I’ve found a clear way of showing how Jupiter swells brighter and slightly less bright, stands higher and lower, over its twelve-year cycle. At least I hope it’s clear.
If you were looking at the Moon through your moon window around two nights ago when it was Full (the night between Feb. 3 and 4), you will have seen a star above it and a star a handspan to the left. The star above was Jupiter and the star to the left was Regulus. The Moon, now beginning to be “decrescent” – that is, with a subtle softening by shadow around its right side – has moved on, passing south of Regulus last night.
Now comes the opposition of Jupiter – the great event of this part of this year, the opposition of the greatest body that can have an opposition. Or we could say that the Anti-Sun position in the sky swept over the Moon where it was two nights ago and now sweeps over Jupiter.
The opposition happens in this coming night between Feb. 5 and 6, at 6 hours Universal Time, which is 6 AM in Britain, 1 AM in the eastern US, and on the west coast 10 PM in Feb. 5. The exact moment matters only theoretically; it’s merely the middle of the good time for observing the planet, and that good time extends two months or so on either side.
You could say that the good span for seeing a planet is roughly the time when it is retrograding (appearing to move backward, because we are overtaking it) minus the times of Moon-glare within that span. Jupiter is face-on to us now but outcompeted by the Moon; observing it will get easier over the next week or two as the Moon gets farther east and rises later.
There is a lot more in Astronomical Calendar 2015 about Jupiter and its opposition, but let’s add something.
When, only six days ago, the subject was Juno (the asteroid named for Jupiter’s wife), I showed you an improved version of my small chart of the “locus” of Juno’s oppositions. Locus is Latin for “place,” but it’s used in mathematics to mean a line connecting all points that meet a certain condition. I was first interested in this when I learned from Jean Meeus’s book that the locus of the places in the sky where Juno can be at opposition happens to form an almost straight line along the sky’s equator. I had already realized that the meaning of this “locus” could be made clearer by including not only the locus curve but the asteroid’s track for a span of time around some actual opposition dates, showing that the midpoint of this track is always on the locus curve.
The same kind of chart can show that other asteroids have opposition-locus curves very different from Juno’s; and can be used not only for asteroids but for the planets. Since the planets orbit in planes close to the ecliptic plane, their opposition-locus curves will depart not much north or south from the ecliptic, and their opposition tracks will just about run along in their locus curves.
However, I realized that the chart for a planet, or any body, could be made richer by including not only the locus curve and the good-observable-time tracks, but symbols (circles) for the body at the opposition dates. These symbols can be graded in size, like those for stars – but, finally, sized not just for brightness but for difference in brightness from the body’s average opposition brightness; or, better, brightness compared with the range between its brightest and dimmest oppositions.
Thus results a useful chart in which, for Jupiter’s oppositions over its twelve-year cycle, you not only can see where they happen but can simultaneously compare their declinations and their peaks of brightness.
Declination makes the greater difference, because it is easier for us northerners to watch Jupiter when it is 22.7 degrees north as in 2014, than when it is 22.4 degrees south as in 2019. But it’s interesting to see that the peak of brightness is magnitude -2.9 when near Jupiter’s 2011 perihelion and -2.5 when near its 2017 aphelion. We could as well give a figure for Jupiter’s angular size in the telescope, which, like brightness, depends on its distance.
The locus of the oppositions is drawn in yellow, and you can see where it curves slightly north and south of the ecliptic. The tracks, drawn in black, are for the 200 days centered on opposition, so as to include the 121 days when Jupiter is retrograding (e.g. from its 2014 Dec. 8 turn back westward to its 2015 Apr. 8 resumption of forward motion). The little arrowhead at the end of the track shows how far Jupiter has retraced its steps forward by the end of the 200 days.
The boundaries of the zodiacal constellations are shown, without their names, which would be too cluttering. Perhaps you can recognize them. Jupiter at its present opposition is retrograding from Leo into Cancer; in 2016, it will be well over in Leo; 2017, in Virgo; 2018, in Libra; 2019, in non-zodiacal Ophiuchus.
For asteroids and comets that don’t hug the ecliptic, these retrograde loops will look clearer! So we’ll get back to locus charts sometime.