Jupiter-Saturn conjunction of autumn 2020

We are approaching the (approx) 20 yr event of Jupiter overtaking Saturn in the solar system -- the actual period is closer to 19.86 years. Every third conjunction is in a similar part of their orbits, with the position slowly advancing in the direction of their orbits, so that after about 747 years, the conjunctions return to almost exactly the same positions.

This coming conjunction will be played out all through the autumn and Jupiter is already quite close to Saturn in the sky (about the same distance as the span of Orion's body). You'll find Jupiter as the most prominent object in the southern skies around midnight this month, and Saturn is the relatively bright coppery hued planet to its left.

The earth passes Jupiter on July 14th and Saturn on July 20th (14.3 and 20.9 UT) which means that Jupiter has not quite caught up to Saturn. This coming conjunction will be a close one as the two planets are just past the point where they recently crossed the earth's orbital plane heading south (they don't do that in exactly the same celestial longitude but it's close to where they crossed the Milky Way in the past year or two).

Since Jupiter is moving at roughly 2.5 times the pace of Saturn across the sky, the current gap will narrow. Parallax will delay this somewhat, since the earth is sprinting ahead of both and our view is off to the side of the heliocentric conjunction line which will be reached in early October. At that point, it will appear to us that Jupiter has some distance left to go. But by November, with the two of them beginning to move into the evening skies and setting 1.5 hours after sunset, the parallax will be decreasing and the conjunction will be rapidly improving until we see a geocentric conjunction in mid-December (although that may be difficult as they will be low in the usually cloudy skies after sunset at that time of year).

Just in the six days (6.6d actually) between Jupiter and Saturn oppositions, Jupiter will have moved to a position where a second earth following this one would pass it on July 14.8 rather than 14.3 as we passed Saturn. So in that 6.6 days, it makes up (0.5/6.6) of the separation towards the heliocentric conjunction. That's (1/13.2) of the separation telling you that in another 12.2 x 6.6 days (from July 20) Jupiter would have caught Saturn -- approximately 80 days or October 8th. On that date, a hypothetical earth near the heliocentric conjunction line would be where we had been on July 22 or 23. From that alternate earth, observers would see Jupiter and Saturn in a very close conjunction. But our parallax off to the side of the solar system (pretty much at right angles to the J-S conjunction line) widens the apparent separation. We see the closest conjunction around December 22nd. We're not quite back to the heliocentric conjunction line extended through the Sun at that point, and Jupiter is a little past Saturn by then (as seen from the vicinity of the Sun).

There will be interesting additions of the Moon to the conjunction event, in recent days the full moon cruised by the two planets, and it will appear to be almost full in early August going by them again, but by November and December it will be less than a half moon making for quite a sight in the southwestern skies on the dates when the Moon appears to pass them (each lunation this will come a bit closer to being in the week after new moon). These events will be around Nov 22nd and Dec 19th (the December new moon is on the 14th so that one will be a fat crescent moon, the November case is almost a half moon in appearance).

Should be some great viewing if clear skies co-operate at those times. (my dates are approximate, better details are probably available now but as the chances of clear skies are quite low, any of the dates close to these may give at least an idea of what better positioned observers might be seeing).

Venus won't join this party, it remains a morning star throughout the conjunction event. As such, it will be most prominent around late August to November of this year in the pre-dawn skies.