Mercury's Tight Kisses on Saturn-Neptune Synodic Cycles

by Frank Hoogerbeets — 7 December 2025

On 11 December 2025 the grand conjunction Sun-Saturn-Neptune is exact. It's a rare, once in a 36 years alignment. Even more rare is the fact that Mercury joins the conjunction opposing the two gas giants on the other side of the Sun, forming a unique near quadruple alignment with an angular separation of only 9.6' (minutes). So how unique is this Mercury-Sun-Saturn-Neptune alignment?

When I looked at the previous grand Saturn-Neptune conjunctions, i.e. 1989 and 1953, I found something I didn't expect: in both cases Mercury was again at near exact opposition! This is unusual, because each planet has its own unique orbital period around the Sun and for Mercury to consecutively kiss the grand Saturn-Neptune conjunction secretly behind the Sun, a very specific orbital-conjunction resonance must be at play that has not been observed or studied before, as there is no literature or documentation about this phenomenon.

Investigation

So what is the nature of this quadruple Mercury-Sun-Saturn-Neptune alignment? How tight is this resonance and does it occur every time? The previous time the grand Saturn-Neptune conjunction occurred was on 18 July 1989 at 03:23:40 UTC. Mercury was extremely close, aligning with both planets on 16 July 1989 at 06:42:16 UTC and was only 5' away from the exact grand conjunction. 36 years earlier the grand conjunction occurred on 18 February 1953 at 18:29:39 UTC and Mercury opposed the conjuction at exactly 16:16:07 UTC on the same day. It had an angular separation of only 0.2' from the grand conjunction, an extremely tight quadruple alignment! However, in 1917 there was no quadruple alignment, as Mercury aligned with Saturn and Neptune on 6 July, while the grand conjunction occurred on 10 August. It's a confirmation that the orbital-conjunction resonance between Mercury and Sun-Saturn-Neptune drifts in time.

Year	Exact heliocentric Sun–Saturn–Neptune	Mercury opposition orb	Notes
1917 Aug 10	125°50′30″ (5°50′ Leo)	~126°	No alignment
1953 Feb 18	202°51′22″ (22°51′ Libra)	12″ (0.2′)	Absolute peak — tighter than 1/100°
1989 Jul 18	281°13′54″ (11°14′ Capricorn)	5′	Extremely close
2025 Dec 11	0°52′23″ (0°52′ Aries)	9.6′	Second peak — still spectacular
2061 May 29	79°36′38″ (19°37′ Gemini)	35′	Visibly widening
2096 Aug 19	156°51′59″ (6°52′ Virgo)	1°27′	Pattern fading

Why Does It Happen?

So why does this resonance occur and by how much does it drift? In order to understand this, we translate the relevant cycles to Earth years. The Saturn-Neptune mean synodic cycle is 35.87-35.88 years, whereby the slight variance is caused by eccentricity of the planetary orbits. Mercury's tropical year is 0.240846 Earth years, as Mercury revolves around the Sun in 87.9692568 Earth days:

87.9692568 ÷ 365.24218967 ≈ 0.24084607

If we divide the Saturn-Neptune mean synodic cycle by Mercury's tropical year, we see that in one synodic cycle Mercury revolves around the Sun almost exactly 149 times:

35.88 ÷ 0.24084607 ≈ 148.97

The difference is only ~5 hours in 36 years — an accuracy of 1 part in ~60,000. This near-commensurability makes Mercury fall short of completing exactly 149 orbits around the Sun by a tiny 0.0006 orbit per synodic cycle, causing its opposition position to drift retrograde by only ~0.216° (13') every 36 years — one of slowest drifts in the entire Solar System.

Neptune's small eccentricity modulates each individual synodic cycle length by ± 0.4 years. Remarkably, the two longest cycles occur from 1953 to 1989 and from 1989 to 2025, both lasting > 36.39 years, exactly when the 149:1 near resonance requires the smallest corrective drift. Because the cycles are slightly longer than average, Mercury falls short by an even tinier fraction, so the retrograde drift per cycle drops to only 0.08-0.09° — effectively freezing the alignment for some 70 years. The shorter grand conjunction cycles before 1953 and after 2025 produce larger drifts, up to 0.7° when the cycle is only 35.2-35.4 years, which is why the quadruple pattern "turns on" dramatically between 1917 and 1953 and then slowly degrades again after 2061.

How Rare Are These Quadruple Alignments?

With an average drift rate of ≈ 0.216° per Saturn-Neptune synodic cycle, we can reliably extrapolate when the pattern with Mercury repeats itself. A full 360° circuit of the relative opposition point takes:

360° ÷ 0.216°/cycle ≈ 1,667 cycles

As the mean Saturn-Neptune synodic cycle ≈ 35.87 years, it takes:

1,667 × 35.87 ≈ 59,800 years

So the same 149:1 geometry repeats roughly every 60,000 years, the time for the phase to drift a full 360°. In human terms, that's longer than two precession cycles (Great Year ≈ 25,920 years). Within that timescale, geometric variations occur more often, alternating between conjunctions and oppositions. A sequence of Saturn-Sun-Mercury-Neptune alignments (Mercury on the side of Neptune in grand oppositions) will occur from 2051 to 2222. These geometric variations in alignments repeat every ~740-880 years, because Mercury's ultra-stable mean heliocentric tropical motion of ~3.08530455°/day (4.09236°/day geocentric minus 1°/day Earth motion) keeps returning it to the slowly drifting Saturn-Neptune axis with clocklike fidelity.

Timescale	What it measures	Actual period (modern ephemerides)	What you see in practice
700–900 years	Time between consecutive sweet spots (decades-long runs of < 15′ kisses)	~740–880 years	1953–2025 (conjunctions) → 2150–2222 (oppositions) → next conjunction sequence ~2900–2970, etc.
~60,000 years	Time for the phase to drift a full 360° and the pattern to repeat exactly the same way	59,600 – 59,900 years	The entire configuration (which centuries have the ultra-tight kisses, drift direction, etc.) repeats identically

Conclusion

The multi-decade sequence of Mercury's extremely tight kisses on the Saturn-Neptune synodic cycles is very rare and won't repeat itself in the same way for another 60,000 years. And we just so happen to experience one in our time from 1953 to 2061, with one of the tightest kisses in December 2025. The next series of "sweet spots" will occur from 2150 to 2222 during Saturn-Neptune grand oppositions.

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