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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

Saturn from Cassini Orbiter. Credit: NASA/ESA/ASI

Jupiter from Hubble's Wide Field Camera 3

Jupiter from Hubble's Wide Field Camera 3. Credit: NASA/ESA

Predicting these Events

The models of the Solar System have shaped how accurately we can predict such events. From Ptolemy’s geocentric view, where everything revolved around Earth in perfect circles (whilst ‘epicycles’ or smaller orbits made by objects were made, explaining retrograde motion -the backwards and forwards motion of objects in the sky); to Copernicus’ heliocentric view, with everything revolving around the Sun, again in perfect circles; to the modern day Kepler view, with objects orbiting around the Sun in an elliptical path (explaining retrograde motion) [11].

Using Tyco Brahe’s observations, Kepler made laws which governed the motion of objects around the Sun. His first described how all planets’ orbits are elliptical (ellipses having two points or foci, whereby the sum of the distances from the foci and edge of the ellipse is constant), with the Sun at one focus.

His second was that, in equal time periods, the area of the ellipse covered by an imaginary line between the planet and Sun was constant (showing that planets speed up their orbit the closer to the Sun they are).

The last law was that the square of the orbital periods (the time it takes for a planet to orbit the Sun) is directly proportional (increasing together at constant ratio) to the cube of the orbit’s semi-major axis (the furthest distance from the edge of the ellipse to its centre) [12].

All these laws put together, plus some extra maths from more recent scientists like Einstein, have brought about the accurate predictions of where everything will be at different points in time in the sky, so we can know when and where to look for phenomena like the Great Conjunction, to enjoy what the universe brings us.

Events like the nearby Great Conjunction are truly once in a lifetime (or twice if you’re as young as I am), so they’re truly unmissable events if you have the chance to observe them.

That said, if the sky doesn’t clear or the horizon is too high, watch the live streams by Prof. Matthew Bate of the University of Exeter’s Astrophysics Group both on Saturday (today) and Sunday on YouTube (happening at 4:15-6:15pm).

Elliptical Orbit of Planets around the Sun (Kepler's 1st Law)

Elliptical Orbit of Planets around the Sun (Kepler's 1st Law).

Credit: Arpad Horvath, CC BY-SA 3.0

by George Abraham, ADAS member.

Click here for the previous news article

Click here for the next news article

Click here to see where Jupiter is currently using Stellarium (or find it yourself using their app)

Click here to see Jupiter and Saturn’s location using The Sky Live’s planetarium

Click here to see the live sky view (from Timperley Village Club) on Heavens Above, to see Jupiter and Saturn, as well as any satellites, so your pictures are clean of satellite streaks.

Click here to see predictions for future lunar occultations

Click here to see the weather forecast, to plan for your observation (and here for this and other widgets for help with planing your astronomy activities)

Click here to see predictions of future eclipses and solar transits

Click here to see Kepler's laws along with Einstein's and Newton's laws in action in this simulation (and here to see other useful resources like this one)



  1. "What is a planetary conjunction". Royal Museums Greenwich. Archived from the original on 19th December 2020.

  2. "Bright Planet & Asteroid Occultations by the Moon for 2021". International Occultation Timing Association. Archived from the original on 19th December 2020.

  3. "What are Eclipses and Transits?" Archived from the original on 19th December 2020.

  4. "Eclipses and Transits Visible". Archived from the original on 19th December 2020.

  5. "The Great Conjunction: History in the Making". December 2020 The Sky at Night Magazine, ISBN 9-771745-986058-12>

  6. "Jupiter and Saturn meet in close 'great conjunction' since 1623". The Guardian. Archived from the original on 19th December.

  7. "The December 2020 Great Conjunction". Archived from the original on 19th December 2020.

  8. "Sunrise, Sunset, and Daylength, December 2020". Archived from the original on 19th December 2020.

  9. "Jupiter". Stellarium Web. Archived from the original on 19th December 2020.

  10. "The 'Great' Conjunction of Jupiter and Saturn". NASA. Archived from the original on 19th December 2020.

  11. "Geocentric and Heliocentric Models". Archived from the original on 19th December 2020.

  12. "Orbits and Kepler's Laws". NASA. Archived from the original on 19th December 2020.

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