When Two Giants Align

The mWhat is a Conjunction?


A conjunction is where two celestial objects come close together in the sky. When specifically looking at planetary conjunctions, they’re due to the elliptical orbits, slightly inclined planes of each orbit, and each object’s varying time taken to complete this orbit [1].


However, if the objects come too close to each other, as seen from Earth, one can partially or fully cover the other, leading to an occultation: something usually observed between the Moon and another object behind, because of the Moon’s size, also making a fun target for astronomers to image, with the next lunar occultation happening on 17th April 2021 by Mars (but only visible in SE Asia) [2].


Then, there are eclipses, where the Sun, Earth and Moon align in various ways. Firstly, a lunar eclipse involves the Earth sandwiched between the Sun and Moon, meaning the Earth blocks the Sun’s light from reaching the Moon, instead going through the Earth's atmosphere, creating a red tinge on the Moon’s surface [3], with the next total lunar eclipse (where the alignment is perfect) visible from Timperley Village Club happening on 20th December 2029 [4].


Conversely, solar eclipse involve the Moon sandwiched between the Earth and Sun, blocking the Sun’s light out from reaching Earth [3], with the most recent one seen in Argentina and Chile on 14th December 2020, and the next partial solar eclipse (where the Sun is partly obscured), visible from Timperley Village Club, happening on 10th June 2021, whilst the next total solar eclipse (where the Sun is partly obscured) will be seen on 14th June 2151 (although total solar eclipses can be seen in the near future if you’re not immortal and can travel) [4].


Then, there are transits. When specifically looking at solar transits, an inferior planet (a planet orbiting the Sun closer than we are) crosses in front of the Sun, seen as a dot travelling across the Sun [3] (if using the correct safety equipment of course), with the next visible from Timperley Village Club being a transit of Mercury on 13th November 2032 (although the next Venus transit visible from here will be on 8th December 2125, being much rarer) [4]. However, there are also planetary transits, whereby a planet would partially obscure another (with Jupiter transiting Saturn next on 17th June 7541… sit tight!) [7].


ESO Very Large Telescope, Moon, Venus and Jupiter in 2009

ESO Very Large Telescope, Moon, Venus and Jupiter in 2009.

Credit: ESO/Y. Beletsky, CC BY 4.0


The Great Conjunction


That said, our main focus right now is what’s called a “Great Conjunction” being a conjunction of Saturn and Jupiter: the two largest planets in our Solar System. It’s especially special due to the separation of the two planets, being just 0.1º (with the next Great Conjunction happening in 2040 incurring a larger 1.1º separation.


Moreover, Great Conjunctions only happen every 19.6 years, and a separation of this amount famously hasn’t been seen since 1623, as documented by Kepler in his book “De Stella Nova” [5]


This dramatic and close proximity to one another in the night sky means that, instead of seeing the two planets as two points of light with the naked eye, they will be seen as one object, until a low power telescope or pair or binoculars are used [6].


And, it’ll be worth it, since the next time Jupiter and Saturn will appear less that 0.2º apart will be 2080, and then 2417 (with these events happening in pairs, as seen with the pair of events in 1623 and 1683) [7].


Illistration from Kepler's De Stella Nova (1606) of great conjunctions 1583-1723

Illustration from Kepler's De Stella Nova (1606) of great conjunctions 1583-1723


Observing the Wonder


You may have seen the classic trick played on astronomers, where the weather turns cloudy as soon as a once-in-a-lifetime opportunity to observe something arrises. However, if you want to be in with a chance of seeing this, make sure you’re available each day from now until 28th December [7], between 5pm and 6pm [8][9].


Then, if it clears, look toward the southwestern horizon, and, if there aren’t any tall trees or houses in the way, you should see a bright point in the sky: Jupiter and Saturn [9]. Then, just as Galileo Galilei did in 1610, just before the Great Conjunction of 1623, you can use a pair of binoculars (large 20x80s would be best, with a wide field of view and high magnification [5]) or a small telescope to see the four Galilean moons: Io, Europa, Ganymede and Callisto [10].


Sadly, they will only be visible during the twilight hours, since, during the Great Conjunction, happening on the 21st December at 6:37pm (but visible only from 4:30pm till 6pm) [6], will only be visible in full darkness on 21st at 5:52pm with just 2º of elevation [5]. That said, the twilight could make images look that bit more dramatic if imaged with a wide field-of-view and a good background.

Saturn as viewed by Cassini Orbiter