Tonight (Dec. 21), grab your binoculars as you'll be able to see an unobstructed view of the bright "star" that is actually two planets appearing as one blip of light in the southwest horizon.
It has popularly been known as the Christmas Star—the brilliant planets Jupiter and Saturn intersecting on the night of 21 December. The viewing is a highly vivid celestial conjunction that is readily visible in the evening sky for the next two weeks.
“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,” said Henry Throop, astronomer in the Planetary Science Division at NASA Headquarters in Washington. “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.”
Jupiter and Saturn remain hundreds of millions of miles distant in space. The planets can appear as separate objects under high magnification. Binocular or telescopic observers may see planets and moons in the same field of view. The solstice signals official winter beginnings in the northern hemisphere.
Only once in 20 years the positions of Jupiter and Saturn are aligned in the atmosphere. Almost 400 years have gone by since the planets spread so close together.
The planets will look very similar together, but will stay in space for hundreds of millions of miles. convergence is only a coinciding moment dependent on the planet's orbits and the Earth's inclination.
“Conjunctions like this could happen on any day of the year, depending on where the planets are in their orbits,” said Throop. “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.”
Here is NASA's guidance on best viewing tips:
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.
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.
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.
Cover photo: NASA/JPL-Caltech