Dim third

Another “trio.” On the evening of March 4, look for Venus, Mars, and Uranus, at their tightest grouping, in the after-sunset western sky.


Here’s what is really happening in space.


Of the three planets, Venus is the nearest, therefore the fastest-moving; Uranus much the most distant and slow. Venus passed fairly close (0.4 of a degree) south of Mars on Feb. 22. Now it passes much closer (only 0.09 of a degree) north of Uranus.

This is a less spectacular conjunction than the one of ten and a half days ago, because Uranus is so distant and dim. You’ll need binoculars, at least, for a hope of seeing it. Uranus is theoretically just visible to the naked eye, but so close to that limit that it wasn’t discovered till 1781.

But the conjunction is noteworthy in that it’s the closest between two major planets in 2015.

The moment of closest approach, or “appulse,” is about 19 hours Universal Time, which is 2 PM in eastern America, so by the time darkness falls the event is past, but the planets will not have separated far.


Close-ups of the three-planet pattern on March 4 (left) and March 5, at 45 minutes after sunset in eastern North America. The pictures are centered on the point (marked by a cross) midway between Venus and Mars. The dashed circle is 5 degrees in diameter. The moment on March 4 is a few hours after Venus passed Uranus, which was also when the planets fitted into the smallest circle. A day later, Venus has moved about 1.2 degrees on. The dots for the planets are sized for their brightness, not for their angular width in the telescope. The real widths of Venus, Mars, and Uranus are only about 12, 4, and 3 seconds (3600ths of a degree), so the real gap between Venus and Uranus when they are closest is 28 times Venus’s width. This is what we call a very close conjunction. You can imagine how rare it is for one planet actually to occult (get in front of) another.

The idea of the “trio” is a handy way of picking out the most interesting concentrations that form and then dissolve in the perpetual slow swirl of the sky’s moving bodies – like knots in a time-space weave. We give this accolade to a grouping of three notable celestial bodies when they come to fit within a circle of diameter 5 degrees or less. The term and the definition were suggested by Jean Meeus, and the choice of 5 degrees results in a list of these best groupings that’s not too short nor too long – from one or two to a dozen a year. The number also depends on which bodies, besides the five bright planets, you consider notable enough to include: the Moon? the twenty brightest stars, or more? the outer planets? Some therefore would exclude this occasion, because the third body is “only” Uranus. It also depends on whether you exclude events too close to the Sun to be observable in practice.

The present scene has come about like this. The gap between hurrying Venus and slower Mars has been widening, but not quite enough to prevent the trio from forming: Mars is still under 5 degrees to the west, though only just under – 4.87 degrees. In fact, since Mars is moving faster than Uranus, by about 0.7 of a degree per day, the trio began some hours back when the Mars-Uranus distance shrank to 5 degrees. The tightest moment, the center of the trio event, comes when Venus passes Uranus, because that’s when the width of the circle containing all three stops shrinking and starts to increase.

Venus is at present 1.36 Sun-Earth distances (astronomical units) from us, Mars 2.25, Uranus 20.84. Translated into kilometers, those are roughly 204 million, 337 million, and 3,118 million. So Venus shines at magnitude -4, Mars 1.3, Uranus 5.9. That means (because the “magnitude” system is a logarithmic one) that the brilliant Venus, even though not as bright as it can be when nearer, is about 9,000 times brighter than the pinprick of Uranus just beside it.

Hard, therefore, to realize that while Venus is nearly as wide as Earth, and Mars about half as wide, Uranus is 4 times wider.

Often I think, Why do we give ourselves all this concern about mere lumps of matter hurtling inertly along their predetermined courses in the void? – are they not like the specks of drifting dust, on a larger scale? What is worthy of our minds is the translation of the two dimensions of what we see into the many dimensions of what we understand.

3 thoughts on “Dim third”

  1. Weather permitting, this’ll be a great one. I’ve seen Uranus a few times in the past, once even without binoculars.
    Spoiler alert! Bad Bad Joke next…..
    You don’t need any telescope or binoculars to see Uranus…..
    Just squat over a mirror….
    Warned ya!….a friend of mine never let’s this inference go when I tell him about the plantes and stuff.\
    Hmmm, proofreading this comment I see I mis-typed and am more reminded of my Bird of Paradise plant which is flowering for me in Brewster; indoors of course. I pressed one of the florets that got knocked off as I worked my way through the jungle of that room to replenish the bird feeder just outside my window….. but I digress….
    Anyway, Winter’s icy grip is still clutching to even the well-travelled streets and avenues in the Bronx….. Brrrrrrr, and even worse in Brewster,,,,,Brrrrrr again.
    Am I detecting a pattern here?
    But the clear, cold nights are excellent for even the brightest, light-polluted places such as here in ‘da Bronx’. Why, last night it was glorious to see the Moon dance around Jupiter.
    more later….
    I’m gonna shoot that ‘gosh-darned- groundhog’.

  2. Thanks very much for this. In anticipation of this conjunction I’ve been looking for Uranus through binoculars. No luck recently. The weather has been a bit hazy, when not downright cloudy. I’ll keep looking this evening and tomorrow evening, and through next week for the conjunction of Mars and Uranus.

    I love what you said about the significance of conjunctions as an opportunity to relate what we see in the sky to our understanding of the solar system. When I teach people skywatching, I like to highlight the irony that astronomers understand more about the universe than ever before, and their knowledge is more easily available to more people than ever before, while, because of light pollution and our busy urban lives, many of us have lost the direct personal knowledge of the sky that was the birthright of all our ancestors until a few generations ago. But with our bodies, eyes, and minds we have all we need to regain an understanding and appreciation of the sky. All we need to do is to get outside on a regular basis, look carefully at the sky, and think about what we see.

    1. Success! Uranus was not visible in binoculars, but through a 70 mm refractor at 21x magnification Uranus was a faint grey dot about half a degree below dazzling Venus. At 89x Uranus showed a tiny disk and a blue tinge in the grey.

      By the way, with a polarizing filter Venus’ gibbous phase is becoming more obvious. I’m looking forward to her crescent (or should that be decrescent?) phase this summer, when I can hope to surprise people by handing them binoculars and suggesting they look at Venus.

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