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Beginners Astronomy: Solstices, Equinoxes and Sighthill Part I

by Duncan Lunan

The plane of the Earth’s orbit around the Sun is the Ecliptic, the center line of the Zodiac, and the Earth’s axis is inclined to it by 23.5 degrees;  as the Sun moves along the Ecliptic over the course of the year, its horizon position varies from its most southerly midwinter rise and set, when it’s overhead at the Tropic of Capricorn, to its most northerly midsummer rise and set, when it’s overhead at the Tropic of Cancer  (Fig. 1). 

Fig. 1. Ecliptic coordinates. Drawing by Nick Portwin

Those events are known as the  midwinter solstice and the midsummer solstice, respectively.  Although they are the shortest and longest days of the year in the northern hemisphere, it’s the opposite in the south, and because the Earth’s orbit is an ellipse and not a perfect circle, the latest winter sunrises and summer sunsets in our latitudes come after the solstices themselves. 

Midway between them lie the equinoxes, on or around March 21st and September 21st, with slight variation year to year, for the same reason that we need an extra day in the calendar every four years.   This year, the autumn equinox is on September 22nd.  At the spring equinox and again at the autumn equinox, the Sun is overhead at the equator and day and night have the same 12-hour length all over the world  (hence ‘equi-nox’).  The terminator, dividing day from night, passes through the north and south poles  (Figs. 2, 2A and 2B), and if the Earth were a prefect sphere with no atmosphere, the Sun would rise due east and due west everywhere.   (The Earth’s equatorial bulge, the local skyline and atmospheric refraction all affect what you actually see on those days.)

Fig. 2. equinox-globe, Wikipedia
Fig. 2A. Equinoxes at right. Solstices at left. Spin axis straight up and down, North Pole at top, South Pole bottom. Images via Geosync.
Fig. 2B. autumn equinox 2013 from Russian Electro-L metsat

The Sighthill Stone Circle

In 1979, at the end of building phase 1 of the Sighthill stone circle, at the point where we had four stones left, the late John Braithwaite and I proposed a phase 2 in which two of the spare stones would be erected as outliers, east and west, to make the equinoctial sunrises and sunsets.  That proposal was incorporated in the plan to renovate and complete the circle drafted in 2001, and when we revived that in 2011, Linda and I started trying to photograph the equinox events, to see exactly where they appeared.  Due to traffic, we just missed sunset in March 2011.  Linda’s photos of the afterglow shining through Glasgow University spire (Figs. 3 and 3A) are in my book The Stones and the Stars  (Springer, 2012), and one of them was on the cover of the first print run, though Springer then replaced it with a daylight photo of Callanish.  

Linda’s photo is also in Dr. David Clarke’s Reflections on the Astronomy of Glasgow (Edinburgh University Press, 2013). As it turns out, we were lucky to get even that.  Unlike the situation at the solstices, where the Sun hangs around its ‘standstill point’ for days either side of the event, at the equinoxes its day-to-day movement along the horizon is at its most rapid and for accuracy you really need to catch it on the day itself.  The weather nowadays is a lot more changeable than it was in the Neolithic, and we didn’t get another clear equinoctial sunset or sunrise all the way to the removal of the stones in 2016.  Since the new site  was allocated there have been efforts to photograph sunrise and sunset around the September equinox, initially so that the stones could be positioned as accurately possible.  We had hoped to conduct a year of observations before the stones were re-erected, but there was a long wait for the site to be ready and practical considerations intervened.  Also, the view to the northeast was not opened up until after the stones had been re-erected, so all the alignments were done by calculation alone, as they all had to be in the ‘Time Team’ style rush of 1979. 

David McGlone photographed the equinox sunset in September 2019, when Linda and I were in Ireland.  Good as his photos are, they show only glimpses of the sunset glow around the University spire, at various points during the event, and it’s impossible to say where the Sun itself is with any precision.  Weather prevented us doing more in 2020 and again in spring 2021, and as the new houses in the north-east quadrant are now blocking it, there may be no more chances until those buildings come down in 30 years or so.  We may be able to measure the ascending and descending tracks, at least, and maybe get horizon observations elsewhere along the alignments.  But it won’t be this year, by the look of it:  the BBC says the 22nd will be cloudy all day, the Met Office says it will be raining, and Accuweather says there may be thunder.

Next Week Duncan considers more about the planets. Until then here’s a short video about the relocation of the Sighthill Stone Circle

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