Tonight's Sky
Sun
Sun
Moon
Moon
Mercury
Mercury
Venus
Venus
Mars
Mars
Jupiter
Jupiter
Saturn
Saturn

Tonight's Sky — Change location

OR

Searching...

Tonight's Sky — Select location

Tonight's Sky — Enter coordinates

° '
° '

Juno's planetary past

Though once considered a full-sized planet, the third asteroid ever discovered faced demotion long ago.
ChapleGlenn

A few months ago, we were privy to a close opposition of Mars — an event that occurs all too infrequently. This month, we set our sights on another favorable but uncommon opposition of a planet: Juno.

“Wait a second,” you protest. “Juno isn’t a planet!” Actually, it was — two centuries ago. 

Here’s the story. By the end of the 18th century, the distances from the Sun to the known planets seemed to obey a mathematical sequence as proposed by Johann Titius and Johann Bode. But there was a glitch: No planet existed between Mars and Jupiter, where Bode’s law predicted one should be. Then, on the very first day of 1801, Italian astronomer Giuseppe Piazzi discovered a body, later named Ceres, in the prescribed location. The solar system was complete, according to Bode’s law.

However, things began to unravel when, during the next six years, three more bodies — Pallas, Juno, and Vesta — were discovered orbiting in the same zone as Ceres. Worse yet, none of these bodies, dubbed “asteroids” by William Herschel, came close to approximating the size of the known planets. A rash of asteroid discoveries in the mid-1800s sealed the deal.

Ceres, Pallas, Juno, Vesta, and the others were reclassified as minor planets. And in 1846, a newly discovered planet — Neptune, which orbits the Sun more than 800 million miles closer than suggested by Titius-Bode — closed the book on that so-called law.

Juno
Throughout the month of November, observers can spot dwarf planet Juno as it treks within a degree of a trio of bright stars. Juno is shown here during its opposition on November 16, when it will sit next to the binary star 32 Eridani.
Astronomy: Roen Kelly
This state of affairs set a precedent for Pluto’s recent fall from planetary grace. Like Ceres, Pluto was originally thought to be the only body of its kind in that part of the solar system. It, too, was much smaller than the traditionally known planets. When more “Plutoids” came to light — including Eris, which is slightly smaller than Pluto — these two, along with Ceres, were reclassified as dwarf planets.

Because Juno appears as a mere telescopic speck, we’ll need to mentally enhance the view with some background knowledge. Juno was discovered in 1804 by German astronomer Karl Harding and named after the highest-ranking Roman goddess. As one of the larger asteroids (10th or 11th in rank), it has a diameter of 150 miles (240 kilometers), roughly the width of New Hampshire and Vermont combined. Juno orbits the Sun once every 4.7 years, with oppositions about every 15.5 months. Like Mars, its eccentric orbit means that not all Juno oppositions are created equal. During a typical opposition, Juno might shine at magnitude 9 or 10. Every 13 years, however, an opposition coincides with Juno’s closest approach to the Sun. Such is the case with this month’s opposition, which occurs on the 16th. Juno will be 96.3 million miles away and an easy magnitude 7.4 binocular target. We’ll have to wait until October 2031 to see Juno this bright again.

Now that you know Juno is much more than a diminutive speck of light through binoculars or a telescope, you’ll want to view it for yourself. First, you’ll need the finder chart on the right side of page 43. If you’re a “star hopper,” focus on those times when Juno passes within a degree of a bright field star. During November, these opportunities occur when Juno encounters 35 Eridani (magnitude 5.3) early in the month, 32 Eridani (magnitude 4.5) at midmonth, and 22 Eridani (magnitude 5.5) at month’s end. Because none of these stars is bright, you want to set up in an area with reasonably dark skies. 

And now I must digress: 32 Eridani is a showpiece double star whose G8 and A2 spectral class components appear distinctly yellow and blue. The magnitudes are 4.8 and 5.9 with a separation of 6.9". It was a easy choice to make my Double Star Marathon list, and it is a definite bonus for anyone who looks for Juno during its close encounter with this pretty pair. 

Now back to Juno. Since it looks like a star, how do you know you’re seeing the real deal? Your first clue is if your suspect is close to where the finder chart indicates. If you’re like me and want to be 100 percent sure, take advantage of the fact that asteroids move — stars don’t. Make a sketch of the field that includes your possible Juno and nearby field stars. Mounting your binoculars on a tripod will help a lot. Through a telescope, use low magnification to encompass as large a chunk of sky as possible, particularly the area where Juno is predicted to be one or two evenings later. On the next clear night, reobserve the area. If the suspect you sketched is no longer in the original spot but has moved to the location predicted for that night, break out the champagne — you’ve captured Juno! 

Questions, comments, or suggestions? Email me at gchaple@hotmail.com. Next month: The treasures of the triangle. Clear skies!

0

JOIN THE DISCUSSION

Read and share your comments on this article
Comment on this article
Want to leave a comment?
Only registered members of Astronomy.com are allowed to comment on this article. Registration is FREE and only takes a couple minutes.

Login or Register now.
0 comments
ADVERTISEMENT

FREE EMAIL NEWSLETTER

Receive news, sky-event information, observing tips, and more from Astronomy's weekly email newsletter. View our Privacy Policy.

ADVERTISEMENT
ADVERTISEMENT
Apollo_RightRail

Click here to receive a FREE e-Guide exclusively from Astronomy magazine.

Find us on Facebook