With the conspicuous exception of Jupiter, all the bright planets are poorly placed at present. Jupiter, though, now rises in the ENE by 23:00 GMT and is unmistakable and brighter than any star as it climbs through our E and SE sky to cross the meridian before dawn.
Our chart depicts a 30°-wide window of sky and plots Jupiter’s motion between Leo and Cancer until we lose it in our evening twilight next summer. We find it to the W of the Sickle, the reversed question-mark of stars that curls above Leo’s star Regulus.
Currently 9°NW of Regulus, Jupiter creeps a degree closer by the time it reaches a stationary point on 9 December. It then retrogrades westwards to another stationary point on 8 April to the ESE of the Praesepe star cluster in Cancer. This backwards motion results from our changing vantage point as the Earth overtakes Jupiter on our inside-track orbit. The two are closest on 6 February when Jupiter is 650m km distant and at opposition, so that it rises in the ENE at sunset, is highest in the S in the middle of the night and sets in the WNW at sunrise.
After 8 April, Jupiter resumes its easterly or direct motion, retracing its steps and continuing to pass only 0.4° NNE of Regulus next August when both are too close to the Sun to be seen.
Jupiter and its system of moons are always fascinating through a telescope. Indeed the four main Galilean moons, three of them larger than our Moon, may be glimpsed through binoculars.
Their configuration evolves from night to night as they orbit every 1.8 to 17 days, sometimes transiting in front of the disc and sometimes hiding behind it in occultation or being eclipsed by the Jovian shadow. At present their orbits are almost edge on to the Earth and the Sun, with the result that they have recently started to eclipse and occult each other occasionally in a series of so-called mutual phenomena that continue into next year.
Jupiter’s disc, 139,800km across, is 37 arcsec wide at present and swells to 45 arcsec at opposition. It shows a wealth of ever-changing cloud detail, with dark belts running parallel to its equator separated by lighter-hued zones. Smaller-scale features, spots and streaks, are carried across the disc as the planet, 11 times wider than the Earth, spins on its axis in under 10 hours – the shortest planetary day in the solar system. The famous Great Red Spot has shrunk in recent years and appears as a salmon-pink oval some 16,500km wide, less than half the size quoted in many references.
Our chart also plots within the Sickle the radiant point for the annual Leonids meteor shower which is active from 15-20 November and reaches a sharp peak at about 01:00 on the 18th. Very swift (71 km per second) meteors flash in all parts of the sky along tracks that point pack to this radiant. With negligible moonlight this year, we may see up to 10 or 20 Leonids per hour after the radiant clears the horizon at about 23:00. These rates are paltry in comparison with high, even storm-force, ones seen between 1998 and 2002 when the parent comet, Tempel-Tuttle, was in the vicinity.
