Article written by: Apostolos Christou, resident Astronomer Armagh Observatory and Planetarium
As pointed out in the Astronotes January edition of the Monthly Night Sky, if skies are clear on the morning of 21st January 2019, sky-watchers will witness a rare total eclipse of the Moon. This will be the last opportunity to view a total lunar eclipse from the British Isles for three-and-a-half years, until 16 May 2022. However, even on that occasion, as seen from Northern Ireland the Moon sets before the end of totality. Indeed, the next total lunar eclipse visible in its entirety from NI will not be until the year 2029.
For the 21st January eclipse, the timings of the different phases of the eclipse, explained in a previous Astronotes blog article by Emeritus Director Mark Bailey, will be as follows:
EVENT (GMT) (FROM BELFAST)
PENUMBRAL PHASE BEGINS: 02:36 GMT 47 deg
UMBRAL OR PARTIAL PHASE BEGINS: 03:34 GMT 40 deg
TOTAL PHASE (TOTALITY) BEGINS: 04:41 GMT 31 deg
MID-TOTALITY: 05:12 GMT 27 deg
TOTALITY ENDS: 05:43 GMT 23 deg
PARTIAL PHASE ENDS: 06:51 GMT 13 deg
PENUMBRAL PHASE ENDS: 07:48 GMT 06 deg
The partial, or main, phase of the eclipse begins at 03:34 GMT when the Moon is 40 degrees above the horizon in a West South-West direction. Totality begins at 04:41 GMT at 31 degrees altitude due West and ends at 05:43, with the Moon now lying in a direction slightly North of West at an altitude of 23 degrees. Our natural satellite then exits the Earth’s umbra completely at 06:51 GMT and at an altitude of 13 degrees above the horizon.
The circumstances of this eclipse are very nearly perfect for observers in NW Europe. In addition to the British Isles, the eclipse is visible in its entirety from the Americas, and in Europe from Portugal and Norway as well as parts of Spain, France, Finland and Sweden. Observers in other parts of Europe will see the Moon set before the end of the penumbral phase, though most of the continent will be able to view the event up to the end of totality.
From the British Isles, the partial & penumbral phases following totality occur at relatively low altitude and provide a wide range of photo opportunities, with a partially-eclipsed Moon silhouetted against buildings, landmarks and other suitably photogenic background scenery. Sunrise occurs at 08:30 GMT, approximately one hour after the eclipse ends.
Although being up early on a Monday morning may not sound an attractive proposition, the upcoming eclipse will be the last one visible from these parts for quite a while. Total lunar eclipses visible from Northern Ireland will take place on 16 May 2022, on 14 March & 7 September 2025, on New Year’s Eve 2028 and on 26th June 2029. However, on those occasions the Moon is very close to the horizon and difficult to observe during the total phase. Indeed, the next total eclipse visible from the British Isles in its entirety will not occur until the 20th December 2029, more than ten years hence.
To view the totally-eclipsed Moon, given suitably clear skies one should begin observing before 05:45 GMT on the morning of the 21st. Those finding themselves outdoors slightly later, say at 6am, will be greeted by a thin sliver or arc of the bright lunar disk above the western horizon, which will become progressively larger as the Moon continues to move out of the Earth’s shadow.
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 bent or refracted into the deepest part of the Earth’s shadow, so providing illumination even at the centre of the shadow.
Indeed, if one were to stand on the surface of the Moon and look back towards the Sun, the view would be reminiscent of a solar eclipse viewed from the Earth’s surface. The above image was taken by the Japanese Kaguya spacecraft orbiting the Moon during a lunar eclipse some 10 years ago. Sunlight refracted through Earth’s atmosphere is responsible for the “ring of light’’ effect, sometimes referred to as the “light of a thousand sunsets’’. The weather conditions in those parts of the Earth’s atmosphere, for example how dusty or cloudy it happens to be, determine how dark the Moon becomes during the period of totality.
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 — again producing a range of photo-opportunities for the keen photographer.
In any event, the lack of direct solar illumination will render the eclipsed Moon about as bright as the brightest stars in the sky and allows to make out faint, so-called “deep sky” objects that would otherwise be drowned out by moonlight. On this occasion (see diagram above), the Moon’s location at the border of the constellations Cancer and Gemini places it about 7 degrees below and to the right of the M44 open star cluster also known as the “Beehive”, creating another interesting photo opportunity. Further to the right lie Castor and Pollux, the brightest stars in Gemini.