The evening sky over the Northern Hemisphere treated stargazers to a once-in-a-lifetime illusion on Monday as the solar system's two biggest planets appeared to meet in a celestial alignment that astronomers call the “Great Conjunction”.

The rare spectacle resulted from a near convergence of the orbits of Jupiter and Saturn that happened to coincide with Monday's winter solstice, the shortest day of the year.

For those able to observe the alignment in clear skies, the two frozen-gas spheres appeared closer and more vibrant — almost as a single point of light — than at any time in 800 years.

A conjunction of the two planets takes place about once every 20 years. But the last time Jupiter and Saturn came as close together in the sky as on Monday was in 1623, an alignment that occurred during daylight and was thus not visible from most places on Earth.

The last visible great conjunction occurred long before telescopes were invented, in 1226, halfway through construction of the Notre Dame cathedral in Paris.

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Header image: Planets Jupiter and Saturn (C, top) are seen above the Los Angeles skyline during the great conjunction as seen from the Griffith Observatory on the same day as the winter solstice on December 21. — AFP