Beginners Astronomy: Solstices, Equinoxes and Sighthill Part 2

by Duncan Lunan

Part 1 can be found here: Beginners Astronomy: Solstices, Equinoxes and Sighthill Part I

The weather would not be a problem on the Moon, but despite the lack of atmosphere, there’s another major problem.  The Moon’s orbit is tilted to the Ecliptic by approximately 5 degrees, and the pull of the Sun and the Earth’s equatorial bulge cause the entire orbit of the Moon to precess around the sky, like the movement of a spinning plate as it settles, with a period of 18.61 years – so creating the solar and lunar ‘standstills’, on either side of the solsticial sunrise and sunset points, which so fascinated the builders of ancient stone circles  (Fig. 4).  

Fig. 4. Megalith alignments by Dave McClymont, 1979, after design by Duncan Lunan

As it does so, the Moon’s axis precesses around its pole star, Zeta Draconis, in the same way that the Earth’s celestial pole  (currently near Polaris)  precesses around the North Ecliptic Pole.  But where the Earth’s precessional cycle takes 26,000 years, the Moon’s takes only the same 18.61 years  (Fig. 5). 

Fig. 5. Lunar precessional cycle, drawing by Duncan Lunan

As a result, according to Cassini’s Laws  (determined by observation in the 17th century), “The Moon’s equator is tilted at a constant angle (about 1°32′ of arc) to the plane of the Earth’s orbit around the Sun (i.e. the ecliptic)”.  This why the Sun is always somewhere in the sky, seen from the Mountains of Eternal Day at the lunar north pole, and the deep craters at both poles are in permanent shadow, holding deposits of water ice which may be very important to future bases and settlers.  And as another consequence, the Moon never has an equinox  (Fig. 6).

Fig. 6. Planes ecliptic lunar equator orbit, no equinoxes. Wikipedia

If you think that’s complicated, consider the situation on Mercury, where the day takes two-thirds of the year and the 88-day orbit is markedly elliptical.  As a result there are ‘heat poles’ on the equator which take it in turns to receive the most intense heating from the Sun, and at other points the Sun will bob and up and down over the horizon, changing size as it does so.  But Mercury’s axial tilt to its orbital plane is only 2 degrees, so for all practical purposes on Mercury it’s always the equinox, all day, every day.  The situation is virtually the same on Venus, with a tilt of only three degrees – but since you can never see the Sun from the surface, due to the thickness of the clouds, it literally doesn’t make a blind bit of difference.

Mars has a day almost the same as Earth’s, an axial tilt slightly greater than ours, and a more elliptical orbit than Earth’s, so the changes of season make a big difference there.  The big Martian dust storms don’t follow the orbital seasons, but occur when Mars is closest to the Sun and therefore, annoyingly, often when it’s closest to the Earth.  The major seassonal interest at the moment is in the spring equinox of the northern hemisphere, because that’s when the orbiters and landers have been picking up methane in the atmosphere – not chemically stable, so being replenished by something, and it could be due to volcanic activity or even the presence of life.

Jupiter too has a tilt of only three degrees, so ‘no change there’.  But becauses it’s always equinox there, and the moons orbit in the plane of the equator, it means there’s a constant parade of eclipses as they transit the face of the planet, preceded or followed by their shadows, occulting one another and disappearing behind the planet.  We can see these events particularly well when the Earth is near the plane of Jupiter’s equator, as it has been for the last two months.

Saturn, however, has an axial tilt of 27 degrees,even greater than Mars’s, and the seasonal effects are compounded by the rings, because they orbit in the plane of the equator.  With a year 29 times longer than ours, it means that the angle at which the rings are presented to us is constantly changing, and there are bands of the southern hemisphere which don’t see the Sun for years at a time, except perhaps through gaps in the rings as their shadow moves south, then north again.  The Astronomy Now book Saturn, Exploring the Ringed Planet  (Pole Star Publications, 2015)  is particularly good on this, following the seasons through the Cassini probe’s 13-year orbit of Saturn.

Every 14.5 years the rings are edge-on to us, and therefore, they’re edge-on to the Sun when it’s over Saturn’s equator at the equinoxes.  Dramatic things happen then.  In 1966, the Soviet spectroscopist Kozyrev detected an atmosphere over the sunlit face of the rings – something NASA tried to claim as a new discovery when it was detected by Pioneer 11 in 1979.  Another source of debate for many years was the thickness of the rings – in the Jeff Hawke story ‘Out of Touch’, written by Harry Harrison and published in the Daily Express in 1957-58, the thickness of the rings varied for story purposes from at least a mile down to just a few inches!  In their 1980 and 1981 flybys the Voyager spacecraft tried to photograph them edge-on but failed to capture the needed images, limiting the thickness of the rings to half-a-mile at most.  The Cassini probe never caught them either, but at the equinox in 2009, particles 100 metres across could be seen casting shadows for hundreds of miles across the rings – showing that the average size of the ring particles had to be much less, while the ‘ring shepherd’ moon Prometheus could be seen to raise huge streamers, ‘fans’ and ‘ladders’ of particles above the ring plane, and Daphnis, within the rings, was pulling up equally huge ridges, only visible when the Sun was on their horizon  (Fig. 8).

The convention is that objects in the Solar System orbiting or rotating anticlockwise, as seen from the north, are in ‘direct motion’, while those going clockwise, like Halley’s Comet or the rotation of Venus, are ‘retrograde’.  Uranus is lying on its side with respect to all the rest and has an axial inclination of 98 degrees, or 82 degrees retrograde if you prefer.  With an orbital period of 84 years, that means that at each solstice the Sun is overhead at the north or south pole, alternately.  When Voyager 2 passed the planet in 1986 the Sun was overhead at the north pole and although the northern hemisphere was fully lit, only a few minor cloud features could be seen.  21 years later in 2007, in the North Lanarkshire Astronomy Project, my colleague Bob Graham and I were able to show school classes that both hemispheres were now at the equinox, with a whole string of new storms along the equator  (Fig. 9).

Fig. 9. Equinoctial storms on Uranus, NASA

The situation may be of use in the future, if the users are possessed of sufficient patience.  There’s a constant outflow of material called the Solar Wind, emerging from huge ‘coronal holes’ in the Sun’s outer atmosphere.  The flow is tenuous, so much so that comets’ tails are pushed away from the Sun not by the Solar Wind  (a common mistake)  but by the mere pressure of sunlight.  Nevertheless, over large distances, it is not insignificant:  if we could collect all the hydrogen that reaches the Moon over a year and combine it with lunar oxygen, it would make enough water to fill an Olympic swimming pool.  And the volume of the Solar System is immense:  if we represented the Sun by half the diameter of the Sighthill stone circle, Uranus would lie far out on Maryhill Road. 

Fig. 10. Ion scoop over moon of Uranus, by Gavin Roberts, 1979

All that huge volume is filled with hydrogen and lesser quantities of heavier elements.  If a magnetic field collector was launched from the moons of Uranus at the equinox  (Fig. 10), it could come back with an enormous cargo 42 years later;  and if you wanted it back sooner, it could make a Jupiter slingshot meantime to put its orbit in the right plane for recovery.  For my Man and the Planets, the Resouces of the Solar System, Gavin Roberts created a painting called ‘Jupiter Industrialised’, showing an artificial sun manufacturing oxygen in the atmosphere of the planet, a magnetic field generator at the Europa L2 point to protect the moon from Jupiter’s radiation belts  (see ‘Jupiter’s Moons’ ), and a passing solar wind  collector using the field to get an extra boost on its way back to Uranus  (Fig. 11).

Neptune has an axial tilt of 28.5 degrees, so it too experiences seasons, with solstices and equinoxes 40 years apart.  Like Jupiter’s, Neptune’s atmosphere is dominated by heat from the interior, so major variations are not expected – though it’s hard to tell at this distance and with only one flyby to date, Voyager 2’s in 1989.  Triton, Neptune’s largest moon, is in retrograde orbit around the planet, with a trapped rotation and axial tilt of 30 degrees, meaning that the Sun passes overhead at the poles on a cycle like Uranus’s.  There’s little variation in solar input, due to the great distance from the Sun, but Triton shows clear signs of internal activity and even liquid, as predicted by the Glasgow artist Ed Buckley in 1975  (Fig. 12).  What happens at the equinoxes remains to be seen.

Fig. 12. Ed Buckley, Neptune from Triton, 1975

Pluto’s axial tilt, as defined above, is 119.5 degrees  (or 60.5 degrees retrograde if you prefer), creating very long seasons, with the further complication of varying distance from the Sun, within Neptune’s orbit at its closest, during which the atmosphere freezes out in winter.  New Horizons managed to pass the planet in 2015 before the freezing was complete, revealing signs of internal activity and possibly even liquid water in Sputnik Planum, below the nitrogen ice crust.  There’s little understanding of the ‘cold chemistry’ that occurs there on those scales, and the equinox on Pluto and its large moon Chiron may also hold surprises for us in the next few decades.

Thousands of exoplanets have been discovered orbiting planets of other stars, and little has been deduced about their seasonal cycles thus far.  We can tell that many of the exoplanets are close enough to their stars to have trapped rotations.  Some are so close that iron is gaseous in the atmosphere and must rain out at the sunset terminators, while it seems that many of the ‘super-earths’ have lower densities than Earth and may have oceans hundreds of miles deep.  There’s plenty of room for speculation about what seasons there may be like!

Part 1 : Beginners Astronomy: Solstices, Equinoxes and Sighthill Part I

Duncan Lunan has written a series for Beginners Astronomy which you can find using The Orkney News search button and the monthly star charts: The Sky Above You – September 2021

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