We’re less than a week away from a spectacular celestial event that hasn’t been so clearly visible for centuries, and skywatching experts say it’s not too early to start looking for the bright spectacle of Jupiter and Saturn drawing nearer to each other.

In what’s called a “great conjunction,” the two largest planets in our solar system will appear closer to each other in the night sky than at any point in almost 400 years.

“Witnessing this special event couldn’t be easier,” says Gary Seronik, a consulting editor at Sky & Telescope magazine. “On Monday, December 21st, which coincidentally marks the solstice, simply step outdoors roughly 45 minutes after sunset, face southwest, and look about 15 degrees above the horizon — that’s a span slightly greater than the width of your fist seen at arm’s length.”

Of course, there’s no guarantee the weather will cooperate on that night, so it’s worth checking out the sight even a few nights before or after the 21st, when the planetary pair will still appear within less than a moon’s diameter on one another.

“So start watching now, tonight — you won’t want to miss every opportunity you can get to witness this amazing sight,” Seronik adds.

This also helps explain how it gets the name the “Christmas Star,” with the event happening so close to the holiday and serving as one possible explanation for the biblical Star of Bethlehem.

The next conjunction isn’t until 2040 and the planets will be further apart then compared to this year.

When you head outside in the evening to spot the planets, Jupiter will appear far brighter, with Saturn above and to the right a bit.

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Virginia Tech astronomer Nahum Arav says it may even be a bit tough to pick apart the two when they are at their closest.

“The two planets can easily be seen with the naked eye. They will be very close to each other, about a fifth of the moon’s diameter. At their closest, some people will need a binocular to separate them.”

It’s all a trick of cosmic perspective, of course. In reality the planets will be around 450 millions miles apart from each other.

Arav also recommends planning at least an hour for a thorough viewing on the 21st if you have clear skies and a telescope or binoculars. It could be possible to make out Jupiter’s four Galilean moons and Titan, the largest moon circling Saturn with an amateur telescope.

The two planets haven’t looked this close from our vantage point since 1623, and even then their positioning closer to the sun wasn’t ideal. So really, this is the most spectacular this conjunction has been from our view since probably 1226.

The next opportunity won’t be quite such a long wait, as the next conjunction is set for 2080.