Description by Dr Frank Prendergast, TU Dublin, frank.prendergast@tudublin.ie
2024 June 21–22
From a Stellarium animation of the Moon rising, transiting and setting – customised for the Ballycroy Visitor Centre, Co. Mayo and available at https://www.mayodarkskypark.ie/what-s-on/news-items/. In higher latitudes, such as in the north of Scotland, the maximum astronomical altitude of the Moon will be less than 3° or six diameters of the lunar disc.
Midsummer celestial lunar spectacle
On the night of Friday 21 June into Saturday 22 June 2024, something special will occur in the Irish night sky. Cloud permitting, you will witness a Full Moon rising and setting as far south on the horizon as it can ever reach. The phenomenon, popularly known as a ‘Lunar Standstill’, is really the South Major Limit of the 18.6-year lunar node cycle. The Moon never stands still. What will be very noticeable is the incredibly low trajectory or path – seeming to hug the horizon as depicted by the green arc in the illustration. When due South at 01:40, the Moon’s lower limb will be only 5° above the Ballycroy horizon in the Mayo Dark Sky Park but a little higher in the sky if viewed further south in Ireland. Depending on the altitude of your local horizon, the Moon will rise about 23:30 and set around 04:30 local time.
To the naked eye, the phenomenon repeats in the three years centred on 2024 but this year permits observing the lowest transit altitude and corresponding extreme rise and set limits on your local horizon.
Moon Illusion
Proximity of the Moon to the horizon creates an impression that it is larger than when it is higher in the sky. This is an illusion proven by measurement of the Moon’s diameter corrected for the effect of atmospheric refraction. As you embrace the Moon’s magical journey until it sets at around 04:30, you will have plenty of time to think about this puzzle!
Moon & Sun watching
By engaging with the night sky, you can hardly fail to notice that in every month the Moon emerges from the glare of the setting Sun in the western sky as a thin crescent. So begins the 30-day lunar month or lunar phase cycle, our primordial clock, the giver of cyclic time, and the regulator of many natural cycles and some religious calendars. While the Moon is so far south on the horizon this June, on the same date the Sun will be rising and setting as far north as it can ever get. This marks the summer solstice. In the historic and prehistoric past, many cultures would have noticed and celebrated these immutable phenomena.