Dark Moon Rising: the total lunar eclipse of 27 July, 2018

Article by Emeritus Director, Professor Mark Bailey

If skies are clear towards the south-east on the evening of 27th July 2018, sky-watchers will be able to see a rare total eclipse of the Moon.  Seen from Armagh, the eclipse begins while the Moon is still below the horizon, shortly before sunset that evening.  It is also just a week after the 49th anniversary of the Apollo 11 Moon Landing, as described in Armagh Director Michael Burton’s recent Astronotes blog article.

The Armagh Observatory and Planetarium are holding a special event to mark this eclipse, coming at almost the same time as the opposition of Mars.  The event has proved so popular that tickets sold out within a couple of hours of being released, so we have written this blog entry to tell you about what will happen if you missed out on obtaining a ticket or are going to try to observe the eclipse from elsewhere.

A total eclipse of the Moon occurs when the Moon in its orbit around the Earth and the Sun passes directly through the Earth’s shadow, that is, when the Sun, Earth and Moon are aligned nearly exactly on a straight line with the Moon on the opposite side of the Earth from the Sun.

Geometry of a lunar eclipse. Credit: Wikipedia Commons

This arrangement means that a lunar eclipse can only occur around the time of full Moon, which this July occurs at approximately 21:20 BST on Friday 27th July.  Unfortunately, as seen from the British Isles, the Sun is still above the horizon and the Moon not yet risen when the eclipse starts, so this year’s lunar eclipse will not be visible in its entirety from the UK. Those parts of Britain farther south will have a better view of the full or total phase of the eclipse than those farther north. For example, seen from Belfast, sunset and moonrise occur around 21:34 BST and 21:26 BST respectively; but from London these times become respectively 20:57 BST and 20:49 BST.

 

The Five Phases of a Lunar Eclipse

A total lunar eclipse has five main phases. The first begins when the Moon first enters the outer, so-called penumbral, part of the Earth’s shadow.  At this time the nearly full Moon is only very slightly diminished in brightness, initially almost imperceptibly.  The second part begins when part of the Moon enters the central and darkest part of the Earth’s shadow, the region called the umbra.  This is called the partial eclipse phase.  During this time, a growing fraction of the Moon enters the dark, umbral part of the Earth’s shadow and develops a distinctive orange, copper-red or rust colour in contrast to the bright yellowish white of the penumbrally eclipsed Moon.  The full Moon gradually becomes significantly diminished in brightness until the third part of the eclipse begins, with the whole Moon within the deepest part of the Earth’s shadow.  This is known as the full or total eclipse phase.

After usually about an hour, depending on the detailed circumstances of the eclipse, the Moon leaves the umbra, signifying the end of the umbral or total eclipse phase and the start of the fourth phase of the eclipse, the second partial phase. The nearly full Moon then slowly returns to its previous form, with the whole Moon initially outside the umbra and then finally outside the penumbra too, this being the end of the eclipse usually some 4 to 6 hours after the eclipse began.

A total lunar eclipse happens when the whole moon enters Earth’s shadow. Some sunlight still reaches the moon, but first it goes through Earth’s atmosphere. The atmosphere filters out most of the sun’s blue light, so the moon looks red. In this time-lapsed image, the moon changes colour as it moves through Earth’s shadow. Image Credit: NASA/Fred Espenak

 

The Special Lunar Eclipse of July 27

The eclipse of 27th July 2018 is unusual in several respects.  First, the Moon makes a nearly central passage through the Earth’s shadow, making this potentially one of the longest lunar eclipses.  Secondly, at the time of the eclipse the Moon happens to be close to the farthest point in its elliptical orbit from the Earth, which means that its angular velocity across the sky is relatively slow.  This, together with its near central passage through the Earth’s shadow leads to an exceptionally long total lunar eclipse, nearly 103 minutes, which is the longest total lunar eclipse in the 21st century. However, as already mentioned, the first parts of the eclipse are not visible from the UK or Ireland, and the Moon is already within the umbral part of the Earth’s shadow when it rises, so the total visible period of totality will be much less than this, as seen from here.

The low altitude of the Moon during the eclipse will also make it difficult to see the eclipsed Moon unless the sky happens to be exceptionally clear in the appropriate direction, which is towards the south-east.  On the other hand, with clear skies such a rising eclipsed Moon can provide a wide range of interesting photo opportunities, with a dark or copper-red Moon silhouetted against different landscapes, buildings or other suitably photogenic backgrounds.

Credit: Armagh Observatory and Planetarium

Every total lunar eclipse is different, and given the right weather conditions is well-worth observing.  Each provides a different and unique visual experience. And even if it is cloudy, you can still admire how dark it becomes during the short period of totality before the Moon returns to full brightness.  If it is clear, the changing visual appearance of the Moon during the different phases of the eclipse can often provide stargazers with great photo-opportunities.

As described in this month’s Astronotes sky notes article by Yanina Metodieva, sky-watchers may also try to identify each of the different planets visible around the time of the eclipse.  Venus may be visible shining brightly low in the west, setting shortly before 23:00 BST; Jupiter shines somewhat less brightly than Venus low in the south-west, with the much fainter Saturn at a similar altitude low in the south-east; and Mars, which rises shortly after the end of the total phase of the eclipse, shines even brighter than Jupiter a few degrees below the Moon.

The conditions for this eclipse provide the following timings for Belfast: Moonrise (21:26 BST), which is already during totality; Maximum Eclipse (21:21 BST), which occurs a few minutes before moonrise; End of Totality (22:13 BST), which occurs when the Moon is less than 5 degrees above the horizon; and End of the Partial Eclipse phase (23:19 BST), when the Moon is about 10 degrees above the horizon.

This is the first total lunar eclipse visible from Northern Ireland for nearly three years, the last one being that of 28th September 2015 , which was visible in its entirety.  The previous total lunar eclipse occurred around sunrise on 21st December 2010, and so (like this July’s eclipse) could not be seen in its entirety from Northern Ireland.  The next total lunar eclipse visible in its entirety from Northern Ireland will occur on 21st January 2019.

 

What will the eclipsed Moon look like?

People sometimes ask why the Moon is visible at all during a total eclipse.  The answer is that some sunlight, passing through the edge of the Earth’s atmosphere as seen from the Moon, is refracted into the deepest part of the Earth’s shadow, so providing illumination even at the centre of the shadow.  The weather conditions in those parts of the Earth’s atmosphere, for example how dusty or cloudy it happens to be, determine the brightness of the residual sunlight in the centre of the umbra and so help to determine how dark the Moon becomes during the period of totality.

The Moon in eclipse. Image taken on 27 September 2015 from California by Alfredo Garcia Jr. Credit: Wikipedia Commons

In short, the colour and brightness of the Moon during a total lunar eclipse cannot easily be predicted.   They depend on how centrally the Moon passes through the Earth’s shadow and on how cloudy or transparent are those parts of the Earth’s atmosphere that enable sunlight to reach the Moon.  During a very dark eclipse the Moon may be almost invisible.  Less dark eclipses may show the Moon as dark grey or brown; or as rust-coloured, brick-red, or (if very bright) copper-red or orange.

However, people should be aware that any cloud, haze or mist lying close to the horizon may make it very difficult to seen the eclipsed Moon around moonrise, and so it may be very difficult to see this particular eclipse at all.