By now many of you have seen the big, bright, blazing “star” in the eastern sky at nightfall or perhaps noticed it low in the west at dawn. Either way, you’re looking at Jupiter, the brightest planet easily visible in the sky this month. Jupiter owes it brilliance to its giant size (88,000 miles or 11 Earths) and perpetual cloud cover. Like Venus, when we look at Jupiter, we see only clouds and atmosphere. Whatever solid surface this world has is hidden beneath thousands of miles of hydrogen, helium, ammonia and water vapor.
This Friday evening the 28th Jupiter is at opposition to the sun in the constellation Aries the Ram. When a planet’s at opposition, it lines up with Earth on the same side of the sun. All the outer planets from Jupiter to Neptune reach opposition once a year, when the speedier Earth laps the
slower-orbiting outer planet. As seen from our planet, Jupiter is directly opposite the sun in the sky, rising in the east at the same time the sun sets in the west. It passes due south around 1 a.m. daylight time, when it’s highest in the sky, and then descends in the west, setting at sunrise.
Not all oppositions are created equal. Because the planets orbit in ellipses with the sun off to one side rather than perfectly concentric circles centered on the sun, Jupiter’s distance from the sun varies around its orbit. At closest, it’s 461 million miles from Sol and 507 million at farthest.
Now it just so happens that Jupiter was at perihelion or closest to the sun this past March. Since Earth is pulling up alongside the planet just 7 months after its perihelion, this opposition will be one of the best ever, with Jupiter blazing at magnitude -2.9, about the same intensity as the International Space Station. That’s not all. Close means big. Jupiter will grow to a diameter of nearly 50 arc seconds or almost 1/30 the size of the full moon. I know that sounds small, but it’s large for a planet, and means you see more detail and color compared to more distant oppositions.
With all that atmosphere to play with, Jupiter’s famous for its colorful and changeable cloud bands. The dark stripes you see in photos and small telescopes are called belts. The North and South Equatorial Belts are thick and dark, making them easy to spy. Separating the belts are the lighter zones. Because the planet is primarily gas and spins rapidly – an entire Jovian day is just under 10 hours long – it bulges noticeably at its equator. Try your hand at seeing this in a small telescope. Once you know what to look for, you’ll might be surprised that Jupiter looks slightly “flattened” instead of spherical. The difference between equatorial and polar diameters is significant – 6,322 miles or 80 percent the size of Earth!
Finally, Jupiter’s four bright moons provide endless fun and interest as they cycle around the planet like a solar system in miniature. Depending how close or far each is from the planet at the time of observation, binoculars will show from one to four moon as tiny stars lined up very close by. Any small telescope magnifying 20x and higher will easily show them all provided they’re not hiding in front of or behind the planet. Medium-sized telescopes from 6-inches and up under steady skies and higher magnifications (200x) will show each moon as a tiny disk.
One last note – I was asked for a chart showing where Comet Elenin can be found. Be aware that it’s extremely faint and to my knowledge has been seen by only two expert comet observers. Still you might like to know that Elenin is still there as a faint remnant dust cloud. It travels from Gemini into Auriga this week.