Dec. 21, 2020 – If skies are clear tonight, make sure to go outside about one hour after sunset and look to the southwest. Jupiter and Saturn will appear to meet in the sky as one very bright spot. The rare celestial event is called a “Great Conjunction.” Being able to experience it will definitely be a bright spot in a very challenging year.

Christmas Star and Winter Solstice
According to NASA, the “Great Conjunction” is popular also as the Christmas star. Christians celebrate Christmas as the birth of Jesus, the son of Mary and Joseph. As Mary and Joseph were walking to Bethlehem the night Jesus was born, a bright star showed them the way. Was it a Jupiter-Saturn planetary conjunction?

Today is also the Winter Solstice in the Northern Hemisphere, officially turning the calendar from fall to winter. It is the shortest day in terms of sunlight because of Earth’s tilt as it orbits around the Sun, a star.

In the Southern Hemisphere, today is the Summer Solstice. People living in Australia can swim outdoors during the holidays! Again, it’s because Earth tilts on its axis as it orbits around the Sun.

Great Conjunction of 2020: Information from NASA

In 1610, Italian astronomer Galileo Galilei pointed his telescope to the night sky, discovering the four moons of Jupiter – Io, Europa, Ganymede, and Callisto. In that same year, Galileo also discovered a strange oval surrounding Saturn, which later observations determined to be its rings. These discoveries changed how people understood the far reaches of our solar system.

Thirteen years later, in 1623, the solar system’s two giant planets, Jupiter and Saturn, traveled together across the sky. Jupiter caught up to and passed Saturn, an astronomical event known as a “Great Conjunction.”

“You can imagine the solar system to be a racetrack, with each of the planets as a runner in their own lane and the Earth toward the center of the stadium,” Henry Throop, NASA astronomer, said on the agency’s website. “From our vantage point, we’ll be able to be to see Jupiter on the inside lane, approaching Saturn all month and finally overtaking it on December 21.”

2020 Great Conjunction – rare celestial event
According to NASA, it’s been nearly 400 years since the planets passed this close to each other in the sky. And it’s been nearly 800 years since the alignment of Saturn and Jupiter occurred at night.

The closest alignment will appear just a tenth of a degree apart and last for a few days. On the 21st, the two planets will appear so close that a pinkie finger at arm’s length will easily cover both planets in the sky. The planets will be easy to see with the unaided eye by looking toward the southwest just after sunset.

“Conjunctions like this could happen on any day of the year, depending on where the planets are in their orbits,” Throop said. “The date of the conjunction is determined by the positions of Jupiter, Saturn, and the Earth in their paths around the Sun, while the date of the solstice is determined by the tilt of Earth’s axis. The solstice is the longest night of the year, so this rare coincidence will give people a great chance to go outside and see the solar system.”

NASA tips for viewing the “Great Conjunction”

1. Find a spot with an unobstructed view of the sky, such as a field or park. Jupiter and Saturn are bright, so they can be seen even from most cities.
2. An hour after sunset, look to the southwestern sky. Jupiter will look like a bright star and be easily visible. Saturn will be slightly fainter and will appear slightly above and to the left of Jupiter until December 21, when Jupiter will overtake it and they will reverse positions in the sky.
3. The planets can be seen with the unaided eye, but if you have binoculars or a small telescope, you may be able to see Jupiter’s four large moons orbiting the giant planet.

People living in areas covered in clouds can access online coverage of the rare show in the sky. It will be trending on social media no doubt with pictures and descriptions.

REVIEW QUESTIONS

1. What is the radius of Saturn, Jupiter and Earth?

2. What is a “Great Conjunction” of Jupiter and Saturn?

3. Who discovered Jupiter’s four moons in 1610?

INQUIRY QUESTIONS

1. Why is today’s Great Conjunction of Jupiter and Saturn such a rare celestial event?

2. Why can people living in Australia swim outside in December?

3. Compare, in quantitative terms, the size of the three planets mentioned in the story – Jupiter, Saturn and Earth. Convert kilometers to miles.

4. Compare the photos in the story of the Great Conjunction taken on Dec. 13, 2020 and Dec. 20, 2020. How do the photos support the text? What would you expect a photo taken on Dec. 22, 2020 to look like?