We discussed on August 18 the spans of time when all five naked-eye planets are visible, or at least above the horizon.
It’s taken me all the time since then to master this topic, since it’s not simple. When are all five observable? It depends on the observer’s location on Earth, and on the number of minutes after sunset or before sunrise at which you consider the low sky dark enough for the dimmest of the planets to be findable.
It would be yet more complicated if we tried to define observability as being, say, 1° above the horizon for Venus and 7° for Mercury. And yes, Uranus qualifies, at a stretch, as a naked-eye planet, but that’s a quibble! Adding more planets to the five would compound the problem further, and make all-five spans of time much rarer.
Taking the location to be the standard USA one I’ve been using for horizon scenes – latitude 40° north, and the longitude of the Central time zone, 90° west – and the time of day to be 30 minutes after sunset, then in 2021 there was an all-five span from August 11 to 27.
The vertical white lines show the beginning and end of the all-five span. The colored curves show the planets’ altitudes, but there is a third dimension that can’t be easily shown: azimuth – position around the horizon. I make the lines thinner when azimuth is less than 180°, so you can see that Jupiter and Saturn are in the eastern half of the sky. Here again is the horizon scene on Aug. 18.
This all-five span of time began when Mercury climbed into the evening sky, having passed behind the Sun at superior conjunction on Aug. 1. And the span ended when Mars became too low, also in the pre-sunset sky, sinking slowly toward its conjunction behind the Sun on Oct. 8.
A longer all-five span will come in 2022, spilling over into the first few days of 2023.
At this time Mars is in the eastern sky. The curves for Jupiter and Saturn change from thin to thick because they pass through moments when they are on the meridian (over the south point of the horizon) at our chosen location and time of day. Here is the broad horizon scene for Christmas Sunday, 2022 Dec. 25.
There aren’t all-five spans in every year. If we take the years 2020-2031, and keep the same location and same time of day, 30 minutes after sunset, the spans are:
2021 Aug 11 to 27
2022 Dec 1 to 2023 Jan 5
2025 Feb 18 to Mar 3
2029 Apr 19 to 24
If, keeping to the same 30 minutes after sunset, and latitude 40 north, we change the location longitude to 0, there is little difference.
2021 Aug 11 to 30
2022 Dec 1 to 2023 Jan 5
2025 Feb 18 to Mar 4
2029 Apr 21 to 25
If instead we use a south-hemisphere latitude. such as 35° south, which is a fair dinkum compromise for southern Australia, northern New Zealand, and South Africa (keeping longitude 0), the result is very different
2021 Aug 13 to Sep 15
2022 Dec 4 to 2023 Jan 2
2025 Feb 24 to Mar 1
South of Earth’s equator, the celestial equator and the ecliptic slant in the opposite direction in relation to the horizon, so planets that happen to be in northerly parts of their orbits set sooner rather than later.
For a location on Earth’s equator, different again:
2021 Aug 12 to Sep 11
2022 Dec 2 to 2023 Jan 4
2025 Feb 20 to Mar 3
Then there’s the number of minutes after sunset. If, keeping the default location (lat. 40 north, long. 90 west), we choose 45 minutes, when twilight is a bit darker but planets set earlier), the all-fuve spans are fewer and shorter:
2022 Dec 13 to 2023 Jan 3
2025 Feb 21 to 28
Final variation (for now, anyway): the default location of 40 north, 90 west, but 30 minutes before sunrise:
2020 Jul 9 to 21
2022 Jun 2 to Jul 11
2028 Oct 9 to Nov 5
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In Florida, USA, a “five-span” should occur on June 20, 2022 at about 5:45 am.
Thank you Guy for the heads up. I’m looking forward to next Christmas!
An additional complication is that you need to be someplace where you have a clear view down to the horizon to see Mercury, and, depending on where the other planets are, perhaps quite far over in the opposite direction. And of course, clear weather.
I have a slightly less rigorous observing challenge that I call solar system bingo. The goal is to observe all the major planets, Earth’s Moon, and an asteroid, with any sort of optical aid (usually binoculars for me) between sunset and the following sunrise. Bonus points for having a good dinner and getting a few hours of sleep.
I always like to add the horizon of Earth and make it 6 planets within one view
Me, too.
Thank you for this excellent post. I especially enjoyed “South of Earth’s equator, the celestial equator and the ecliptic slant in the opposite direction in relation to the horizon…” because I had to think about that for a few minutes before it clicked. It gave me a wonderfully different perspective from my usual northern hemisphere viewpoint. I’d give this post 5 stars but it’s really 5 planets.
Witty point, Shannon!
Yes indeed. My Astronomy 2022 Australia book notes as a highlight that all five naked-eye planets will be visible in the evening sky around Christmas.
6 naked eye planets if you count Uranus or even 7 if you add Earth,look at your feet!I saw Uranus naked eye in November from the Northumberland National Park right up against the Scottish border and after finding it walked 5m north and saw it again from Scotland!I used my 2×40 Helios Starfield binoculars to locate it and figure out how it stood in relation to dim stars before having a go naked eye.You need somewhere dark probably Bortle 4 or less I think that where I was is 2 or 3 but as there’s nothing there to type in to Clear Outside it’s hard to tell exactly without spending a lot on an SQM meter but Bryness the nearest hamlet,8km west,is 3.