July 2001 Sky from Keeble Observatory

It may well be time to revisit the question of whether or not Pluto is a planet. From the time you were in grade school, you've probably memorized the names of the Sun's nine planets - all together, now! Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto. The International Astronomical Union (IAU) is the official body which names and categorizes celestial objects. In 1999 this group considered and then rejected a proposal to reclassify Pluto as a "trans-Neptunian object" - demoting it from planet status and making it merely the largest and best known denizen of the fringe region of our solar system known as the Kuiper Belt. Historically, Pluto was discovered only in 1930, and although its discovery was the result of a deliberate search for a planet beyond Neptune, that Clyde Tombaugh was looking for it where he found it was the result of a set of lucky coincidences (actually mistakes!).

Consider - Pluto is smaller than Earth's moon, a mere 2300 km in diameter. It's neither rocky like the inner planets, nor a gas giant like the outer planets. It's icy, more like a comet or one of the satellites of Uranus and Neptune. We used to be able to argue that it was a planet because it possesses its own satellite, a smaller still icy ball called Charon. But, we have now identified another Kuiper Belt object, known only as 1998 W31, which has its own moon, too. A newly discovered object at about the same distance from the Sun is called Varuna, and it's diameter is about 40% that of Pluto. Of the 377 Kuiper belt objects (most are probably comets) discovered since 1992, four share Pluto's orbital period and its 3:2 resonance with the orbital period of Neptune. Those four have been dubbed "Plutinos."

NASA had scrubbed plans for a Pluto/Kuiper probe because of cost overruns. However, public outcry has surprised them into soliciting proposals for a less expensive probe, possibly to launch within the next few years, and probably taking up to a decade to reach its distant target: 40 times further from the Sun than Earth orbits.

Lunar phases for July 2001: Full Moon on July 5th, 11:04 am; Last Quarter on the 13th, 2:45 pm; New Moon at 3:44 pm on the 20th; First Quarter at 6:08 am on the 27th. There will be a partial lunar eclipse on the 5th, but you'll have to go to Australia to see it!

Evening planet watchers will have to content themselves with watching Mars, which should be visible in evening twilight to the south-southeast. It should still be the brightest object in that part of the sky, having passed a mere 42 million miles from Earth on the 21st of June. The pre-dawn sky is filled with planets, however. Mars is still visible to the southwest. Venus is brilliant above the eastern horizon, Saturn starts the month about 14 degrees below and to the left of Venus, by month's end it will be about 17 degrees above and to the right. Jupiter starts the month even further below Venus, but finishes only 5 degrees away. Mercury is within a few degrees of Jupiter at mid-month.

An overhead view at midmonth, at about 10:30 pm has the constellation Hercules at zenith. This grouping looks like a distorted square - binoculars and a clear night may permit you to see an impressive globular cluster near the northernmost star in this "square." To the northwest lies Ursa Major (the Big Dipper) with its familiar bowl and handle shape. Follow the arc of the handle back towards the west and you'll encounter bright Arcturus, in Bootes. To the southwest, that bright star is Spica, in the constellation Virgo. To the south, don't confuse bright red planet Mars with only a little less bright red star Antares (anti-Mars), in the constellation Scorpio. Pluto, which you can't see but which we discussed above, is about 10 degrees higher in the sky than Antares and Mars, in the constellation Ophiuchus. Back overhead, but now turning toward the east, you'll note a bright triangle of stars. Highest and brightest is Vega, in Lyra. Deneb is to the right and below Vega, marking the tail of the Swan (Cygnus). The "head" of the Swan is to the left, marked by the beautiful binary Albireo. Altair is below and a bit to the left, in the constellation Aquila (the Eagle). The plane of the Milky Way Galaxy is marked by the line from Deneb to Albireo. The Sun's motion around the center of the Galaxy takes us in the direction of Cygnus.