As Jupiter Approaches Saturn

I got home about 11 last night from work, and it was clear (the sky is taken from Heavens Above, as are all charts in this):

Jupiter was sinking in the west, while Saturn was getting higher in the east.

And, as the decade unfolds, they will be getting closer.

A bit of maths. If one planet takes A time units (days, years, centuries) to go round the Sun, and another one takes B time units, then the number of time units (C) that it takes for them to line up compared to the Sun is given by:

1/C = |(1/A - 1/B)|

The vertical lines are there to denote the absolute value of something, e.g |1| and |-1| are both 1.

Jupiter takes 4,332.59 days to go round the Sun, and Saturn takes 10 759.22 days. So, the values we have are:

1/A = 0.000 230 809

1/B = 0.000 092 944

This gives us:

1/C = 0.000 137 865

And hence:

C = 7,253.47 days

Using an average year of 365.25 days, this works out as about 52 days short of 20 years.

This isn't going to be exact as the planets do not orbit the Sun in nice, regular circles, but in ellipses, travelling faster when nearest the Sun (perihelion) and slowest when furthest away (aphelion). In addition, the Earth is itself in motion around the Sun, but 19 to 20 years should be a ball park figure.

And if you're a cosmologist, getting the correct date a billion years or so out is accurate enough for you.

The last time that Jupiter and Saturn lined up was 31 May 2000, when they were low down before dawn, but they were close for several weeks either side. The day that I really remember was 6 April 2000, when I was living in St Andrews (sky at 9pm):

Mars and Jupiter were so close together it is hard to separate them on the chart. Add in the Moon and Saturn and you have the second most impressive sight in the sky that night - at the time I had no idea that only a couple of hours later I would witness the most impressive display of aurora borealis I have ever seen.

Nature puts on a free show and most people are walking along with heads slightly down not noticing it.

So, we should look for around 2020 for the next time Jupiter and Saturn are close again.

One way we can consider this is to look at the constellations the planets are in - and I mean the real constellations in the sky, not the "starsigns" you see in magazines.

Date	Jupiter	Saturn
currently	Taurus	Libra
13 May 2013	Taurus	Virgo
27 June 2013	Gemini	Virgo
1 September 2013	Gemini	Libra
7 July 2014	Cancer	Libra
14 October 2014	Leo	Libra
17 January 2015	Leo	Scorpius
4 February 2015	Cancer	Scorpius
12 May 2015	Cancer	Libra
10 June 2015	Leo	Libra
16 October 2015	Leo	Scorpius
1 December 2015	Leo	Ophiuchus
9 August 2016	Virgo	Ophiuchus
23 February 2017	Virgo	Sagittarius
18 May 2017	Virgo	Ophiuchus
15 November 2017	Libra	Ophiuchus
19 November 2017	Libra	Sagittarius
20 November 2018	Scorpius	Sagittarius
13 December 2018	Ophiuchus	Sagittarius
16 November 2019	Sagittarius	Sagittarius
22 March 2020	Sagittarius	Capricornus
3 July 2020	Sagittarius	Sagittarius
15 December 2020	Sagittarius	Capricornus
18 December 2020	Capricornus	Capricornus
25 April 2021	Aquarius	Capricornus

Saturn remains in Capricornus until it enters Aquarius on 13 February 2023 - by which time Jupiter will be in Cetus.

Another way is to look at the dates of opposition - when the Sun, Earth and planet line up:

Year	Jupiter	Saturn	Difference (days)
2013	None	April 28	146*
2014	January 5	May 10	125
2015	February 6	May 23	106
2016	March 8	June 3	87
2017	April 7	June 15	69
2018	May 9	June 27	49
2019	June 10	July 9	29
2020	July 14	July 20	6

[* Jupiter was last at opposition on 3 December 2012]

This tells us that Jupiter and Saturn will be close through 2020. Now, they will be at their closest on 21 December. This is the sky at 4.30pm that day:

Indeed, Jupiter and Saturn will be so close that the chart shows a single object!

Notice Mars over in Pisces and the Moon. If we go back to 16 December (again at 4.30pm) we find the Moon - only about 48 hours after New Moon - below and to the right:

While on 17 December (again at 4.30pm) we find the Moon now to the left of Jupiter and Saturn:

Now, this is all the action happening low down in the south to south-west. However, let's look at midnight on 20 July - the day Saturn is at opposition, and only 6 days after Jupiter is:

(Meanwhile, over in Cetus, Mars is rising).

As I noted in a previous post, the late summer and autumn evening sky is stable - you only lose stars and constellations slowly into the evening twilight. So, as we might expect, Jupiter and Saturn hang around in the evening sky through summer and autumn, being the sight people might notice for several months - if they look up.

And people might notice something else, planet related, which I have hinted at. On 13 October 2020, Mars is at opposition. It will be closer than most oppositions (and brighter) - although the July 2018 opposition is closer and brighter, Mars will be low down in Capricornus, while at the 2020 opposition it will be higher up, in Pisces. At 9pm on opposition day, the sky will be:

Mars will be the brightest object in the night sky - with these close oppositions it will actually be brighter than Jupiter (although only for a few weeks).

It will be later March/early April 2020 that Mars is close to Jupiter and Saturn, with the sky at 5.30am on 1 April, as it is starting to get light, giving us:

and a close-up on the planets shows:

Worth getting up early for.

What about Venus - does it play a part in this gathering?

Venus starts 2020 as an evening object. In 2019 it is at superior conjunction - when the Earth, Sun and Venus form a straight line in that order - on 14 August. At superior conjunction, Venus cannot be seen, but it marks the point when it slowly reappears in the evening sky. In 2020 it is at greatest eastern elongation (when it is at its furthest from the Sun in the evening sky) on 24 March, then inferior conjunction (when the Earth, Venus and the Sun form a straight line in that order) on 3 June, reappearing in the morning sky and reaching greatest western elongation on 13 August, and then in 2021 it is back at superior conjunction on 26 March.

For a close approach, really we are looking at late 2019 - the sky at 4.30pm on 1 December is:

So you need a very clear south-western horizon to get this gathering. As Jupiter and Saturn move into the evening twilight, Venus moves away from them.

So 2020 will be a year of interesting planetary action.