The two largest planets in our solar system, Jupiter and Saturn, are separated by an average distance of over 600 million kilometres at their closest – more than four times the distance between Earth and the Sun. Even at distances of around 700 million kilometres and 1.2 billion kilometres from Earth respectively, Jupiter and Saturn appear as bright pinpoints of light in our night skies, currently appearing in our south-western skies just after sunset.
This year, skywatchers the world over shall be treated to a great conjunction of the two gas giants. Although the planets are far apart, they will appear to approach each other from our point of view on Earth’s surface. On December 21, Jupiter and Saturn will appear closer to each other in our skies than they have appeared since July 16, 1623, just a few years after Galileo’s famous observations of the four largest moons of Jupiter. Even then, however, the two planets were located much closer to the Sun from our point of view, making it extremely likely that the conjunction was not seen by many. Indeed, the last truly observable great conjunction with the two planets getting this close to each other occurred way back in the year 1226!
Great conjunctions of Jupiter and Saturn, although not as spectacular as the one in 2020, occur rather frequently – approximately once every 20 years. These conjunctions occur because of Jupiter’s and Saturn’s differing orbital distances and speeds. Jupiter orbits the Sun once every 11.9 years, while Saturn takes 29.4 years to complete one orbit around the Sun. For this reason, Jupiter ‘catches up’ with Saturn in its orbit, and overtakes it, appearing to get closer to Saturn before once again getting farther away.
The phenomenon can therefore be observed to some extent throughout the entirety of the upcoming weeks! Even now, the two planets are relatively close to each other in our evening skies.
As the month progresses, the angular distance between the two planets will narrow from around two degrees to 0.06 degrees on December 21, before slowly getting farther apart again.
This means that they will gradually appear to get closer to each other over the course of the entire month, turning all clear December evenings into opportunities to observe this event unfold. Being bright planets, being in a dark location is not a necessity to observe this phenomenon – an unobstructed view of south- western skies will be enough!
Josef Borg is currently a PhD student within the Institute of Space Sciences and Astronomy, University of Malta, and also the president of the Astronomical Society of Malta.
Did you know?
• Jupiter has some of the largest storms in the solar system. In particular, the Great Red Spot is a massive anticyclone, currently around 1.5 times the size of Earth, which has been observed for over three centuries. The persistent storm is estimated to have been significantly larger in the past, but seems to have shrunk over time. New storms appear and disappear over time in Jupiter’s cloud tops!
• Galileo’s observations of Jupiter’s moons date back to the early 17th century. With the observation of Ganymede, Europa, Io and Callisto, Galileo confirmed that bodies in the solar system can orbit bodies other than Earth, thus bringing the start of the true end for geocentrism. Interestingly, his observations of the moons of Jupiter also led him to unknowingly observe Neptune, which he marked as a background star! Neptune was then discovered much later in the 19th century.
• You don’t need a telescope to observe the five closest planets to Earth! Mercury, Venus, Mars, Jupiter and Saturn are all visible to the naked eye from most locations, when the planets are not too close to the Sun in the sky. Mercury is probably the hardest to observe of the five, since it is always relatively close to the Sun in the sky, appearing only briefly just after sunset or just before sunrise.
For more trivia, see: www.um.edu.mt/think.
Sound bites
• The Sun fires off its biggest solar flare in more than three years on November 29: The solar flare, which is a sudden, bright explosion of electromagnetic energy, measured as an M4.4 on the scale astronomers use for sun storms. M-class flares are medium-sized eruptions (compared to small C-class flares and large X-class flares) and rank on a scale from one to nine, with larger numbers representing stronger flares.
Source: https://www.space.com/sun-fires-off-solar-flare-m4-worst-three-years
• China’s Chang’e 5 poised for historic moon landing to collect lunar samples: The Chang’e 5 spacecraft launched on November 23, intends on becoming the first mission to bring lunar samples to Earth since 1976. The mission reached lunar orbit on November 28. According to China’s state-run news agency Xinhua, the mission’s orbiter/return vehicle and its lander/ascender vehicle separated in lunar orbit on November 29. That move sets the stage for a landing near the peak of Mons Rümker, a mountain in the Oceanus Procellarum (‘Ocean of Storms’) region of the moon.
Source: https://www.space.com/china-chang-e-5-moon-lander-separates-from-orbiter
For more sound bites, listen to Radio Mocha every Saturday at 7.30pm on Radju Malta and the following Monday at 9pm on Radju Malta 2 https://www.fb.com/RadioMochaMalta/