lunar face facing towards us.

For more trivia see:

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• To find out more interesting science news, tune in to Radju Malta on Saturday mornings at 11.05am and listen to Radio Mocha.

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