The great conjunction the “Christmas Star”

Tonight starwatchers in Gibraltar will be treated to an event that only happens once every 19.6 years, on average, Jupiter and Saturn will appear in the same place in the night sky in an event called a Great Conjunction.

This phenomenon has been dubbed ,by some astronomers, the "Christmas Star" in reference to the celestial light that guided the Three Wise men to Jesus in the Bible's Nativity Story.

“These two gas giants of our Solar System, which are usually bright enough to see with the naked eye even from the light-polluted heart of Gibraltar, will align, as seen from Earth, to look like one extremely bright planet,” said William Recagno the President Gibraltar Astronomical Society.

“This year, the pair will be just 0.1 degrees apart in the sky.(the width of your little finger at arms length is 1 to 1.5 degree(s)), making it the closest such event since 1623.The next time these planets will be visible this close together in the night sky will be the year 2080.”

“The 2020 conjunction is especially rare — the planets haven't been this close together in nearly 400 years, and haven't been observable this close together at night since medieval times, in 1226,” he added.

He also noted that due to the fact that Jupiter and Saturn are the furthest from the Sun of all the naked-eye planets, they orbit the slowest.

It takes almost 30 years for Saturn to do a lap of the sun, while Jupiter takes about 12. This is why conjunctions between the two are the rarest of all the easily visible planets. In reality the planets are still millions of kilometres apart but from Earth’s perspective they will appear as a single bright “star”.

“You can differentiate Saturn and Jupiter from the stars because the planets will appear brighter and more solid in the night sky,” he said.

“From Gibraltar, the pair will become observable at around 6:29pm ,as the dusk sky fades, above our south-western horizon. They will then sink south west towards the horizon, setting two hours and 19 minutes after the Sun, at 8:29pm.”

Today is also the winter solstice in the Northern Hemisphere, meaning it will be the shortest day of the year in terms of sunlight.