Oct 22, 1999

A Close Encounter with Jupiter



A close encounter with Jupiter


This weekend the Solar System's largest planet will be brighter and nearer to Earth than at any time in the past 12 years.


UPDATE: The closest-ever pictures of Jupiter's volcanic moon Io have been released today.
See for details.


Jupiter and the Moon on
Oct 23, 1999
October 22, 1999: Lately, you may have noticed a bright light in the sky that rises above the eastern horizon around dinnertime. It's not an airplane or a UFO -- it's Jupiter, the largest planet in the solar system. This weekend Jupiter will be brighter and closer to the Earth than it has been for a dozen years.

Right: Duane Hilton's rendering of Jupiter and the full moon on October 23, 1999 as seen near 10 p.m. local time from Bryce Canyon.

On October 23, 1999 Jupiter will be "at opposition." That's an astronomical term that simply means you can draw a perfectly straight line between the Sun, the earth, and Jupiter as shown below. This geometry isn't unusual; Jupiter is at opposition about once a year. However, there is something special about this year's opposition. Jupiter is passing through the part of its elliptical orbit that is closest to the Sun. That means it will also be closest to Earth. On October 23 Jupiter will be 368 million miles away from our planet -- just a hop, skip, and a jump by cosmic standards.
Jupiter at opposition
Left: When Jupiter is at opposition it is directly opposite the Sun as seen from Earth. Compared to Earth, which zips around the sun once a year, Jupiter completes one circuit around the Sun rather slowly, once every 12 years. Opposition takes place about once per year as the Earth speeds by the slower-moving giant planet.
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To see Jupiter this week, simply go outside shortly after the sun sets and look toward the east. Jupiter, blazing at magnitude -3, will be rising above the eastern horizon. The giant planet will be visible all night long. It crosses the southern meridian at local midnight and sets in the western sky at dawn.

This is a good time to look at Jupiter through a telescope. Jupiter will appear to be nearly 50 arc seconds in diameter, approximately 36 times smaller than the full moon. That's big! Even inexpensive department store telescopes will be powerful enough to reveal light and dark bands crossing Jupiter's equator and Jupiter's four largest moons: Io, Europa, Ganymede and Callisto. It's also a good time to view Jupiter's famous Great Red Spot, a gigantic storm system that has been churning for at least 300 years. Tonight, October 22, the Red Spot will move across the center of Jupiter's disk, as seen from Earth, around 9:19 p.m. EDT. A six-inch or larger telescope and good viewing conditions are recommended for seeing the red spot. The Galilean satellites can easily be seen through a good pair of binoculars. (As an interesting exercise, see how steadily you can hold the binoculars as you're viewing Jupiter. It's not as easy as it sounds!)
Left: This Hubble Space Telescope image of Jupiter shows a hurricane twice the size of the Earth. It has been raging at least as long as telescopes could see it, and shows no signs of slowing. It is Jupiter's Great Red Spot, the largest swirling storm system in the Solar System. Like most astronomical phenomena, the Great Red Spot was neither predicted nor immediately understood after its discovery. Still today, details of how and why the Great Red Spot changes its shape, size, and color remain mysterious. [More]

If you're out this weekend admiring Jupiter, don't forget that NASA is watching the giant planet, too, although with a better view. NASA's Galileo spacecraft has just marked the tenth anniversary of its launch aboard the Space Shuttle Atlantis on October 18, 1989. Galileo entered orbit around Jupiter in December 1995. Since then it has revolutionized our understanding of Jupiter and its moons by sending a probe into Jupiter's atmosphere, finding new evidence for water on Europa, and most recently zipping a mere 600 km above the volcanoes of Io. Many of Galileo's impressive accomplishments are described at JPL's new Galileo home page: The current Galileo Extended Mission will conclude later this year with a daring pass less than 300 km above Io, possibly flying through the plume of a sulfurous volcano. For more information about the Io flybys, please visit




Web Links


This week's sky at a glance -- from Sky & Telescope

Jack Star Gazer -- Jack Horkheimer's naked eye astronomy web site

Jupiter - from the SEDS Nine Planets web site

Saturn - from the SEDS Nine Planets web site

More NASA Science News


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For more information, please contact:
Dr. John M. Horack , Director of Science Communications
Author: Dr. Tony Phillips
Curator: Bryan Walls
NASA Official: Frank M. Rose