Jupiter and Saturn have actually been incrementally approaching since the summer of this year.
Professor Hartigan revealed how between December 16 to 25, the duo will be separated by less than the diameter of a Full Moon.
He said: “On the evening of closest approach on Dec 21 they will look like a double planet, separated by only a fifth the diameter of the Full Moon.
“For most telescope viewers, each planet and several of their largest moons will be visible in the same field of view that evening.”
How to see the December 2020 Jupiter and Saturn double planet?
Astronomers agree the best conditions for viewing the double planet phenomenon will be close to the equator.
However, the event will be observable from everywhere on Earth – weather-permitting.
Professor Hartigan confirmed the planetary duo will continue to loom low in the western sky for approximately an hour after sunset each evening.
He said: ”The further north a viewer is, the less time they’ll have to catch a glimpse of the conjunction before the planets sink below the horizon.”