Jupiter closely touched by two

The crescent Moon will make a show with distant Jupiter, low over the sunset horizon on Friday, August 5.

And there is, in a sense, a third body in this line-up, a human-made one called Juno.

Every month the path the Moon burns around the sky re-tilts (precesses) slightly, like a spinning coin, and a result this year is that for the first six months the Moon passed south of Jupiter, by decreasing gaps; then, the next four passages are closer:

July 9: 0.81° south
Aug. 6: 0.22° south
Sep. 2: 0.36° north
Sep. 30: 0.86° north

They are close enough to hit.  That is, the half-degree-wide Moon occults (hides) the planet, as seen from at least some latitudes of the Earth, and from some longitudes depending on the time of day.

After that, for the rest of the year, the Moon will more widely miss Jupiter on the north.

As you can see, the Aug. 6 passage is the most central.  Indeed it is the most central Moon-planet conjunction of the year.  However, it happens at about 4h Universal Time, which in eastern North America is midnight, and on the west coast 8 PM back in August 5.  So the occultation is observable, if at all, across the southern Pacific and mostly in daylight.

Detail from the “Occultations” section of Astronomical Calendar 2016.  The dotted line of Jupiter’s passage is as seen from the center of the Earth; from farther north, the passage is too far north to be behind the Moon.

Skilled telescope-users in northern Australia may, even in the daytime sky, find the spark that is Jupiter and time its disappearance, then reappearance shortly afterward from the crescent Moon’s bright limb.  Along a certainly line across Australia this will be an even shorter occultation and of the grazing kind, in which Jupiter winks through valleys in the Moon’s northern (lower, as seen from Australia) profile.

Nor are the year’s other three Jupiter occultations, except possibly Sep. 30, any better for northern countries.

However, this month’s conjunction is fine enough, 39° out from the Sun, though at an angle that puts it rather low in our evening sky.  You’ll see the Moon close to the right of Jupiter on the evening of Friday August 5, more distantly to the left of it next evening.

And as you gaze at Jupiter, you’ll see, if you have the eyes of Lynceus, a three-winged insect zooming around it in a pole-to-pole orbit.

This is spacecraft Juno.  Its wings are solar arrays, and also help to stabilize it.  Other missions to the outer planets have been nuclear-powered; this is the first to be, like many Earth-orbiting satellites, solar-powered.  It was launched from Cape Canaveral on 2011 Aug. 5, used re-passings of Earth to boost its orbit to greater length, and, this July 4, entered orbit around Jupiter.  After 20 months and 37 orbits it will be told to descend into the atmosphere and burn up.  The reason for this is to make sure that no radioactive debris will contaminate Jupiter’s satellite Europa, under whose ice there could be life.

The mission was at first termed JUpiter Near-polar Orbiter, which could be forced by one of the pseudo-initials methods into acronym JUNO.  Some classically-minded members of the public scoffed: there is already an asteroid Juno; and the god Jupiter, perpetually under suspicion from his wife Juno because of his infidelities, would hardly allow her to come sniffing around him.  But NASA’s explanation was that Jupiter drew a curtain of clouds about himself to hide his misdeeds, and Juno peered keenly through them – as JUNO will peer through the planet’s mighty layers of cloud, to uncover the mysteries of its deep interior.

There will be a future Jupiter-system-investigating acronym-tweaking mission named JUICE (Jupiter Icy Moon Explorer), not to be confused with JIMO (the Jupiter Icy Moon Orbiter).