Answered by Brian Tonks – BYU-I daho Astronomy Professor
The ancients noticed that seven celestial objects move relative to the stars: the Sun, the Moon, and the five visible planets (the word “planet” comes from the Greeks meaning “wanderer”—the Greeks knew them as wandering stars). Astrologers attributed special meaning to these objects. It is clear that the Sun plays an important role in our individual lives, providing the energy we need to live on this planet. The role that the other six moving celestial objects play in our lives is not as clear. These six objects move at different speeds, but they trace nearly the same path as they move through the stars. Jupiter travels faster along its path (taking just under twelve years to complete one trip) than Saturn (which takes about 29½ years); Mars, Venus, and Mercury travel much faster. Because they travel along their paths at different speeds and their paths are nearly identical, the planets will inevitably come near each other (as we view them from Earth). Conjunctions, especially between two planets, are relatively common. Conjunctions between three planets are extremely rare.
Even though Jupiter and Saturn appear to be next to one another in the sky, they are really a long way apart, just over four times the Earth’s distance from the Sun. We will see the conjunction best about one hour after sunset, fairly low in the southwestern sky. Remember that this is a two-dimensional diagram with their orbits projected onto the plane of the Earth’s orbit around the Sun. The two planets won’t be in exactly the same position vertically when they are in the same position horizontally (as we see them from our vantage point on the Earth).
Conjunctions between Jupiter and Saturn occur about every twenty years, the longest spacing of any conjunction between two of the visible planets. From a human perspective, a Jupiter-Saturn conjunction is quite rare. This year’s conjunction brings Jupiter and Saturn very close together. You should be able to see both planets in the same field of view through a pair of binoculars or small telescope. It will be the closest conjunction between these planets for a long time. This conjunction will be the best for at least the next 60 years! For most of us, it will be a once in a lifetime event, and an eye-catching sight.
On 21 December, the day of Winter Solstice, the two planets achieve conjunction. A conjunction is when two or more planets appear to approach one another in the night sky. In the days after the conjunction, the two planets will separate as Jupiter passes by. This conjunction prompts several questions including what causes this to happen, does it have any significance, and does its occurrence on the night of Winter solstice have any special meaning?
Find an observing location with a clear view of the southwestern sky. Make sure that trees, houses, and hills won’t obscure your view; the conjunction will only be about 1 fist width held at arm’s length above the southwest horizon an hour after sunset. On the 21st arrive at your spot by about ½ hour after sunset. Being that close to the SW horizon, the two planets will set within about two hours of sunset.
Some people claim that the two planets will “merge” and produce an incredibly bright “star”, similar to depictions of the Star of Bethlehem Don’t be disappointed; it won’t look like that. The two planets won’t “merge”; you will still see them as two separate and distinct star-like lights, although very close together. Combined, the two planets’ light will still be less than the brightness of Venus. Because astrologers believe that the positions of the heavenly objects directly impact our lives and this very close conjunction occurs on the first day of Winter, astrologers are predicting that unpleasant events that will occur (ancient astrologers believed that Jupiter-Saturn conjunctions signaled bad things to come). First, there is no observed correlation between Jupiter-Saturn conjunctions and bad things happening. Second, there is no cause-effect mechanism between the position of the planets in their orbit around the Sun and direct effects on the Earth. Because the two planets are aligned to the Earth, some claim that their combined gravity could pull on Earth with enough force to trigger earthquakes or other potential disasters. That is just not true. These forces are way too small to have an impact on Earth.
Hopefully the weather will be clear enough for you to enjoy this once-in-a-lifetime celestial event.
The apparent merging of these two planets is an example of FORCED PERSPECTIVE.
Similar to this age-old optical illusion with the Leaning Tower of Pisa.