Unless you have been living under a rock, you have probably heard there will be a “Great Conjunction” between Jupiter and Saturn on Dec. 21. Jupiter and Saturn are already very close to each other in the southwestern sky at sunset, and on Dec. 21, they will appear within 0.1 degree of each other. That is about one-fifth of the diameter of the moon.
Jupiter orbits the sun in just less than 12 years. Saturn is about twice the distance from the sun as Jupiter, and it orbits in just less than 30 years. This means Jupiter laps Saturn every 19.86 years, and they appear close together in the sky from our perspective.
Because both orbits are slightly tilted with respect to the orbit of Earth, the conjunctions vary in their closeness. This effect of tilted orbits is exactly the same reason we don’t get solar eclipses every month. This conjunction is the closest one since 1623 and will be the closest one in any of our lifetimes. In the year 7541, Jupiter will actually pass directly in front of Saturn in an exceptionally rare eclipse.
The time a planet takes to orbit the sun is a direct result of Newton’s law of gravity. By the way, Isaac Newton (1642-1726) developed that, along with calculus and optics when he was sent home from college to isolate because of the Great Plague of London in 1665. What did you do when you were sent home to isolate because of a pandemic?
Newton was trying to understand the reasons for the observed motion of the planets in a more fundamental way. His predecessors Copernicus (1473-1543), Galileo (1564-1642) and Kepler (1571-1630) had advocated for and sometimes been persecuted for the heresy that the sun was the true center of the solar system.
Kepler’s very accurate laws of planetary motion that Newton was trying to understand were empirical. In other words, they provided an accurate description, but without any underlying theory as to why that was the way things worked.
Kepler’s first law was the revolutionary statement that orbits were actually elliptical, rather than circular, and that the sun was at one focus of the ellipse. As an aside, a circle is a special kind of ellipse, in the same way that a square is a special kind of rectangle, which means planetary orbits can in fact be circular. Most are very nearly circular.
This monthJan. 2 is Earth’s perihelion. That is when Earth’s elliptical orbit will bring it closer to the sun than any other day of the year. This bit of trivia debunks the misconception that it is colder in the winter because the sun is farther away and supports the reason for the seasons as being the tilt of the Earth’s axis. If the sun doesn’t look much bigger at this time of year, it is because our ellipse is so close to being a circle.
If you don’t know where to look for Jupiter and Saturn, they are the brightest things in the southwestern sky at dusk. At the conjunction, they will be close enough that they might be hard to resolve into two objects with your naked eye. If you have access to a telescope, this is the time to use it, as even the smallest telescopes can show the Galilean moons of Jupiter and the rings of Saturn in the same field of view.
Mars is also very bright in the southeastern sky and makes another excellent telescope target.
The Geminid meteor shower, peaking Dec. 13 and Dec. 14, and the Quadrantid meteor shower, peaking Jan. 2 and Jan. 3, are a couple of the best annual showers. The new moon Dec. 14 should make the Geminids extra good, but the full moon Dec. 30 will likely make the Quadrantids harder to see.
Charles Hakes teaches in the physics and engineering department at Fort Lewis College and is the director of the Fort Lewis Observatory. Reach him at email@example.com.