By Guest Author 26/07/2018

Dr Duncan Steel
A few minutes after 8am this Saturday those further south in New Zealand will have a once-in-a-lifetime chance to witness a rare celestial event: a selenelion (or selenehelion).

What is that? It’s when the eclipsed Moon can be seen on one horizon, whilst the rising Sun can also be observed near the opposite horizon. One might think this to be impossible – because an eclipse occurs when the Sun, Earth and Moon are in a straight line, and so if the Moon is above the horizon then the Sun must surely be below it. But the bending (refraction) of the rays of light caused by our atmosphere makes it feasible to see both the eclipsed Moon and the Sun at the same time — so long as you are in the right place.

The areas of our planet from which this can be experienced are very limited, because the total lunar eclipse must be ongoing at the time of moonset/sunrise. In this case the further south you are in NZ the better, with Otago and Southland being favoured. In Auckland the eclipse is still partial (i.e. totality is yet to begin) as the Moon disappears below the horizon. From Whanganui to Wellington there is just a slim chance of glimpsing the totally-eclipsed Moon as the Sun peeks above the horizon. The opportunity is longer in the southernmost parts of the country. In Invercargill there is five minutes between sunrise at 8:12 and moonset shortly thereafter, with the Moon entirely within Earth’s shadow. In Dunedin the interval is reduced to four minutes; in Christchurch, three minutes.

During the total eclipse (which begins at 7:30 am, NZST) the Moon is much fainter than it usually appears, and is coloured between dark brown and blood red, leading to various superstitions about the ‘Blood Moon’. This coloration is due to some sunlight leaking through Earth’s atmosphere and reaching the Moon, red light standing a better chance of doing so (for the same reason that the sky is blue: shorter wavelengths are scattered more efficiently by the molecules in our atmosphere).

To witness this selenelion event, obviously clear skies are needed, and you need to be in a location with a view to where the Moon will set in the WSW and also to where the Sun will rise in the ENE. The higher you are, the better, because the Sun will be barely clearing the horizon as the Moon sets in the opposite direction. The effect of the atmosphere is to bend the light from each celestial object by about half a degree, which also happens to be their apparent diameters in the sky.

Having the opportunity to observe an event like this is quite rare. Although there is a lunar eclipse visible from NZ every couple of years, on average, being located in the right place to see a celestial peculiarity like a selenelion is much less frequent, and very few people have witnessed one. The first record of any such event dates from 1666.

Dr Duncan Steel is the Chief Systems Architect at the Centre for Space Science Technology in Alexandra.

The map showing the eclipse circumstances can be found here.