Observing and reading are lovely ways to learn astronomy, but doing some maths can be equally enjoyable.

"Jupiter-Saturn conjunction" in December involves Jupiter and Saturn, which have different orbital speeds. When they will meet again after the "goodbye kiss" in 2020? Let's find out by doing some simple calculations.

It takes Jupiter and Saturn approximately 12 and 30 Earth years to complete their orbits around the sun respectively. Planets' orbits are elliptical, but the orbits of Jupiter and Saturn have such a small eccentricity that they are more or less circular. Each year, Jupiter advances about 30° (360°/12) while Saturn about 12° (360°/30) along their orbits.

We want to know "when they will meet", we could think of "Jupiter pulling ahead of Saturn by 18° (30°-12°) in a year". Additionally, as they will meet again as soon as Jupiter completes a circle more than Saturn (i.e. 360°), the calculation becomes straightforward.

The answer of "When will they meet again?" is: 360°/18° = 20 Earth years later!