Planet Jupiter. Credit: NASA, ESA, A. Simon-Miller (GSFC),
I. de Pater, M. Wong (UC Berkeley)

Opposition is the best time to see a planet like Jupiter: it's at its largest visual size, it's nowhere near the glare of the Sun, and it's in the sky all night long. Score on three counts—how many things in life work out so that the best of all worlds occur at the same time?

The Opposition of Jupiter occurs about once each year, the timing governed mostly by Earth's relatively speedier orbit around the Sun. Earth takes one year to revolve, while Jupiter takes closer to 12 years. So, opposition actually happens every year plus one month; in the time Earth has taken to come back around to the previous point of opposition, Jupiter has moved along one twelfth of its orbit, so it takes Earth an extra month to "catch up."

This year, the date of the Opposition of Jupiter is September 21, at which time the planet will be only 3.95 Astronomical Units away—about 591 million kilometers (369 million miles). Sounds faraway—and it is—but Jupiter's size helps make up for this. With a diameter of 142,984 kilometers (over 11 times that of Earth), even at this distance there's a lot to see, even through a small telescope.

I remember observing Jupiter through my first telescope—a 4-inch reflector. Jupiter's four "Galilean" moons are easy to see even through a telescope that small, but I could also make out, just barely, the parallel streaks across Jupiter's face that are formed by its cloud belts…ah, my young eyes then! At Chabot, we'll be observing Jupiter in the weeks ahead—come up and take a look through some very large telescopes, Friday and Saturday nights!

Right now, Jupiter rises after 9:00 PM, but by mid-September it will rise closer to 7:30 PM. We should be picking it up toward the end of September, and then onward into the Autumn.

Current Event from Jupiter: Another "big whack" observed, on August 20th 2010. Any planet is potentially subject to the occasional impact by a large object—an asteroid or a comet. Even Earth. But, it appears, Jupiter has been hit by comets and/or asteroids rather frequently of late (three times in the last 13 months), leading some to suggest that this kind of thing may go on more often that at once thought.

So as the recent clustering of the planets Venus, Mars, and Saturn ride off into the sunset to our west, we can now turn the other way and welcome the return of Jupiter, and maybe give our thanks to this huge debris-vacuuming gravity trap of our solar system that may have taken some cosmic bullets for us….

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Mars as seen through Chabot Space & Science Center’s 20-inch telescope near the 2003 close encounter. Credit: Conrad Jung/Chabot Space & Science Center

As August approaches, the ghost of Mars returns to haunt us, in the form of emails and phone calls from people asking if it's true that Mars is about to get closer to Earth than it has been in a gazillion years—so close that it will look as big as the Full Moon.

I say "ghost" because it simply isn't true, here in 2009. I say "haunt" because, six years ago, it was true—at least, partly.

The time: August 27, 2003. The scene: Earth and Mars. The event: Mars is coming into opposition—the time when Earth passes directly between Mars and the Sun, and consequently Mars is closest to us and at the opposite point in the sky from the Sun—hence "opposition." A routine encounter, one that happens about every 2.2 years. But what's different with this Mars opposition is the distance between Earth and Mars at closest approach: the two planets are closer together than they have been in a very long time: a bit less than 35 million miles.

This was a very big deal, you may recall. We remember it very well at Chabot: On one of the evenings that weekend, we had 2000 people who came up to see Mars through our telescopes…. A close opposition is the best time to see a planet, and this was closer than average for Mars by maybe 10 million miles. (It was at another very close opposition of Mars when Percival Lowell made his famous "Martian canals" observations and Martian civilization hypothesis, back in 1894.)

At the time of the 2003 opposition, there were a lot of reports—emails, websites, blogs—flying around describing the event, in some cases with exaggeration. One exaggeration is the amount of time since the previous closest encounter with Earth. Different accounts suggested a thousand years, ten thousand years, even one hundred thousand years. Technically this may have been true, if one were calculating down to the inch. Practically speaking, however, the opposition in 1924 was almost as close, by a difference of only 12,000 miles (one and a half Earth diameters).

The other (gross) exaggeration was a statement made that at opposition Mars would appear as large as the Full Moon. That would be spectacular! However, at some point a piece of information was lost from the original message: the part about needing to look at Mars through a telescope to achieve the advertised view.

The final piece of information missing from that message—which gave birth to the annual Mars Hoax –was the year, 2003, omitted along the way and making every August 27th a day to view the splendor of Mars in all its glory. But, alas, the ghost of Mars.

For the record, the next extra-close opposition of Mars will occur on August 15th, 2050, when it will be only 200,000 miles farther than the 2003 near-miss….

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