1623: The Great Conjunction of Jupiter and Saturn

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(NASA Photo)

The 2020 Great Conjunction of Jupiter and Saturn is the closest conjunction of these giant planets since their conjunction in 1623.

by Jeffrey L. Hunt

The Great Conjunction of 1623  was a very close conjunction of Jupiter and Saturn.  Since that time, many Jupiter – Saturn conjunctions have occurred . 

What were the circumstances of that conjunction?

Was the 1623 conjunction observed? (Did anybody see it?)

2016, August 27: The Venus-Jupiter conjunction

The Venus-Jupiter conjunction of August 27, 2016 had nearly the same separation as the predicted separation of the Jupiter-Saturn Great Conjunction of December 21, 2020.

The Jupiter – Saturn conjunction of 1623 occurred in the wake of the invention of the telescope, so observing was in its infancy; yet, the sky was full of planetary activity. A partial lunar eclipse (April 15, 1623) was visible throughout the Americas and in Central Europe, where the moon was setting as the eclipse reached its 90% magnitude. Venus passed Jupiter and Saturn in late June and Mercury passed the planetary pair less than two weeks later, when the planets were about 22° east of the sun. With the inner planets in the vicinity of the impending Great Conjunction and Mars reaching opposition (July 4, 1623), surely sky watchers were observing the planets’ locations to test and revise their planetary motion equations.

Other articles:

  • Feature article on about the 2020 Great Conjunction on When the Curves Line Up
  • Full-length semi-technical article about the Great Conjunction of 2020.

By the time of the Great Conjunction on July 16, 1623, the planetary pair was less than 13° east of the sun. By Civil Twilight, the pair was near the horizon at mid-latitudes. Without optical help, the conjunction likely went unobserved, even for those with recently minted telescopes. Even then, the observer needed some luck to find the conjunction.

In later years, two British publications stated that the 1623 conjunction was not observed. In 1886, the Monthly Notices of the Royal Astronomical Society state that the February 8, 1683, Jupiter – Saturn conjunction was the first observed “since the invention of the telescope” and that the 1623 passing went unobserved. The same statement was written in the Journal of the British Astronomical Association in 1897. Perhaps the conjunction was observed without optical aid and recorded from more southerly latitudes, when the planets were higher in the sky.

Did the two British publications make the statements out of parochialism, rather than from factual observations made around Europe regarding the first Great Conjunction observed with a telescope, or was this the first time that the conjunction fit into an eyepiece since the telescope’s invention? The February 24, 1643, conjunction was visible in the western sky during mid-twilight as well as the October 16, 1663, conjunction. At the second conjunction the planets were about 10° up in the southwest at one hour after sunset. However, at both conjunctions, the planets were nearly 1° apart. At the 1683 conjunction, the planets were close, about 0.2° apart, twice the separation of the upcoming event. While the two previous conjunctions were visible to the naked eye and individually in a telescopic eyepiece, the 1683 conjunction was the first observed with both planets simultaneously in an eyepiece. With a separation of 0.1°, the 1623 conjunction would have fit into telescopes eyepieces of that generation, but certainly those early telescopes were unwieldy to steer and hold steady, and the telescope operator needed some persistence during the days preceding the conjunction to follow the converging planets into bright twilight while they had sufficient altitude to observe them. So, while the British publications are accurate about viewing the planets simultaneously through a telescope, the two preceding conjunctions were visible to the unaided eye and individually through a telescope, and this does not speak to the issue as whether the 1623 conjunction when unobserved across all of humanity.

In recent times, Great Conjunctions occurred February 18, 1961; followed by a triple conjunction of the two planets in 1980-81; and the last occurred May 30, 2000, although this was difficult to observe.

Read the Great Conjunction of 2020 Article.

November 29, 2020 Update:  Patrick Hartigan from Rice University has generated a list of Great Conjunctions spanning 3000 years.  The dates may be off a day or two from the actual conjunction dates. His list includes the following close conjunctions:

  • March, 1226, separation 2.1′, one-third the separation of 2020
  • August, 1563, Separation 6.8′, slightly larger than 2020
  • July,1623, separation 5.2′, slightly less than 2020, but not likely visible.

So how do we properly describe this?  Closest since 1623? Yes, although not likely observed.  Closest since 1563?  Yes.  This was easily visible in the morning sky.  Closest observable since 1226?  Yes, this was clearly visible as well. 

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2021: Mars During February

During February 2021, Mars parades eastward in the dim starfield of Aries and moves into Taurus, nearing a March conjunction with the Pleiades star cluster.



Categories: Astronomy, Feature, Sky Watching

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5 replies

  1. My American Ephemeris for the 21st Century, 2000-2050 (half as many years for $3 more than the 20th Century Ephemeris – figures) lists Jupiter at 0*32.3′ Aquarius and Saturn at 0*30.7′ for noon UT on Dec. 22, 2020, a separation of only 1.6 arcminutes. Is my ephemeris wrong? If right, it’s even wider than Mar., 1226. And I wonder when it was closer. If true, I would appreciate correcting your article. I hope to get an authoritative answer from you. Thank you.

    • Jeffrey L. Hunt – Jeffrey L. Hunt is an educational technologist living in suburban Chicago. When he's not learning about and implementing technology in classes, he's running or looking at the stars.

      From reading a description of the source, it appears that it lists only the celestial longitude. The planets do not move exactly along the ecliptic. During December 2020, both planets are south of the ecliptic by a fraction of a degree. So when the difference of latitude is combined with the difference in longitude, the math yields the numbers that are being written here and in other media. So the actual gaps between the planets are accurate, when including the celestial latitude and the celestial longitude.

  2. Christine – www.starfires.com

    Thank you for your Reply! So astronomically, the two planetary bodies are actually closer in space than normal, not just aligned in line of sight in a conjunction from earth’s perspective, is that correct? As an astrologer ( not an astronomer), yes this conjunction occurs frequently, even in Aquarius! yet people are asking me what’s the big deal about this one? Wishing i had mastered physics and astronomy.

    • Jeffrey L. Hunt – Jeffrey L. Hunt is an educational technologist living in suburban Chicago. When he's not learning about and implementing technology in classes, he's running or looking at the stars.

      The planets were closer to each other on November 2, when they had their heliocentric conjunction as viewed from the sun, compared to their separation on December 21. Not sure what is meant by “normal.” Closer than their average distance? Yes. They appear close in the sky, but they are millions of miles apart in space. Jupiter passes Saturn every 20 years, so it is infrequent or generational. The planets moved into the constellation Capricornus several days ago and they are approaching the star Sigma Capricorni, although the starfield is getting more difficult to see with the planets appearing low in the southwest during twilight. The big deal is the infrequency and that they haven’t been this close together since 1623 although there have been great conjunctions in between then and now. The 1623 conjunction likely was not visible. The 1980-81 grouping was a triple conjunction. The 2000 conjunction was very difficult to observe because its proximity to the sun. During the Middle Ages there was a very close conjunction as well as one during the 16th Century. Great Conjunctions are infrequent. The very close ones are extremely rare.

  3. Christine – www.starfires.com

    Got it, Thank you for the the time you put into your response -Christine

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