After nearly 400 years, Saturn and Jupiter – the two largest planets in our solar system – will be brought closest in the night sky by an astronomical event called the “great conjunction” and popularly referred to as the “Christmas Star”.

Try this PYQ:

What is a coma, in the context of Astronomy?

(a) Bright half of material on the comet

(b) Long tail of dust

(c) Two asteroids orbiting each other

(d) Two planets orbiting each other

What are the Conjunctions?

• A conjunction is not unique to Saturn and Jupiter however, it is the name given to any event where planets or asteroids appear to be very close together in the sky when viewed from the Earth.
• In June 2005 for instance, as a result of the “spectacular” conjunction, Mercury, Venus and Saturn appeared so close together in the sky that the patch of sky where the three planets were could be covered by a thumb.
• Astronomers use the word “great” for the conjunction of Jupiter and Saturn because of the planets’ sizes.

The “Great Conjunction”

• It happens once in about 20 years because of the time each of the planets takes to orbit around the Sun.
• Jupiter takes roughly 12 years to complete one lap around the Sun and Saturn takes 30 years.
• This is because Saturn has a larger orbit and moves more slowly because it is not as strongly influenced by the Sun’s gravitational force as planets that are closer to the Sun.
• As the two planets move along their orbits, every two decades, Jupiter catches up with Saturn resulting in what astronomers call the great conjunction.

A ‘rare alignment’

• Jupiter and Saturn are bright planets and can be typically seen with the naked eye even from cities.
• But during conjunction, they appear to be close to each other, which is what makes the event noteworthy.
• The event will coincide with the winter solstice (shortest day of the year in terms of hours of sunlight received) in the Northern Hemisphere and summer solstice in the Southern Hemisphere.
• This year, however, the event is rare because the planets will come the closest to each other in nearly four centuries; in what astronomer Henry Throop described is a result of a “rare alignment” of the planets.

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