This coming Monday, December 21st, skywatchers are in for a spectacular sight in the night sky- a rare planetary alignment that hasn’t been seen in the night sky in nearly 800 years!
This astronomical event has gotten a lot of press recently as it happens to fall on the winter solstice, even though this is just a coincidence.
The solstice is determined by the Earth’s tilt, while this planetary alignment is made possible by Jupiter’s, Saturn’s, and Earth’s orbits. Consider a circular racetrack- the cars in the innermost lanes are the fastest, passing those in the outer lanes often. But eventually all of the cars will line up as their different lap times match. Our solar system behaves the same way with Earth, Saturn, and Jupiter’s orbits lining up roughly every 20 years. The last time this happened was in 2000, another alignment (called a conjunction) will happen again in 2040.
But then how has this not happened in 800 years?
A conjunction of Jupiter and Saturn will happen every 20 years but this event, called “the great conjunction,” is special as both planets will be closer in our night sky than they have been in nearly 400 years.
The last time the two planets were this close together was back in 1623 when Galileo was studying the heavens. In fact, this event allowed him to determine that Jupiter had several moons by observing the planet through a rudimentary telescope. You’ll be able to see the same Monday night if you happen to have a telescope or binoculars on hand. This conjunction, or “meeting,” of the planets was obscured by the sun’s glare, which won’t be a problem as this will be the closest meeting of Saturn and Jupiter to occur at night in nearly 800 years.
Look to the southwestern sky just after sunset Monday for a bright “star.” This bright dot, the brightest object in the night sky other than the moon, will be both Jupiter and Saturn- separated by less than 0.1 of a degree or roughly one-fifth the diameter of a full moon. It’ll be visible through most of the night if weather permits. Both planets will be close to one another in the night sky for the remainder of the year but will slowly drift away from one another as each planet, including our own, races around the sun at different speeds.
Storm Team 2 Meteorologist David Dickson