SKY REPORTER: Big and Bright Jupiter

Jupiter and its four largest moons, discovered by Galileo.
Credit: Jan Sandberg

While walking on a recent frigid evening, I spotted Jupiter’s bright point of light in the eastern sky beyond a foreground of tree branches. A quarter turn to the right revealed Orion’s starry figure neatly placed halfway up the sky. That southerly view included many celestial objects intrinsically big and bright including the Hunter’s stars Betelgeuse and Rigel, its immense Great Nebula star factory, and remarkable Pleiades and Hyades star clusters of Taurus. However, it was Jupiter that most captured my thoughts that night. 

Seeing and considering the giant planet is enjoyable on many levels. To unaided eyes, Jupiter’s bright punctuation of light outshines all nighttime stars and in planetary brilliance is second only to Venus, these weeks low in the southwest for a while after sunset. Unlike Venus’s attenuated evening or early morning appearances, Jupiter is now visible most of the night. We’re presently catching up with Jupiter as our faster orbiting Earth overtakes the larger planet. The situation makes it appear Jupiter is gradually moving east to west relative to stars in the zodiac, reversing that planet’s usual motion during most of the year. This retrograde motion began the 9th of December and continues until April 8, when Jupiter’s west to east apparent motion resumes. Between now and February 6, when Jupiter is opposite the Sun from the viewpoint of Earth, our actual distance is diminishing and that planet appears to gradually brighten. Using Regulus, the only first magnitude star in Leo as a benchmark, we can measure increasing apparent separations between it and Jupiter until the 8th of April, when Jupiter resumes its usual motion towards the east and its span to Regulus diminishes.

Even near times of opposition the disk of Jupiter is several times too small to be discerned by eye alone, and we are tempted to bring out binoculars and/or telescopes to better appreciate Jupiter’s visual treats. A magnification of seven is usually sufficient to show the Jovian disk and may also give glimpses of the planet’s largest moons, discovered by Galileo early in the 17th century. Stepping up the visual ladder, a small telescope providing about 50 magnifications, with decent optics and a sturdy mounting, allows us to notice not only the equatorial elongation of the Jovian disk caused by centripetal effects of the planet’s rapid rotation, but also to see its major equatorial belts, and clearly view the Galilean moons. Io, the innermost of those big satellites, revolves so rapidly that even over the course of a few hours its position relative to the planet’s orb may be noticed with a small telescope. 

Adding to millennia of Jovian visual records, a new era began in 1955 with detection of radio bursts from the big planet. Astronomers Kenneth Franklin and Bernard Burke discovered electro-magnetic emissions, now believed to stem from currents generated by interactions between ionized effluent from Io’s sulfur spewing volcanoes and Jupiter’s vast magnetic field. Ken served many years on staff and as chairman of the American Museum-Hayden Planetarium. I remember him fondly for his rigor and graciousness providing information and advice to me, and for being a genuine good guy!

More perspectives on Jupiter were provided by a progression of space missions giving data and images from up close observations at many wavelengths. 

Pioneer 10, first spacecraft to travel through the asteroid belt, made its closest approach to Jupiter in December 1973 as it passed within 81,000 miles of the planet’s cloud tops taking what were then the most detailed images ever made of that world. A year later, Pioneer 11 moved by the planet. During that period of the American space program, it didn’t take decades before increasingly intrepid explorations were accomplished. Voyager I and II flew past Jupiter in March and July of 1979. Their high resolution cameras revealed remarkable surface details of Galilean moons, the planet’s cloud tops, and its Great Red Spot.

The Galileo mission went into orbit around Jupiter in December 1995 and continued surveying the planet and environs until September 2003. It parachuted an instrument probe deep into the Jovian atmosphere, made many passes of Galilean moons, and investigated the planet’s magnetic field. While en route, when it provided unique views of devastating Earth sized fireballs as comet Shoemaker-Levy 9’s fragments impacted Jupiter’s atmosphere in July 1994. Images of mountain sized comet pieces having such immense effects evoked thoughts about what will happen if a comet is allowed to hit Earth.

During late morning of January 26, asteroid 2004 BL86, with an estimated width of about 1,800 feet, will pass our planet at about three times the Moon’s distance. That night, for viewers around New York City, the faint ninth magnitude asteroid will be in the eastern sky about 15 degrees of arc northeast of first magnitude star Procyon. Barely visible in binoculars and small telescopes, it will then be moving north at the apparent rate of about four Moon diameters per hour. 
Researchers at Purdue University have developed a simulation showing effects of hypothetical collisions between Earth, meteors, asteroids and comets. Enter parameters to see how far you should be from such a smash-up to be safe.

NASA’s Juno mission spacecraft is scheduled to arrive at Jupiter next year. It will orbit the planet’s poles with nine instruments designed to measure the planet’s water content, study interior structure, including the possible presence of a high-density core, and will extensively map Jupiter's magnetic and gravitational fields.

Temperatures here in New York are expected to be up above freezing this evening. I look forward to going for a walk around 9 p.m., look east to see Jupiter – and savor its marvels.  

Moon

Planets

Mercury may be glimpsed in evening twilight during the middle of January, but may be seen with increasing difficulty as it moves closer to the Sun’s direction as the month wanes. On Jan 14 it is at its greatest eastern elongation with a separation from the Sun of 19 angular degrees. Thereafter this span diminishes and toward the last days of the month Mercury becomes lost in bright evening twilight. 

Saturn is about one degree south of the waning crescent Moon during predawn hours of January 16. 

Venus, although low in the southwest for about an hour after sunset nevertheless is so bright that when we have clear views above the horizon in that direction, it may be identified quite easily. On the evening of January 21 a day old, extremely thin waxing crescent moon forms a triangle with Venus and Mercury. Binoculars provide considerable help when looking for both the elusive lunar crescent and Mercury near bright Venus, for about an hour after Sunset.    

The waxing crescent Moon appears near Mars and Neptune early on the next evening January 22. Faint Neptune is then two degrees from Mars and three and a half degrees from the thin crescent Moon.