SkyEye

What is a planetary conjunction?

To the Heavens above us
O look and behold
The Planets that love us
All harnessed in gold!
— Rudyard Kipling, "An Astrologer's Song", 1910

A conjunction of two planets is often taken to mean any close pairing of the objects but strictly speaking, two celestial bodies are only in conjunction when they have the same apparent ecliptic longitude, normally as seen from Earth. It is also correct to speak of a conjunction in right ascension when two bodies share the same apparent right ascension.

The Moon is in conjunction with the planets every month. Conjunctions of Mercury and Venus with most or all of the the superior planets occur every year as the two inferior planets follow the Sun around the sky. Mars also usually meets most of the gas giants annually. However, conjunctions between the large planets of the outer solar system are rare events; these planets have large orbital periods and move slowly across the stars.

Like the classical planets, Uranus is visible to the naked eye but it is never brighter than sixth magnitude. Its faintness and slow movement across the sky (its orbital period is 84 years) allowed it to evade detection until 1690 when the Astronomer Royal, John Flamsteed, catalogued it as the sixth-magnitude star 34 Tauri. However, it wasn't until 13 March 1781 that astronomer William Herschel was able to resolve Uranus sufficiently to reveal a disk rather than a point-like star. Herschel originally thought that he had discovered a new comet but subsequent observations confirmed it as a new planet.

Neptune is too faint to be seen by the naked eye so its discovery had to wait until the invention of the telescope. Indeed, Galileo Galilei himself actually observed that distant planet in 1612–1613 when it was undergoing a triple conjunction (4 January 1613, 23/24 January and 31 July) with the planet Jupiter but the Italian scientist mistook the faint object for a fixed star. Deviations in the orbit of Uranus led astronomers to speculate that an as-yet-unknown object was perturbing the newly discovered seventh planet. Mathematicians and astronomers went to work on the problem, trying to predict the position of the mystery planet. Finally, on the night of 23–24 September 1846, German astronomer Johann Gottfried Galle finally found the elusive planet using the predicted position provided by French astronomer and mathematician Urbain Jean Joseph Le Verrier.

Conjunctions occur either singly or in triplets. If the two planets are on the opposite side of the Sun from Earth, a single conjunction results. These are difficult to observe if the planets are near solar conjunction at the time. However, if all three planets are on the same side of the Sun, then a triple conjunction can take place, with the retrograde motion of one or both of the gas giants contributing to the multiple meetings. It is also possible for there to be a triple conjunction in terms of one coordinate system but only a single in the other. In addition, if the ecliptic latitude (declination) is zero at the instant of conjunction in ecliptic longitude (right ascension), then the closer body (as seen from Earth) passes in front of the more distant body, blocking it from view. This is known as an occultation; lunar occultations of the planets are commonplace.

Jupiter and Neptune

Jupiter revolves around the Sun every 12 years and comes into conjunction with Neptune (orbital period 165 years) approximately every 13 years. Listed below are all of their conjunctions since the discovery of Neptune in 1846.

Jupiter and Uranus

Conjunctions between Jupiter and Uranus are slightly less common than those between Jupiter and Neptune, occurring approximately every 14 years. Listed below are all of their conjunctions since the discovery of Uranus. Note that the conjunction in the mid-twenty-first century is only single when measured by right ascension but triple when measured by ecliptic longitude.

Jupiter and Saturn

Conjunctions of Jupiter and Saturn, known as 'Great Conjunctions', occur every 20 years or so and have been observed for millennia. Listed below are the Great Conjunctions of the nineteenth, twentieth and twenty-first centuries. Notice the extremely small elongation during 1881 event. Both Jupiter and Saturn were also in conjunction with the Sun on 22 April of that year.

Saturn and Neptune

Saturn takes 29 years to complete a revolution around the Sun so it has far fewer encounters with its outer neighbours. Conjunctions between Saturn and Neptune take place about every 35 years. Their first conjunction in the twenty-first century is a triple in right ascension but only a single in ecliptic longitude. Interestingly, the two planets were engaged in a triple conjunction when Neptune was discovered in 1846.

Saturn and Uranus

Because Uranus moves faster than Neptune, it takes longer for Saturn to overtake it. Their synodic period is approximately 44 years. Unfortunately, the first conjunction of the twenty-first century will take place only 9° away from the Sun, rendering it unobservable.

Uranus and Neptune

The two ice giants have approximately 171 years between meetings. The first conjunction listed below is after the discovery of Uranus but before the discovery of Neptune. Despite the two objects undergoing a triple conjunction in 1821, observations of Uranus at the time failed to notice nearby Neptune. Their first conjunction since their respective discoveries occurred in 1993 (a triple in ecliptic longitude) and the next will not take place until 2164/5 (another triple but in right ascension).