The Night Sky in July
Atotal eclipse of the Moon takes place on 27 July, the first such event to be visible from the British Isles since September 2015. Unlike its solar counterpart, a total eclipse of the Moon is visible anywhere the Moon happens to be above the horizon at the time of the eclipse. Having stated this, the Moon’s July eclipse will be challenging because it will be in full swing at moonrise. This may make the eclipsed Moon initially hard to see, but circumstances do improve with time as the sky darkens.
A total lunar eclipse happens when the Moon moves into the Earth’s shadow, which appears like the bull’s eye at the centre of a dartboard. The outer, penumbral shadow is a region where only part of the Sun’s disc is blocked by the Earth. The inner umbral shadow is where the Sun’s light is completely blocked from view.
Visually, the Moon’s passage into the penumbral shadow shows little indication that anything has happened. It’s only when the Moon gets close to the umbral shadow that one side of it appears slightly darker than usual. In the case of a total lunar eclipse, the umbra slowly creeps across the Moon’s disc until it engulfs it completely. You might think the Moon would disappear from view, but this is typically not the case. The Earth’s atmosphere acts as a lens, refracting or bending light to infill the otherwise dark umbral shadow. Earth’s atmosphere is good at scattering blue light – which is why the sky appears blue – and consequently it is mainly wavelengths towards the redder end of the spectrum that make it through to illuminate the umbracovered Moon.
From Earth, a totally eclipsed Moon typically appears to glow an orange-red colour, the depth and darkness of which varies depending on how clear Earth’s atmosphere is at the time. As the totally eclipsed Moon rises on 27 July, it’ll be positioned within another, more local example of Earth’s shadow.
When the Sun goes down on any clear evening, look toward the opposite horizon where you will see a thin pinkish band atop a darker grey region. The pink band is known as the Belt of Venus and represents the local effect of the red-coloured sunset in the opposite direction being projected into the atmosphere. The dark grey region below it is the local version of the Earth’s shadow – the blanket of night that moves up over the sky.
The eclipsed, rising Moon on 27 July will be engulfed by the Earth’s shadow twice – once at the distance of the Moon and once because it will appear to be rising in the dark grey region below the Belt of Venus.
From the centre of the UK the Moon rises at 21:10 BST in the South East as the Sun sets at the opposite horizon. Maximum eclipse occurs at 21:22 BST, when the Moon is deepest within the umbral shadow.
Totality ends at 22:13 BST with the subsequent partial lunar eclipse continuing until 23:19 BST. The penumbral shadow slips off the Moon’s disc at 00:29 BST on 28 July. As the end of totality approaches, about an hour after sunset, look out for the bright orange planet Mars rising below and to the right (as seen from the UK) of the Moon. Mars is at opposition on 27 July.
The excitement of a lunar eclipse and Mars at opposition is a convenient distraction from the fact that July nights never get truly dark. Although our compromised view of the stars should end towards late July and early August, this year, a bright Moon will mean we will have to be patient for a while longer.