Jul 24, 2000

Camping Out with the Planets


Camping with the Planets


Earth's slender crescent Moon will glide by two brilliant planets in the dawn sky this week as it heads for a close encounter with Mercury on July 29.


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July 25, 2000 -- In Bill Watterson's comic strip Calvin and Hobbes, young Calvin is appalled every summer when his father takes the family on their annual camping trip. Sleeping on cold ground, fishing in the rain, waking up at dawn -- none of these hold as much appeal to Calvin as a good long spell watching TV indoors at a reasonably civilized hour.

This week, early-rising campers who can't sleep because of a rock underneath their sleeping bag or bears rustling through their food sacks can enjoy a spectacle that even Calvin might appreciate. As dawn breaks, Jupiter and Saturn will be sparkling brightly above the eastern horizon. The two are so brilliant that they're easily seen against the brightening blue sky as late as 5:30 a.m. in some places. On Wednesday morning, July 26th, something special happens: the Moon will glide by the pair of planets to form an arresting celestial trio.

Above: Artist Duane Hilton created this rendition of Jupiter, Saturn and the Moon as they might appear to a mid-latitude stargazer in the northern hemisphere on July 26, 2000. The Pleiades, a beautiful asterism of seven faint stars that look like a mini-Big Dipper, appear in the upper left corner of the image. A finder chart for southern observers appears later in the article.




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"For observers in the northern hemisphere the planet Saturn will lie less than 3 degrees from the Moon," says Mitzi Adams, a Marshall Space Flight Center astronomer. "In what should be a beautiful sight, the Moon, Jupiter, and the Pleiades will form a sort of triangle in the pre-dawn sky. Just look due east about 35 to 40 degrees above the horizon at about 4:30 or 5:00 a.m." "

Fortunately, you don't have to be camping to enjoy the show. Jupiter (magnitude -2.2), Saturn (magnitude +0.2) and the crescent Moon (magnitude -9.0) will be so bright that they can be spotted from urban areas where light pollution normally makes stargazing difficult.



If you oversleep on Wednesday morning, you'll have another chance the next day. At dawn on Thursday, July 27th, the Moon will still be close to Jupiter and Saturn, but even closer to the red giant star Aldebaran, the fiery eye of Taurus the Bull. Aldebaran is not as bright as Saturn or Jupiter, but it is a 1st magnitude star that stands out clearly before sunrise.




Jupiter in the News


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Last week astronomers from Harvard and the University of Arizona announced their discovery of Jupiter's 17th satellite. It's the first finding of a new Jovian moon since the Voyager spacecraft spotted three of them in 1979. Although Jupiter is a giant planet -- it's 11 times wider than Earth -- many of its moons are remarkably tiny. This one, provisionally designated S/1999 J1, is only about 10 km wide. It could be the smallest moon circling any of the gas giants.


Right: These discovery observations (credit: Spacewatch Project) show the 20th magnitude moon S/1999 J 1 moving upwards and to the left in a three-frame sequence of images. [more information]

S/1999 J1 belongs to a group of outer moons that travel around Jupiter in irregular orbits at an average distance of 15 million miles. The Sun's gravitational influence makes these orbits highly erratic. The satellites orbit in the opposite direction to the other Jovian satellites and have undoubtedly been captured long ago from orbits about the Sun.

As the week progresses and the Moon moves closer to the Sun, its crescent will become increasingly slender. On Saturday morning, July 29th, the sliver-thin Moon will pass less than two degrees from the elusive planet Mercury. Of all the planets that can be seen with the unaided eye, Mercury is the most difficult to spot because it is always near the Sun. This Saturday morning Mercury's solar elongation (its apparent distance from the Sun in the sky) will be approximately 19 degrees. It won't be so far from the glare of the Sun again until October 6, 2000. To find the pair from mid-northern latitudes, just look low on the eastern horizon before dawn. The bright zero magnitude object near the Moon is the first planet from our Sun.

Dark-sky observers watching the Moon travel from Jupiter and Saturn on Wednesday to Mercury on Saturday will also be able to see the ghostly outline of the full Moon cradled in the arms of the slim crescent. It's a dim glow that astronomers call "Earthshine."


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Above left: Northern hemisphere observers at middle latitudes should look due east for the Moon, Jupiter and Saturn at 4:30 to 5:00 a.m. local time on July 26th. Above right: In the southern hemisphere the constellations will appear to be rotated with respect to their northern counterparts. This finder chart shows what the sky will look like over Australia on Thursday, July 27th.

Like all the planets we see in the night sky the Moon does not shine by its own light. It reflects sunlight. The side of the Moon facing the Sun shines brightly; the side facing away is nearly dark. The only significant illumination on the "dark side of the Moon" is due to Earthshine -- sunlight that bounces off the Earth and falls on the lunar surface. A slender crescent Moon with Earthshine is widely regarded as one of the most delicate and beautiful sights in the night sky.

"The phase of the Earth as seen from the Moon is nearly full when the Moon is crescent," explains George Lebo, an astronomer at the NASA Marshall Space Flight Center. "Because the Earth is four times bigger than the Moon and is a ten times better reflector, the 'Full Earth' is 160 times brighter than the 'Full Moon.' That's why earthshine is so noticeable."

Web Links


A New Moon for Jupiter - Harvard CfA press release

"A pre-dawn sky for the last week of July" - Jack Stargazer, Episode #00-29

This Week's Sky at a Glance -from Sky & Telescope


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