On December 21, celestial giants Saturn and Jupiter appeared the closest they have been in the sky for 800 years, aligning so close they appeared as one bright light dubbed by many media outlets as the "Christmas star" (despite occurring on the winter solstice). The last time they were this close was 1623, and the last time they were this easily observable (instead of being rendered invisible by being too close to the Sun) was back in 1226.
We call it a "great" conjunction because it involves the two largest bodies in the Solar System, and to ancient observers, they were the two slowest moving planets in the sky. It takes Jupiter 11.86 years to complete a trip around the Sun, and Saturn 29.5 years, their respective orbits bringing them close together in alignment in our field of view from Earth – known as a conjunction – every 19.86 years. Due to their relatively slow movements, and the respect these gas giants inspired, ancient skywatchers dubbed it not just a conjunction, but a "great" one.
The video from the National Astronomical Institute of Thailand, shared as NASA's Astronomy Picture of the Day, features two time-lapse sequences as the planets passed within about a 10th of a degree from each other. The first sequence shows a close-up of the planets – complete with moons, cloud bands, Saturn's rings, and Jupiter's red spot all visible – shot over five days. The second sequence is zoomed out and was shot over nine days.
On their closest alignment on December 21, the planets appeared just one-fifth the diameter of the Moon, or 0.1 degrees, apart. Although they looked incredibly close to each other, the gas giants are of course billions of kilometers from each other.
They won't appear this close again for another 60 years, so mark March 15, 2080, in your diary. If you can wait that long – and medical science jumps forward in leaps and bounds – mark the year 7541, where not only will Jupiter pass in front of Saturn, it will do it twice in one year.
If a planet partially obscures another planet or the Sun it is a transit, but if it completely covers another object like a star or another planet it is called an occultation. According to Time and Date, Jupiter is expected to transit Saturn on February 16, 7541, and then on June 17, an occultation will occur and the king of planets will completely obscure its celestial dance partner.