A Tri-Planet Opposition Year
The three bright outer planets — Mars, Jupiter, and Saturn — appear at opposition during a span of 79 days during 2018. The opposition for bright Jupiter occurs on May 8, among the stars of Libra. Saturn’s opposition is above the Teapot stars of Sagittarius on June 27. The final opposition of bright planets is Mars on July 27.
This article summarizes the events that are explained in these articles:
- 2017-2019: Mars Observing Year with a Perihelic Opposition, July 27, 2018
- 2018: Mars Perihelic Opposition
- 2017-2018: Jupiter’s Year in the Claws of the Scorpion, A Triple Conjunction
- 2018: Saturn With the Teapot
Figure 2: A montage of outer planets’ retrograde motions. (Click an image to see the diagram close up and the caption.)
The first event of the “Tri-planet opposition” is the conjunction of Mars and Jupiter on January 7, 2018. Mars appears to move more rapidly eastward than either Jupiter or Saturn. The Red Planet passes about 0.3 degree from Jupiter on this morning. They are close the previous morning (January 6), only slightly farther apart. Jupiter is 20 times brighter than Mars at this conjunction (Figure 3). After this pairing, Mars continues its eastward motion heading toward Saturn.
Mars reaches slow-moving Saturn on April 2, 85 days after its Jupiter conjunction (Figure 4). The pair is separated by about 1 degree. Mars is only about 25% brighter than Saturn at this point, but rapidly grows in brightness during the next three months as our planet begins to catch the Red Planet. As Mars moves away from Saturn, Jupiter is approaching its opposition.
Opposition 1: Jupiter, May 8
Jupiter reaches opposition on May 8, 2018, near Libra’s bright star, Zubenelgenubi . The chart above (Figure 5) shows the three planets hours before the precise opposition time. Jupiter is in the southwest and Mars is in the southeast. The three bright planets are scattered across the southern sky. Saturn is 50 degrees to the left of Jupiter and Mars is 18 degrees farther to the left (east).
Jupiter starts retrograding on March 8 and continues until July 17.
Opposition 2: Saturn, June 27
Saturn reaches opposition on June 27, coincidentally the night of the June full moon. Saturn starts retrograding on April 17 and concludes September 6 (Figure 6). As with the previous diagram, Jupiter is in the southwest; Mars is in the southeast.
On this night Saturn is 52 degrees from Jupiter; Mars is 34 degrees to the lower left of Saturn. Compared to the previous chart, Jupiter’s noticeably moved westward past Zubenelgenubi; Saturn is close to the “lid” star in the Teapot and it continues to slowly retrograde above the Teapot stars. Mars is farther to the east and begins its retrograde just two days before the date of the diagram.
During the next 3 weeks watch Saturn inch closer to the lid star in the top of Tea Pot. The Ringed Planet passes within 3 degrees on June 17. (Saturn passes this star, Kaus Borealis, three times — a triple conjunction during this apparition: February 18, June 17 and November 18.)
Opposition 3: Mars, July 27
Mars reaches its opposition on July 27, just 79 days after Jupiter reached opposition. It is now 15 times brighter than Saturn and nearly twice as bright as Jupiter. Jupiter is still retrograding near Zubenelgenubi. Saturn continues to retrograde away from the lid star of the Teapot (Figure 7). Mars retrogrades until August 27. On the Mars retrograde loop in Figure 2 that this planet ends its retrograde near four stars, known as the “Dog Kingdom.” The stars (Omega Sagittarii, 59 Sagittarii, 60 Sagittarii, and 62 Sagittarii) resemble a small kite or diamond. See more about this asterism at the end of this article.
Mars is near its closest point to the sun, perihelion, when it reaches that point on September 15th. Such oppositions are called “perihelic oppostions.”
Saturn is 31 degrees to the right (west) of Mars. Jupiter is 50 degrees beyond Saturn.
Future Tri-Planet Oppositions
(This section revised, April 16, 2018)
The trio of bright outer planets continue to reach opposition about every two years for the near future. When Jupiter and Saturn have a conjunction in 2020, the duration between the three planets’ oppositions increases to 91 days, then it begins to lengthen as noted here:
2022 — Saturn, August 14; Jupiter, September 26; Mars, December 8; 116 days
2024/2025 — Saturn, September 8; Jupiter, December 7; Mars, January 6; 130 days
2026/2027 — Saturn, October 4; Jupiter, February 11; Mars, February 19; 138 days
For the next decade the span between a tri-planet opposition interval lengthens. During 2030 and 2031, the time span of the tri-planet oppositions is 200 days, when it spans from November 27, 2030 to June 15, 2031. It begins to shorten again. By 2040, the time interval shortens again to 86 days. Jupiter and Saturn have another conjunction in 2040 (November 5), after the three planets reach their oppositions.
The main difference between the 2018 interval and the 2040 interval is that Mars is at its perihelic opposition in 2020. In 2040, it is 2 years from its aphelic opposition (February 6, 2042) when it is farthest from the earth and Sun. So it will not appear as large through a telescope or appear as bright as it does at its 2018 opposition.
During the 2018 tri-planet opposition time and then again in 2020, look for these three bright outer planets in our skies.