Get ready for the longest eclipse of the century this Friday 27th July 2018 when the full moon will not be like any old full moon.

We see a full moon when the path of the Moon about the Earth lines up with the path of the Earth about the Sun, such that the illuminated face of the Moon is visible from Earth. This full moon alignment happens approximately every 29 days and is technically known as syzygy of the Sun-Earth-Moon-system. When the Moon passes directly behind the Earth into its shadow, we see an eclipse. However, when this alignment is exact, such that the Suns light is completely blocked by the Earth’s shadow, we see a total eclipse. During a total eclipse, the only light that we see reflected from the Moon is light that has passed through the Earth’s atmosphere. In an effect known as Rayleigh scattering – where light is scattering with no effect on its wavelength – the Sun’s rays interact with the Earth’s atmosphere scattering the light in different directions. The more energetic blue rays, with a shorter wavelength, are the most affected, whereas lower energy rays, with longer wavelengths, pass straight through the Earth’s atmosphere where they are refracted around the Earth to the Moon. It is these refracted rays that give the Moon a reddish-orange colour during an eclipse – which is sometimes referred to as a blood moon.

Tomorrows total eclipse – blood moon – is extra special as it will be the longest lunar eclipse of the century. The reason for this is because of the geometrical alignments of the Sun-Earth-Moon system, which results in a small, slow moon and a big shadow.

The Earths shadow is determined by the alignment and proximity to the Sun, which approximately 3 weeks ago on July 6th was at aphelion – the farthest point, in its orbital path, from the sun – at a distance of 94.5 million miles (152.1 million km). The umbra shadow – the darkest part of the Earth’s shadow – is thus the largest in angular size at this time, and the bigger the shadow the longer time it spends in darkness.

Another contributing factor is the moons proximity to Earth, which on Friday will be at its furthest approach – at a distance of 252,415 miles (406,223 kilometers). The moon orbits the Earth in an elliptical orbit, when it is at apogee – furthest from the Earth – it moves slower, so not only does the Moon appear smaller, but it also moving at a slower rate.

It is this combination of a small, slow moon and a big shadow that results in a long eclipse, that is 1 hour and 43 minutes to be precise – which is almost an hour longer than a typical lunar eclipse.

This event, lasting 4 hours in its entirety and peaking at 20:22 (GMT) will not be visible from the United States, with maximum visibility from England and Wales.

Amira Val Baker