Looking low in the southwest sky at dusk this December, two bright objects stand out. The lower and brighter one is the planet Jupiter. Higher and a bit to the left is Saturn. During this past spring and summer, the apparent distance between the planets has been decreasing. On December 21, 2020, the two planets will be at their minimum separation, about 0.15° apart. This is only about one-third the width of the full moon. Jupiter and Saturn have not been this close since the year 1226 A.D. In historical perspective, Francis of Assisi died in that year and Genghis Khan ruled the Mongol Empire.


A conjunction occurs when two objects, usually in our solar system, appear close together. This generally includes planets, asteroids, and the sun, assuming an observer standing on Earth (stars don’t move very fast, so they don’t usually count). The word “conjunction” is the noun form of “conjoin” from the Latin “conjungare” (roughly meaning joined or yoked together). This begs the question, “How close can they get?”


When Jupiter comes between the observer on Earth and Saturn, we say that Jupiter has occulted, or hidden, Saturn. The last time this happened was in 6858 B.C. The next time this will happen is in the year 7541 A.D. Some simple mathematics tells us that a conjunction between Jupiter and Saturn will occur about every 20 years (OK, closer to 19.85 years, but read on). Why is it, then, that Jupiter does not occult Saturn each time there is a conjunction? The answer is the same reason there is not an eclipse of the moon during every full moon, or an eclipse of the sun during every new moon. The orbit of the moon around Earth is tilted, or inclined, about 5° from the orbit of Earth around the sun. Thus, the shadow of the moon often misses Earth and the shadow of Earth often misses the moon, and eclipses do not usually occur. It takes a special combination of the positions of the sun, moon, and Earth to produce an eclipse. Both ancient and modern astronomers were and are able to calculate and predict these eclipses.


Now when we consider Jupiter and Saturn, Jupiter’s orbit is tilted by 1.31° to the orbit of Earth; Saturn’s orbit is inclined by 2.48° to the orbit of Earth. Adding the fact that Jupiter orbits the sun every 11.86 years and Saturn does so every 29.46 years (much slower than the moon around Earth), we get some idea of why it is very rare for Jupiter to eclipse (or rather occult) Saturn. There is worse news. The planets travel in orbits that are not circular, but are rather ovals, or ellipses. These ellipses change shape ever so slightly over thousands of years, but enough to cause a change in exactly when these occultations take place.


We can express the positions of celestial objects as seen from Earth in different ways. All of these ways are somewhat like latitude and longitude on Earth’s surface. One common method is using “right ascension” (which is like longitude in the sky) and “declination” (which is like latitude in the sky). So when two objects have the same right ascension (think of Wausau and Stevens Point with the same longitude on Earth), they appear to those of us on Earth to be as close as they will get in their orbits this time around, and this is called a conjunction. Thus Jupiter and Saturn could be as far apart as about 3.8° (about 7 widths of the full moon) and still be in conjunction. By the way, this maximum separation also happens only about every 14,400 years, just like the occultations.


Although we see objects in the sky from Earth in two dimensions, we must remember that the solar system and the universe exist in at least three dimensions. Even though Saturn appears very close to Jupiter on December 21, the two planets will actually be about 450 million miles apart, with Saturn much more distant than Jupiter.  One last caveat: On December 21 it will be winter in Wisconsin (yes, the Winter Solstice, at 7:30 A.M.). The weather may not cooperate. People will take photographs of this event and post them on the Internet. If you can’t see it in person, look for it online. Meanwhile, please enjoy that which is given to us. Good seeing to you!