Jupiter and Saturn align to create the first 'Christmas star' in 800 years

For anyone who loves stargazing and studying planets, the winter solstice—December 21, 2020—is a date you want to pen down in your diary. The Solar system's largest two planets —Jupiter and Saturn —will be aligned from the Earth's point of view, becoming a super-bright point of light called the "Christmas Star" or "Star of Bethlehem." It'll be the first time since the Middle Ages that this phenomenon is happening, reported The Forbes. This is the first time it's happening in nearly 800 years. For the people on Earth, it'll appear as if the large planets are colliding but in reality, they won't be close at all. The distance between planets is often measured in astronomical units (au), which is roughly the distance between the Earth and the Sun. One astronomical unit is roughly 93 million miles and the Jupiter is 5 au from us while Saturn is 10 au from us. 

With the Earth orbiting the Sun, the planets will appear aligned from the Earth's point of view on December 21, the first time it can be viewed since the year 1226. Jupiter and Saturn aligned in 1623 but its proximity to sunset made its visibility unlikely. “Alignments between these two planets are rather rare, occurring once every 20 years or so, but this conjunction is exceptionally rare because of how close the planets will appear to be to one another,” said Patrick Hartigan, an astronomer at Rice University. “You’d have to go all the way back to just before dawn on March 4, 1226, to see a closer alignment between these objects visible in the night sky.” When two objects line up in the sky, it's called a conjunction. People will be able to see the rare conjunction from anywhere on Earth as long as they have clear skies. The next conjunction between the largest planets similar to this one will next happen on March 15, 2080. According to EarthSky, Jupiter and Saturn meet every 20 years, but on Dec. 21 they will be just 0.1° apart. 

Saturn is the farthest and slowest-moving planet in the solar system visible to the naked eye, followed by Jupiter. While it takes Saturn 30 years to orbit around the sun, it takes Jupiter 12 years. Given the time it takes for them to orbit the sun, such conjunction of the biggest planets are rare. According to EarthSky, the conjunctions between Jupiter and Saturn happened or will happen on:

May 28, 2000
Dec. 21, 2020
Oct. 31, 2040
April 7, 2060
March 15, 2080
Sept. 18, 2100

Jupiter and Saturn’s orbital resonance means they align in a “great conjunction” every 19.6 years, but it happens rarely from the point of view of the Earth. On December solstice, the distance between Jupiter and Saturn will be less than the apparent diameter of a full Moon. As legendary German astronomer Johannes Kepler once suggested, the “star of Bethlehem” in the story of the Magi or “three wise men” might have actually been rare triple conjunction of Jupiter, Saturn, and Venus.

You can look for the celestial event throughout the month but they will be closest on December 21, 2020. After sunset, the planets can be seen low in the western sky for an hour after sunset when viewed from the northern hemisphere. The event can be seen sinking towards the horizon but will be bright enough to be viewed in the twilight. If you have clear skies and look southwest, you will be able to view the celestial event within 45 minutes after sunset. And those using a telescope will be able to all planets and their largest moons in the same field of view that evening on December 21.