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Great conjunction of Jupiter and Saturn, appears as bright star before Christmas

Fasi mo e Afi, Tonga

It is quite special that this year all the planets are easily visible and also in a configuration which happens infrequently. In December 2020, Jupiter and Saturn will come closer together, until their great conjunction on 22 December 7:37 Tonga time. It will be a spectacular phenomenon, a few days before Christmas. They will do so again in 2080 but then for centuries to come, not any more.

By Firitia Velt

The word 'planet' hails from the Greek where it means wandering star. Because that is exactly what (most) planets look like: bright stars in the sky, but they move compared to the so-called fixed stars.

From classical times we have known of seven planets: Moon, Mercury, Venus, Sun, Mars, Jupiter and Saturn; in the order of the speed with which they move along the sky from fast to slow.

It was also customary in old Babylonian times to assign every hour of the 24-hour day to successive planets. Thus, if the first hour was (for example) for the Moon (which would then remain 'king' for the whole day), then the second hour was for Mercury, the third for Venus, and so on. The planetary cycle would be repeated where needed, so it was easy to calculate that the 24th hour would be under the Sun again. Then the next day would start with Mars as king, and so on again, until one week later it was Moon day once more.

After these thousands of years we still use these names in this order for our weeks:

  • Monday for the Moon;
  • Tuesday for the Germanic god Ziu, the god of war and thus considered equal to Mars, the Roman god of bloody war, aptly assigned to the red looking planet Mars;
  • Wednesday for the Germanic god Wodan, the messenger of the gods, like the Roman Mercury, also apt for this swift planet;
  • Thursday for Thor, the Germanic and Norse king of gods, yielding his lightning hammer, as did Jupiter to the Romans;
  • Friday for Frija, the old Germanic goddess of love, wife of Odin; a job for the goddess Venus to the Romans.
  • Saturday and Saturn need no further explanation;
  • Sunday and Sun.

In the Romance languages, French for example, we see the same thing more clearly, where most names directly go to the Latin instead of Germanic roots.

The Solar System. Image: commons Wikipedia.

Visible planets

But back to the stars. Nowadays the Sun and Moon are no longer considered planets, but the Earth has been added to the list, taking the place of the Sun. Of course, as we sit on Earth, we do not see this planet in the sky. We saw the Sun as such. But now we know that the Earth revolves around the Sun, rather than the other way around.

After the invention of the telescope two more planets were discovered, Uranus and Neptune, as well as a multitude of lesser bodies, the planetoids. Nowadays, Pluto is also considered as a planetoid, a sub-sized planet. All of them appear as stars in the sky, but only the five classical planets (Mercury, Venus, Mars, Jupiter, Saturn) are bright enough to be seen with the naked eye.

When are these planets visible? It depends. First they have to be above the horizon. Just like the Sun or Moon, they have at any time about a 50% chance to be above or below. If they are above the horizon, it must be during night-time, because during the day the sky is too bright, so that is another halving of the half, or more than that if you take twilight into account. Furthermore, you may be prepared to quickly look up the sky at 9 in the evening, but less so at 3 in the morning. Another halving, reducing the time a planet is practically visible to maybe less than 10%.

Special year

So it is quite special that this year all the planets are easily visible and in addition in a configuration which only happens infrequently.

Let us start with Venus. The easiest and most difficult at the same time. Easy because it is by far the brightest 'star' in the sky. Difficult, because you have to wake up early in the morning before it gets light, which is about 6 o'clock. But there it is, high above the northeastern horizon, where it will remain until about the end of this year.

Mercury in September

Mercury is more difficult to find despite its brightness, as it is almost always so close to the Sun. But the coming apparition will be a favourable one: almost for the whole month of September and beginning of October it will be above the western horizon an hour after sunset. If you still cannot find it, the Moon crescent will be close on 19 September.

Mars in October

Mars at this moment is a morning star, rising in the east after midnight. But that happens every night a few minutes earlier and by October it rises in the evening, reaching overhead at midnight. If you take the effort to watch the planet during the coming months, notice that it will become brighter and brighter, as the planet is approaching the Earth, coming closest at 14 October when it will appear as a big red star as brilliant as Jupiter.

After that the distance between Mars and Earth will increase again, and the planet will become fainter. Albeit Mars will not come as close to the Earth as it did in 2003, it still comes about as close as it ever can get. You will have to wait 15 years to see it like that again.

Jupiter and Saturn

Meanwhile, you may have noticed for the last few months, a bright star high in the east in the early evening with 5° below it (that is half a hand-width or 10 moon diameters) a somewhat less bright one. Having a quick look outside by the time you go to bed (assuming you are not a night owl), they are straight above your head. Jupiter and Saturn. And by the way, in between them you can see Pluto; if you have telescopic eyes that is, and a big telescope for that matter. Likewise, with such eyes you could see Neptune in between Saturn and Mars, and beyond Mars you could see Uranus. All planets accounted for.

Great conjunction in December

But keep on watching Jupiter and Saturn in the months to come. They will rise earlier every night and move westwards, and they also will come closer together. Until their great conjunction on 22 December 7:37 Tonga time. It will be a spectacular phenomenon. Even though by then their visibility will have been reduced to evening twilight only (so look on the evening of the 21st and 22nd).

The great conjunction - the name given to a conjunction of Jupiter and Saturn. Jupiter revolves around the Sun in almost 12 years, Saturn needs almost 30. So you can easily calculate (1 / (1/12 - ⅓0)) that it takes 20 years before Jupiter overtakes Saturn. Not a daily occurrence, and not all great conjunctions are equally favourable to see. The previous one, 1 June 2000 took place when the planets where very close to the Sun in the morning sky. Nevertheless, I remember that I rose early that morning and saw them there close together in the twilight. Well, not really that close together, they were more than two moon diameters apart (1°11'), but still.

The happening in 1981 was in one way not much better, the planets never came closer than two moon diameters, but at least it happened around the opposition, which gave rise to what one calls a greatest conjunction. Due to the opposition loops there was not one but three conjunctions over the timespan of several months. Such triple conjunctions only happen once in a century, although there were two in the previous one (the other in 1940) and there will be none in this century.

But what makes this year special is the distance. The two planets will pass each other on a distance of only 6', that is one fifth of a moon-diameter.

They will do so again in 2080 but then for centuries to come not anymore; or for centuries in the past for that matter.

South Pacific

Unfortunately, we will miss the closest approach, to see that you would have to live in Africa, as far away from Tonga as you can get. But do not mope too much, the planets will remain closer together than the size of the Moon for at least a week around the date. It will also be worthwhile looking on 17 December when the moon crescent passes them on 3° distance, a conjunction best seen in the South Pacific.

Star of Bethlehem

This great conjunction - a few days before Christmas. We can now already predict that a lot of nonsense about the Star of Bethlehem will fill the social media at that time. Yes, there was a greatest conjunction in 7 BC, centred around September. Not really the right moment, and it was also not really close; the planets remained as far away from each other as they did in 1981 or 2000. No, it is unlikely that this would have excited Babylonian priests to undertake a trip to Jerusalem for a phenomenon that they could see every 20 years.

The one of 26 BC was much better with the planets a little less than a moon-diameter apart and as extra bonus both Venus and Mars close by. It was two moon-diameters for the great conjunction in 14 AD, but which interestingly was on 25 December (Julian calendar).

Firitia Velt is an astronomer and science lecturer, who resides in Tonga