The Triple Conjunction of Jupiter and Saturn

Ancient astronomers believed that the Earth was the center of the universe. They thought that the sun, moon and the five visible planets—Mercury, Venus, Mars, Jupiter and Saturn—circled around the Earth at different distances and speeds. Farthest away was the great celestial sphere in which the stars, and the constellations they formed, were embedded. The constellations on the celestial sphere thus appeared as a backdrop to the sun, moon and planets as they moved across the heavens.

The group of 12 constellations in front of which the sun, moon and planets appear to move are probably the best-known features of the night sky: They are the constellations of the zodiac—Aquarius, Pisces, Aries, Taurus and so on. The band along which these constellations appear is called the ecliptic. In the drawing below, for example, the sun is “in” (really, in front of) Virgo. Of course, the ancients could never really see the sun in any particular constellation since the stars aren’t visible in the daytime; but since they were familiar with the order of the zodiac and the sun’s rising and setting throughout the year, they could easily calculate which constellation the sun was “in,” even if they couldn’t see it. It is much easier to see a planet in a constellation at night, as shown in the photo taken on December 16, 2000, which shows a conjunction of Jupiter and Saturn in Taurus (see photo of conjunction of Jupiter and Saturn).

Whereas the sun makes a smooth journey across the ecliptic, spending exactly one month in each of the zodiac’s 12 constellations, the planets behave much more strangely. Their motion is erratic—at times changing speed, and even changing direction. In fact the word planet comes from the Greek word planeµteµs, meaning “wanderer” or “nomad.” The outer planets Mars, Jupiter and Saturn, for example, spend most of the year traveling eastward against the backdrop of stars, but every so often, and almost always at separate times, each planet will appear to make a loop or switchback in its journey. It will slow down, “stop” and then travel back toward the west for a short distance—in what astronomers call retrograde motion—then it will slow down again, stop and resume travel in an easterly direction.

Early astronomers came up with ingenious theories to explain the planets’ strange behavior. In about 87 C.E., the Alexandrian astronomer Ptolemy suggested that each planet moved in small circles or “epicycles” as it moved along its greater orbit around the Earth. Like wheels-within-wheels, these epicycles helped explain why sometimes a planet would appear to move relatively quickly across the sky from night to night, while at other times it would slow down and change direction for brief periods.

It wasn’t until the 16th century that the much simpler truth of the matter was finally recognized. The Polish astronomer Copernicus realized that the retrograde motion of planets was actually an illusion due to the fact that the sun, not the Earth, was the center of the solar system, and that the Earth was just another planet circling it.

To understand Copernicus’s insight, and to grasp why the planets aren’t really traveling backward even if they seem to at times, imagine a racetrack with several lanes, with the sun in the center. Each planet is a car, racing around the sun in the same, counterclockwise direction, and always staying in its own lane—Mercury on the inside track, Venus next to it, then Earth, Mars, Jupiter and finally Saturn. (Later astronomers discovered more planets, of course, but they aren’t visible without telescopes.) And no racetrack would be complete without the grandstands that surround it, where people sit and watch the race. These are the stars, farther from the sun than any planet, and (for all practical purposes) motionless.

This planetary race is fixed: Not only do the cars/planets in the inner lanes always go faster than those in the outer lanes, but every car also has an added advantage over the car immediately to its right: It has less far to go to complete a lap. So in this imaginary racetrack, Mercury always wins, Venus always comes in second, Earth third, and so on. (Poor Saturn!) And since every car travels around this racetrack counterclockwise, passing is always on the left.

Now, if you’re driving the third car and watching one of the slower cars on the outer tracks to your right—say car five—as you overtake it, your opponent will seem to slow down and actually move backward relative to the fans sitting farther away in the bleachers. But when you’ve put some distance between you and car five, you will again be able to see clearly that it’s moving forward the same way you are. Its backward movement was just an illusion produced by the relative motion of your two cars and a more distant point of reference. (You can observe this phenomenon on an ordinary highway too: While you’re passing a car next to you, it will briefly seem to move backward relative to the more distant trees even though you know it is actually going forward.)

The apparent backward motion of the planets that are farther from the sun than the Earth—Mars, Jupiter and Saturn—against the backdrop of stars is no different. It happens each time the Earth overtakes or “laps” these slower planets as we make our yearly journey around the sun. The Earth overtakes an outer planet when it is in opposition to the sun; that is, the sun, Earth and planet are aligned with the Earth in the middle, as shown in the drawing below. In this drawing, Earth and an outer planet are shown at various stages in their orbits around the sun. The looping line at top shows how the outer planet appears to travel across the sky, starting in the west (at right), when viewed from Earth. As Earth overtakes the planet, the planet appears to loop backward, although of course it’s still moving forward. (This December, both Jupiter and Saturn will be visible moving in retrograde motion. For details, visit our Web site

Jupiter, with its 12-year orbit, and Saturn with a 29.5-year orbit, always appear to move slowly through the night sky—but as the Earth is overtaking them, they appear to slow down even more. What makes the events of 7 B.C.E. so unusual, astronomically speaking, is that the Earth overtook both Jupiter and Saturn at the same time, when they were in conjunction. It’s very rare for these two planets to come into conjunction at all—it only happens once every 20 years—and it’s even less common for this to happen when they are in opposition and the Earth is passing them, making them both go into apparent retrograde motion at about the same time.

This meant that in 7 B.C.E., the two slowest planets lingered for a long time—11 months—together in the same small corner of the sky (in Pisces). While in Pisces, they came into conjunction—passed especially close to each other in the sky—three different times.

The diagram (above) depicts the similar, looping paths these planets took that year starting in the west (at right) and moving, at first, in an easterly direction (to the left). The Babylonian year began in the month of Nisan (our April). Saturn first appeared on about the third of Nisan (April 4 in the drawing), and moved slowly eastward from night to night. Jupiter appeared a few weeks later, on April 24. Jupiter was moving more quickly in its smaller orbit and caught up to Saturn on May 27—the date of the first conjunction.

For the next couple months, both planets appeared to travel in the typical easterly direction, but Earth was gaining fast. As the Earth began to overtake Saturn, Saturn appeared to slow its eastward advance, stop and then switch direction on July 6. Jupiter did the same thing when Earth passed by it: On July 16 it appeared to stop and change direction. Now both planets appeared to move from east to west across the sky, with Saturn in the lead once again. On October 6, 7 B.C.E., at 10 p.m., Jupiter caught up with Saturn. This was the second conjunction of the year, made even more striking by the full moon that shone nearby that evening.

The planets continued to travel west until November. As the Earth moved farther ahead, the planets appeared to pause for a second time: Jupiter on November 7, Saturn on November 20. They then appeared once again to travel in their usual, easterly direction. Saturn again was in the lead, but Jupiter quickly caught up. On December 1, at 9 p.m., the two planets came into conjunction for the third and final time that year. It would be another 20 years before they would meet again, another 200 years before they met in Pisces and another 800 years before they would meet in a triple conjunction.