Juno, the Third Asteroid, is at a fairly good opposition.
“Opposition” means the middle of the best time to see a body, and “fairly good” means, for Juno, that it reaches magnitude 8.1. This is 16 times fainter than the faintest star the naked eye can see in average conditions (magnitude about 5), but Juno is findable, with skill, in binoculars or small telescope, using this chart from page 53 of Astronomical Calendar 2015.
The loop of stars forming the “head” of Hydra the water-snake is the most easily found pattern between Orion and Leo. Then you have to search in the field about two degrees west and a little north from 4.5-magnitude Sigma Hydrae. The 274-kilometer-wide asteroid is a passing molecule sensed by the Water Snake’s forked tongue.
Juno is at right ascension 8h 31m, declination 3° 40′. Or rather, it was, at the moment of opposition, Jan. 29 15h Universal Time (10 AM EST). The day being now Jan. 30, because I’m late getting this ready, Juno is now a little farther along its track: moving northwest at a third of a degree a day, it is between 8h 30.7m 3° 43′ and 8h 29.8m 3° 53′, depending on when in Jan. 30 you look. A day won’t make the asteroid appreciably dimmer. The Moon, just past First Quarter, is 50 degrees away, getting low in the west as Juno’s part of the sky climbs in the southeast.
Considering where Juno will be the day after opposition brings to mind a question. Could it instead have been at opposition at one of these points farther along its track, such as where it is today or tomorrrow? Or could it, some other year, be at opposition at a point a few degrees north or south of where it is now? An asteroid can venture to almost any place around the sky, that is, any point within the swath swept out by the tracks it makes over the years; but are all of those points potentially ones where it could be at opposition?
The answer is no. It can be at opposition only in a limited set of places: indeed, along a certain line of points.
Jean Meeus asks himself this question (about asteroids and outer planets in general) on page 240 of his Mathematical Astronomy Morsels (1997). He asks the question in the simple form: where can a body’s oppositions be? It depends on the inclination, eccentricity, size, and orientation of the body’s orbit, and he gives a page of formulae, which I managed in 2008 to reduce to a 15-line “subroutine” of programming. What my subroutine does is find, for any longitude at which the body may be along its orbit, the geocentric latitude at which it would have to be at opposition.
The interesting thing about Juno is that the line of points along which it can be at opposition – the “locus” of its oppositions – is almost flat along the celestial equator. In other words, every time Juno arrives at opposition it has come down to, or up to, a point within about 5 degrees north or south of the equator. This January it is 3° 40′ north, in Hydra; in 2016 April it will be only 43′ south, in Virgo; and so on.
Here is a chart showing the loci of oppositions for the first four asteroids, Ceres (gray), Pallas (green), Juno (orange), Vesta (cyan). The actual tracks of the asteroids are traced in black for spans of two months around some sample oppositions.
You might think the hilliness of the curve would depend mainly on the orbit’s inclination. Yet Juno’s inclination (13 degrees) is greater than those of Ceres (11) and Vesta (7). Pallas’s inclination of 35 is unusually great for an asteroid, so its oppositions range far from the ecliptic – but, because of the orbit’s orientation, not so far from the equator. These relations make sense because inclination means how much the body can get away from the ecliptic, not the equator. It also seems surprising that little effect is made by the large eccentricity of Juno’s orbit – it can come nearer in than all the others and almost as far out as Pallas.
And despite coming nearer in it is not a star asteroid. As we never tire of remarking about poor Juno, though named for the supreme Roman goddess it in most years fails to become as bright as the other three of what used to be called the Big Four, and often is outdone by several that were discovered much later. This time, however, it is more than a magnitude brighter than Pallas will be in June, and worth looking for.