The recent and much-ballyhooed "planetary parade" was an interesting celestial event that likely spurred even infrequent skywatchers to seek out the seven planets stretched out across the sky.
Some of the planets — like Venus, Jupiter and Mars — were relatively easy to spot because of their great brilliance. Others, like Mercury and Saturn, were a bit more difficult to see because they were quite low to the west-southwest horizon against a rather bright twilight sky. And the two remaining planets, Uranus and Neptune, could be sighted only by observers with good binoculars or a backyard telescope. Plus, they needed to know exactly where in the sky to look.
Was this gathering of planets rare? It depends on how you define "rare." The last time seven planets were simultaneously in the sky was actually not so long ago: July 2020.
For that event, you had to get up before sunrise, because all of the planets were visible at dawn. So 4.5 years had elapsed between the last "great alignment" and the one that just occurred. For comparison, a given location on Earth experiences a total solar eclipse only once every 360 years, on average.
Planet summit meetings occur annually
A wide variety of conjunctions and configurations involving the planets occur during any given year. It is highly unusual, however, for three or more bright planets to appear in the same small area of the sky.
From our Earthly vantage point, we can readily observe Mercury, Venus, Mars, Jupiter and Saturn with our unaided eyes as they revolve around the sun. Each of these planets appears to move against the starry background at its own speed and along its own track. It is obvious that, because they are constantly moving at different speeds, the positions of all five planets at any particular time is unique to that particular moment.
Related: The brightest planets in March's night sky: How to see them (and when)
All of the naked-eye planets, as well as the moon, closely follow an imaginary line in the sky called the ecliptic. The ecliptic is also the path the sun appears to take through the sky as a result of Earth's revolution around it. Technically, the ecliptic represents the extension or projection of the plane of Earth's orbit out toward the sky. But because the moon and the planets move in orbits whose planes do not differ greatly from Earth's orbital plane, these bodies, when visible in our sky, always stay relatively close to the ecliptic line.
Twelve of the constellations the ecliptic passes through form the zodiac. Their names can be readily identified on standard star charts and are familiar to millions of horoscope users.
Portent of disaster?
Ancient humans probably noticed that the planets, which resemble bright stars, had the freedom to wander in the heavens, while the other "fixed" stars remained rooted in their positions. This ability to move would have seemed to have an almost magical, supernatural quality. In fact, evidence that the planets came to be associated with the gods lies in their very names, which represent ancient deities.
The skywatchers of thousands of years ago must have deduced that if the movements of the planets had any significance, it must be to inform those who could read these celestial signs of what the fates held in store. Indeed, even to this day, some people firmly believe that the changing positions of the sun, moon and planets can have an effect on the destinies of individuals and nations.
But is there any chance that any sort of planetary lineup might have any influence on our lives in any way?
Forget it.
No astrologer can predict from planetary alignments or any other celestial configuration when a specific event, good or bad, will occur on Earth.
Catastrophes? Apocalypses? (Oops! Never mind …)
In May 2000, the sun, moon and the five naked-eye planets were contained within a space measuring just 25.9 degrees across. There had been considerable publicity about this gathering, with some suggesting that it was an omen of worldwide disasters and cataclysms that would befall Earth. Others wondered if such a cosmic arrangement might lead to dangerous geophysical effects, like volcanic eruptions, huge storms or tsunamis.
Such concerns are not new. Whenever an unusual celestial gathering occurs, it's nearly always accompanied by both a sense of wonder and outright fear. But the gravitational, magnetic and tidal effects of such a clustering of heavenly bodies are, at most, minuscule.
Was there any validity to the claims that the clustering of planets a quarter century ago could have effects on Earth? Even if all of the planets were exactly in line with Earth and at their shortest possible distance to us, the combined planetary tide would equal only 0.00015 (or 1/6460) the average tide due to the sun. The planetary tide in May 2000 was, in fact, far less than this, because the five naked-eye planets were positioned on the far side of the sun.
The planets in the night sky are always shifting in and out of celestial liaisons, and astronomical amnesia allows us to forget the last time we saw them assembling for such a performance. We also usually fail to recall that none of the influential magical thinking attributed to the previous event ever materialized.
Indeed, the "Great Celestial Alignment of 2000" had no effect whatsoever on our planet.
When is the next alignment?
As mentioned earlier, it's highly unusual for three or more bright planets to appear in the same small area of the sky. For three planets clustered together, renowned Belgian celestial calculator Jean Meeus calls this a "planetary trio." In his 1997 book "Mathematical Astronomy Morsels, Meeus compiled a list of the times from 1980 to 2050 when three planets fit in a patch of sky with a minimum diameter smaller than 5 degrees. (For comparison, your clenched fist held at arm's length measures 10 degrees.)
On April 20, 2026, Mercury, Mars and Saturn will squeeze into a space measuring less than 1.7 degrees. Unfortunately, the planets will be very low, near the eastern horizon, against a bright twilight sky just a half hour before sunrise. Binoculars will likely be needed to view the tight triangle formed by these three worlds. Observers in the Southern Hemisphere will have an advantage in that this trio will appear somewhat higher and rise about an hour before sunup, making the planets somewhat easier to see.
Great alignment redux (almost)
On Oct. 22, 2028, we will have a near-replication of the great alignments of 2020 and 2025. One hour before sunrise, seven planets will be strung out across the sky from east to west, starting with Mercury and Jupiter very low to the eastern horizon. Much higher up will be brilliant Venus, while high in the southeast sky, yellow-orange Mars will hover just above the bluish star Regulus in the constellation Leo, making for a striking color contrast.
Uranus will be high in the southwest, north of the Hyades star cluster and the bright orange star Aldebaran in the constellation Taurus. At magnitude 5.6, this greenish world will be barely visible to the naked eye under a dark, non-light-polluted sky. Otherwise, you'll need binoculars or a small telescope to see it. Finally, about a quarter of the way up in the western sky will be Saturn. The only planet "out of the loop" will be Neptune, which will be below the horizon.
Planetary quintuplets
Finally, there are planetary quintuplets — five planets crowded within an area of sky with a diameter of 10 degrees or less. Amateur astronomer Jerald V. Uptain, of Aberdeen, Mississippi, (quoted in Jean Meeus' Mathematical Astronomy Morsels, 1997 Willmann-Bell, Inc.) found that nine such five-planet groupings will occur over a span of 3,992 years. One of these will take place on Sept. 8, 2040, when the five brightest planets — Venus, Jupiter, Mercury, Saturn and Mars — will fit inside a circle 9.3 degrees in diameter.
Prior to 2040, the last planetary quintuplet occurred in the year 1186, and according to Uptain, records show that the close grouping of five planets that year "caused near panic among the citizens of Europe after an 'authority' predicted that world-wide disasters would result."
In 2040, there will be a thin waxing crescent moon hovering just below the five planets. This incredible display will take place in the evening sky. Unfortunately, it will be very low in the west against a bright twilight sky, likely making it difficult to see. But once again, viewers in the Southern Hemisphere will be able to see this amazing array a bit higher and setting later, in a darker sky.
Considering this will be the first planetary quintuplet in over 800 years (now that's rare!), it just might be worth contemplating a journey Down Under to see it! Mark your calendars!
Joe Rao serves as an instructor and guest lecturer at New York's Hayden Planetarium. He writes about astronomy for Natural History magazine, Sky and Telescope and other publications.
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