CHICAGO (NewsNation Now/Nexstar) — Go ahead and mark December 21 on your calendar. That’s when we can all witness something not seen in nearly 800 years.

During the upcoming winter solstice, Jupiter and Saturn will line up to create what is known as the “Christmas Star” or “Star of Bethlehem.”

These two planets haven’t appeared this (relatively) close together from Earth’s vantage point since the Middle Ages.

NewsNation team talked to a NASA astrophysicist about the phenomenon. Dr. Amber Straughn broke down what’s happening and what you need to do to ensure you can see it. Straughn explained how NASA tracks the history of phenomenon’s like the “Christmas Star.”

“The solar system works like clockwork,” said Straughn. “We can track how the planets move in the sky. And from that we can run the clock backwards and figure out how long ago the planets were lined up like this.”

Stargazers in the northern hemisphere should turn their heads and telescopes to the southwest portion of the sky about 45 minutes after sunset to see the planets align Dec. 21. However. appearances from the phenomenon could be seen throughout the week.

Straughn said the next time we could expect this occurrence is in the 2080s.

“This really is a once in a lifetime occurrence. The planets line up fairly close together, Jupiter and Saturn, about every 20 years,” said Straughn. “But they haven’t been this close together in 800 years. The next time they will be this close together is about 60 years from now.”

You can watch the full interview with Straughn in the player above.