TORONTO -- Jupiter and Saturn are expected to be closely aligned throughout the month of December, appearing from Earth as one giant celestial body or a 'Christmas star' at their closest point.

According to , Jupiter and Saturn have been gradually moving closer together since the summer in what is called a conjunction.

The space agency says conjunctions occur every 20 years between planets, with the "great conjunction" between Jupiter and Saturn being the rarest.

York University astronomy and physics professor Paul Delaney told CTVNews.ca this meeting of Jupiter and Saturn will be an "especially close one," with the planets expected to be around one-fifth of a full moon apart.

"Jupiter and Saturn [have] been putting on a fine show for several months now, but the climax will be their close approach on Dec. 21," Delaney said Thursday in an email. "On that date, they will be much closer than the apparent diametre of the Moon, a separation of 0.1 degrees."

Delaney said the two gas giants haven't been aligned this closely since 1623 and won鈥檛 be matched again until 2080.

The closeness of the planets will create a radiant point of light, often referred to as the "Star of Bethlehem" or the "Christmas Star." Unlike stars, NASA says sky watchers will be able to easily spot Jupiter and Saturn as they both shine steadily.

Dec. 21 is also the , which NASA says is when the sun reaches its southernmost position in the sky from any point on Earth.

Delaney says the rare cosmic phenomenon should be "quite a striking sight" for anyone stargazing in the late afternoon during the Jupiter and Saturn鈥檚 closest approach.

Delaney said the conjunction will also be mostly visible across the world, including in Canada, from Dec. 12-29. But those looking to catch a glimpse will have to be quick, he says.

"The snag of course is that they are visible around sunset and will set less than 2 hours later. They are only 30 degrees east of the Sun so evening twilight and the low altitude will reduce the viewing spectacle," Delaney said.

Weather permitting, Delaney said the climax of the conjunction should be "easy enough to see with the naked eye and binoculars" by looking above the western horizon. He added that the event will look like a "double planet" for those viewing with a telescope.

Despite appearing close together from the view on Earth, NASA says Jupiter and Saturn will still be hundreds of millions of miles apart in space.