As the slender Young Moon emerges into our evening sky, after its New moment on July 2 when it eclipsed the Sun, it passes an array of lights in the background: Pollux; Mars; Mercury; the Beehive or Praesepe star cluster; and Regulus.
See the end note about enlarging illustrations.
And this results in a feast of “trios”: close groupings of three objects. We express this as moments when they fit into a circle with a minimum diameter. That idea is due to the Belgian expert Jean Meeus. (He defined the minimum diameter for a trio as 5 degrees, but I’m allowing a degree more.) Using chapter 19 of his Astronomical Algorithms, I programmed years ago how to find these trios. It’s a heavy process for a year, because the computer can’t just look at a picture as we can and see obvious candidate situations: it has to run through all the permutations of objects 1, 2, and 3 (each of those being. say, 17 objects – the Moon, seven planets, and chosen stars and clusters) and find for every date whether two of them are close and, if so, whether a third is also. For some reason, when I ran this for 2020 it bombed out, so I spent about a week trying to find the bug. I never did, but rebuilt the whole stretch of programming. And in doing so I saw a concentration of trios in this July:
Jul 4 Thu 9 UT Moon, Mercury, and Mars within circle of diameter 3.80°; 20° east of the Sun
Jul 4 Thu 14 Moon, Mercury, and Beehive within circle of diameter 4.73°; 23° east of the Sun
Jul 4 Thu 15 Moon, Mars, and Beehive within circle of diameter 5.89°; 23° east of the Sun
Jul 7 SUN 14 Mercury, Mars, and Beehive within circle of diameter 5.08°; 20° east of the Sun
The choice of objects is somewhat arbitrary. A trio may not really be good for observation if the objects differ too much in brightness; Neptune and Uranus are not for the naked eyes, and I’m no longer including Pluto. The flying Moon is the most abundant producer of trios, but including it might be questionable: its brightness may outcompete nearby objects; and its nearness to us means that its positions from the center of Earth and from from your location differ and one could be in and the other outside that minimum circle..
The ideal trio is one involving three naked-eye planets, or two of those plus a first-magnitude star, and these are uncommon. This year:
Jun 20 Thu 11 Mercury, Mars, and Pollux within circle of diameter 5.58°; 24° east of the Sun
Aug 21 Wed 9 Venus, Mars, and Regulus within circle of diameter 2.08°; 2° east of the Sun
And the second of those will be only 2° from the Sun – unobservable.
Obviously important are both elongation – angular distance from the Sun – and, if the elongation is fairly small, the angle of the ecliptic to the horizon at the time.
Here is how Moon’s travel in early July appears in our Zodiac Wavy Chart for 2019:
– remembering that in this chart the Sun is depicted at the middle of the month; on July 4 it is near the right-hand edge of the “glow.”
We can see that, when on July 2 the Moon blotted out the Sun, Mercury, Mars, and Regulus may have pricked into view on their eastern side, though it is far more certain that Venus did on the east.
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ILLUSTRATIONS in these posts are made with precision but have to be inserted in another format. You may be able to enlarge them on your monitor. One way: right-click, and choose “View image”, then enlarge. Or choose “Copy image”, then put it on your desktop, then open it. On an iPad or phone, use the finger gesture that enlarges (spreading with two fingers, or tapping and dragging with three fingers). I am grateful to know of what methods work for you.
I saw the Mercury / Mars / Pollux trio last month, first with 10×42 binoculars and then naked eye. I’ve been looking for the more recent line-up of Mercury, Mars, Pollux, and Castor, without success. I was pleasantly surprised to see the waxing crescent Moon on July 3, but the planets and stars were hidden by bright haze here in San Francisco. Oh well, Jupiter and Saturn are easy to see, and well worth looking at! I’ve seen Uranus once during this apparition, only with binoculars. My aging eyes and brutal urban light pollution give me a limiting visual magnitude of fifth magnitude on an exceptionally good night.
Close conjunctions are worth noting whether or not they can be observed.
Thank you for pointing out these trios; I wouldn’t be able to keep up with them otherwise. I can’t imagine how long it would take to run this search for an entire year! Plate solving routines still impress me and they aren’t dealing with targets that move so quickly.
Well Uranus is just for the naked eye….just!it and often Mercury are unusual in that they are planets that you first look for with an optical aid,I use a monocular most people binoculars, and then try and find naked eye.
Comments like this help me to choose some numbers.
For instance, in deciding whether to give emphasis (boldface type) to trios in a list of events, I decided to do if the magnitude of all three bodies is brighter than 5. But do you think I should change that to 6, which would include Uranus at all times? Or, if the dividing line is 5.85, Uranus would be included except for a month or two near its Sun-conjunction.
Another criterion is whether to exclude trios involving the Moon if they are more than, say, 10 degrees from the Sun, because the fuller Moon is too bright. Or is a trio of Moon, bright planet, and bright planet or star or a conjunction of the Moon with one of these – good for observation even if the Moon is Full?
Yes I’d include Uranus among the naked eye planets not sure about the Moon when so close to the Sun 🌞.it always amazes me that millions of people must have seen Uranus prior to Herschel’s discovery of it in bath but nobody would have noticed that the very dim object moved against the stars.i use the following techniques to see it naked eye.1/go to a very dark place!2/I locate using my vortex 8×36 monocular and reference it to stars of similar brightness near it.3/then observe the area naked eye picking out the stars and Uranus.its not too difficult to see but it’s knowing it apart from the dim stars of similar magnitude.i use a similar technique for Mercury just after sunset but going up a hill facing in the right direction.i hold that Neptune is the only non naked eye planet.