Jupiter correctly salutes his grandfather

The giant outer planets overtake each other in these cycles of time – the average periods between their heliocentric conjunctions:

Jupiter-Saturn      20.0 years
Jupiter-Uranus      13.5
Jupiter-Neptune      12.8
Saturn-Uranus        44.1
Saturn-Neptune      36.4
Uranus-Neptune     171.6

The explanation of these timings in our web page on the so-called “Great Conjunctions” applies to the Jupiter-Saturn case but helps in understanding the others.

“Jupiter’s orbital period – the time it takes to circle the Sun – is 11.86 years, and Saturn’s is 29.46 years.  So, after overtaking Saturn, Jupiter comes around to the same region of the sky about 12 years later; but Saturn has moved on, and Jupiter takes about another 8 years to overtake again. In other words, conjunctions follow each other at intervals of around 20 years.  And each next conjunction happen about 2/3 of the way around the circle of the sky. These are only approximate, because the geometry differs.  The planets may both be advancing, as seen from Earth, or one or both may be in their apparent retrograde loops.  Sometimes the conjunctions are triple, because Jupiter overtakes, then appears to fall back in its retrograde loop, and overtakes again.  So we should really say that 20 years is the approximate interval between conjunctions or conjunction groups.  There are more conjunctions than 5 per century; including the members of triple conjunctions, there are about 6.2 per century.”

Similarly the timing of the Jupiter-Uranus cycle results from the nearer planet’s orbital period and the slow rate at which the farther planet has advanced along its orbit.

Here is a space view of the cycle of Jupiter-Uranus conjunctions, for comparison with the Jupiter-Saturn one. The alignment of 2010 Sep. 24 was only a degree and a half away from the vernal equinox direction, the zero point for mapping positions in the sky

In the Jupiter-Uranus case, the resulting geocentric conjunctions – as seen from Earth – are single in 1997, but triple in 1983 and 2010-2011.

Mea Culpa Department

This blog post is meant to atone for the previous one’s hasty mis-statement about the cycle of conjunctions.

The moral is that I should break out of my own cycle of alerting myself too late to astronomical events therefore trying to cram into three hours the thinking-work that should have been started three days earlier. Needed is a bell to ring and tell me that a bold-faced event in Astronomical Calendar 2024 is four days ahead

In other news of Sunday March 17

0:00 (Universal /time) Day and night equal, at latitude 40° north

4:11 First quarter Moon

11    Neptune at conjunction with the Sun; 30.897 AU from Earth; latitude -1.26°

15    Moon 4.2° N of M35 cluster; 95° from Sun in evening sky; magnitudes -10.3 and 5.3

17    Mercury at perihelion; 0.3075 AU from the Sun

And it’s St. Patrick’s Day. Wear green, and help Earth to wear green.

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