The Lunar Standstill

I am sticking my neck out a bit here.

I am not an astrologer and the nomenclature which opens this feature is hardly scientific in any true sense.

But then, astronomers too take liberties at times. We speak of "morning" and "evening" stars when referring to planets, we have "stationary points" in our language of planetary motions and we have the word "magnitude" that is used to express brightness and not size, and so on.

Dia. 1: The diagram is for the latitude and longitude of Orkney UK. See text for full explanation. Click for enlargement.

Readers will find by searching the internet adequate explanations as to why the event which happens on 15 September this year is referred to as The Lunar Standstill.

Here I shall confine my observations to purely astronomical matters, and local ones in particular.

Of course the Moon will not appear to halt its motion against the background of stars on this day. What it does do is to reach a maximum northerly declination which will take it over 5 arc degrees north of the ecliptic. This maximum departure from the ecliptic takes place about every eighteen years.

The Moon's distance from Earth is on average a mere 384,400 kilometers. Therefore its position as seen against the star background differs markedly from place to place on the Earth's surface. In almanacs and elsewhere the Moon's position may be given in both geocentric terms or for topographic locations.

Referred to the Earth's centre, the Moon on September 15 will reach a maximum declination of +28° 43'.

On the other hand, as seen from the latitude of Orkney, the maximum declination will be as much as the Moon's apparent diameter less at 28° 13'. (Note: all figures have been rounded up to the nearest 1 arc minute. Effects of atmospheric refraction have not been taken into account since at this altitude they are insignificant for the present discussion.)

From Orkney this maximum is reached at 05h 52m approximately on the morning of the 15th September.

The Moon transits our local meridian under an hour later close to 06h 41m UT. Then the Moon will have a neck-crippling altitude of a little over 59°. This is around five degrees higher than the Sun's altitude at the summer solstice (see Dia. 1)

Because of its high declination the Moon will move out of the constellation Taurus when it will spend just under 9 hours in the constellation Auriga before entering Gemini (Dia. 1)

Thus, during this lunation the Moon will have passed through two non-Zodiacal constellations (Auriga and Ophiuchus) in addition to the familiar twelve. (Currently, every year, the Sun takes eighteen days to traverse Ophiuchus whereas it spends only six days in Scorpius.)

The Moon rises at 19h 55m on the 14th and sets 17h 25m on the 15th September. This means the Moon will be above the horizon for nearly 21.5 hours throughout the 24 hour period.

The position (azimuth) at which the Moon rises above the horizon is a feature giving rise to the harvest and hunter's moons at this time of year - see archives.

The table below shows the rise and set times together with the phase of the Moon throughout this interesting period. Note the small difference between the rising times between 5th and 12th September.

              Date                    Rise           Set             Phase
                                           h  m   s      h   m   s
             05 Sep    	18:51:57    00:34:42    0.889  
             06 Sep    	18:54:29    02:28:58    0.952  
             07 Sep 	18:55:35    04:20:15    0.990  
             08 Sep    	18:56:10    06:07:58    0.999  
             09 Sep    	18:56:47    07:53:46    0.977  
             10 Sep    	18:58:02    09:39:52    0.927  
             11 Sep    	19:00:49    11:27:47    0.852  
             12 Sep 	19:06:58    13:16:59    0.760  
             13 Sep 	19:21:03    15:02:21    0.658  
             14 Sep    	19:54:35    16:29:49    0.551  
             15 Sep    	21:00:19    17:22:01    0.446  
             16 Sep 	22:29:13    17:44:47    0.345  
             
Note: Timings have been computed for Rousay which will differ very slightly from Kirkwall.

JV 30/08/06