Sluggan and Glennan


#12 SLUGGAN. Thom - ,  Ruggles AR2: NM8403 0757
(and #18 GLENNAN ) (Minor Standstill)

Note: All sites with horizons nearer than 1 km. (as here) must be treated with caution. Having said that these results are offered for the record.

Slab 2.5m x 0.8m x 0.25m , oriented approx 178°± 3°.  This is a few degrees to the right of a hilltop about 580m distant. What was found to be a probable alignment for the moon at minor southern standstill is marked.  At 170°.6 this is some distance from the indicated alignment.  Either the alignment is chance, or the stone has not been placed accurately.

Observations showed a possible alignment for the minor southern standstill (upper limb with 'wobble' ).  However viewed from the standard 2m behind the stone the alignment was some 3'.7 low (or the horizon 3'.7 high) for the alignment suggested.   Given the proximity of the skyline (580m) and the relatively high elevation (14°.2), distance from it would be critical.  Simple trigonometry suggests a decrease of elevation of nearly 1' arc for each metre further away.   So it seemed possible that the stone was placed not only with regard to position (and orientation) but also height.   Thus it was hypothesised that the observing position was where the top of the stone was aligned on the slight dip near the hilltop.  This was found to be 4.3m behind the stone.   Observations from this position now showed the alignment to be 1'.4 'out' instead of 3'.7.      (See below)

The following results were found:-

Observing position with the top of the stone on the alignment (horizon of hill) i.e. 4.3m back

At the southern minor standstill, 9.3 years after the maximum declination, the moon reaches its least negative declination.   I.e. in the moon would be only just below the horizon considered at the time of the standstill.

All of this must be somewhat speculative but the likelihood of it being correct is strengthened by a similar situation being observed at Glennan (see below and comments at the end).

Now in this case the moon would not be visible before the time of observation which would be considered desirable.  However observation could be done by having several observers standing in a line behind and away from the stone, and finding which observers see/ do not see the moon as it passes the alignment point.  Except at the Minor Standstill the moon at its maximum south declination would always be further south.  What would be sought would be the time (day) when the moon just failed to clear the horizon. These observers during the preceeding lunations would have observed the moon slowly moving north at its least southern declination each month - evident by having to move gradually closer to the stone while still  just seeing the moon pass the dip. (and presumably marking the position with stakes)  Thereafter it would, each month, retreat a little further south again.

Note that the upper limb at the top of the 'wobble' would be the most easily observed. (See diagrams after Glennan)

#18 GLENNAN Thom A2/24, Ruggles: AR10, NM8573 0113

A slab 2.0m x 1.0m x 0.4m orientated about 25°/205°.  The stone is built into what is now an old wall.


To the NE there is a steep nearby hill. Lunar maximum path shown. There is no alignment here and no suitable ground or features round to the south.

However Examination of the skyline to the SW and SSW does yield alignments:-

Observations gave:-

The situation here is similar to that at Sluggan. A steep nearby hill (~500m.) has been used and because of the high elevation the observing position towards/away is critical.  For similar reasons to those given for Sluggan the foresight (at 208° ) was observed over the top of the stone***. I.e. 3.4m back instead of the 'normal' 2m behind. This increases the (south) declination by about 1'.5.

( *** Viewing the foresight in line with the top of the stone would effectively determine the elevation.  For high elevation, short sightlines this would be important as it makes variation in the height of the observer irrelevant.)

Although the observed declination is then close to the theoretical value there remain considerable uncertainties.   Not least the question of vegetation on the horizon.  At 500m every 15cm of vegetation will increase the elevation by about 1' of arc.  However as long as the observer(s) were consistent regarding their OP the above factors would not prevent the time (day) for the top of the 'wobble' from being found.

The unindicated alignment at 191 being more distant will be affected less by these issues.  For discussion of this apparent alignment with no 'wobble' see later

The following are the observed and theoretical Lunar declinations viewing with the top of the stone on the alignment i.e. 3.4 m. back

Azimuth Elevation Observed Declination Theoretical Declination
Alignment at 191° 191°.188 13°.856 -18° 30'.5 (-18º 30'.1)   see later
Alignment at 208° 208°.105 10°.706 -18° 21'.4 -18° 21'.6


Minor Standstill Discussion

During the minor standstill the moon is not in a unique position except inasmuch as it normally reaches greater declinations.  Thus at the minor standstill the moon reaches the least positive and negative declination.

The dotted line shows the least negative declination reached.  Measuring the minor standstill would be awkward, but it would seem that, in the south, the upper limb with negative wobble would be the least difficult. (and the opposite in the north). Perhaps therefore it is to be expected that both Sluggan and Glennan are for that situation. I.e. for -(ε -ι - s - Δ).

(See Thom & Thom 1980, p 78S ; A.S. Thom 1981, p17 )