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.