Giant Planets, Dwarf Planets

Jupiter is visible all night throughout July, transiting a bit before midnight. It only gets up to about 30 degrees at its highest point, though. So this isn’t a great year for Jupiter watchers looking for those tiny subtle details in the bands.

This has been a great year for Venus watchers. It’s hard to believe, when the evening star has been so high late into the night for so long, but in July we’ll finally lose it, as Venus slips between us and the sun. At first it will get brighter since it’s moving closer to us:brightest on the 12th. Since it’s getting closer to us, its disk as viewed in a telescope will get much larger as its phase shrinks to a slim crescent. At the same time, Venus is sinking lower in the sky, ever closer to the sun. It’s also fairly far south, so by the end of July it will be tough to find it at all.

Saturn, too, is visible in the early evening, drawing closer to the sun and becoming increasingly hard to find as July progresses.

Mars continues to edge marginally higher in the morning sky.It’s joined this month by Mercury, which makes an appearance just before dawn in the second half of the month.

Uranus (in Aquarius) and Neptune (in Capricornus) rise a little after sunset and are visible all night. Neither one gets very high, but they should be fairly easy targets if you’re out observing late at night and want to take a break from small green disks of planetary nebulae to see small green disks of planets. They’ll only get better over the next few months.

Finally, if you’ve ever thought about chasing down an asteroid, the next few months offer a great time to look for (4) Vesta or (1) Ceres. Why? NASA is about to launch a mission, called “Dawn”, to explore these two large and bright asteroids, and they’re soliciting amateur sketches and images. Dawn is scheduled to launch sometime in early July.

Asteroids are generally referred to both by number (in order of discovery) and name (chosen by the discoverer). So (1) Ceres was the first asteroid every discovered, and its discoverer Giuseppe Piazzi decided to name it after the goddess of the harvest. (4) Vesta was the fourth asteroid to be discovered, by Heinrich Wilhelm Olbers, who named it after the goddess of the hearth, perhaps because of its brightness. More recently discovered asteroids have been given all sorts of names, like (9885) Linux, (3834) Zappafrank or my favorite, (22338) Janemojo, named after two SJAA members.

(4) Vesta is the brightest of all asteroids. It sometimes gets bright enough to see with the naked eye, though this month it’s a bit past opposition now, and only magnitude 6.6. That’s still brighter than Neptune. It spends the month of July conveniently located just above the head of Scorpius. It should be possible to see its motion over a period of a few days or a week if you make sketches of it against a pattern of background stars. If you do make sketches, images, or animations, send them to the NASA Amateur Observers’ Program at http://dawn-aop.astro.umd.edu.

(1) Ceres, the largest asteroid, is large enough to be considered a “dwarf planet” (depending on whose definition you use). It’s in Cetus this month, and it’s quite a bit fainter than Vesta at about magnitude 9. But if you try for it and strike out, don’t lose hope: it’ll brighten steadily from now until its 7th magnitude opposition in November.

And don’t lose heart if you don’t manage to see Vesta or Ceres this month. You’ll have plenty of time to catch them before Dawn makes its first encounter with Vesta, in September of 2011. It won’t get to Ceres until February 2015. You can find out more about the Dawn mission at http://dawn.jpl.nasa.gov.