Voyager 2 will photograph Neptune during the 1989 planetary year.
PHOTO: JULIAN BAUM/SCIENCE PHOTO LIBRARY
The planetary year simply refers to the positions of the other eight planets through the course of an Earth year and where you can expect to see them. As the only two major bodies closer to the sun that us, Mercury and Venus have shorter orbital periods and thus come in and out of view as they zip past and circle behind the sun. Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto all have longer orbital periods than Earth and thus come in and out of view as we pass them and circle behind the sun.
Mercury never moves far from the sun in either space or our sky, making it the most difficult to see of the bright planets. But this year Mercury reaches greatest elongation (largest apparent separation) from the sun no less than seven times. On some of those occasions, U.S. viewers will get a chance to see the planet either about 60 to 30 minutes before sunrise or about 30 to 60 minutes after sunset. Mercury's best showing at dusk, low in the west, will occur for a few weeks around May 1; it'll be at its morning-twilight best during the weeks around October 10. Mercury has some rather interesting conjunctions (close meetings) this year, including one with Jupiter before dawn on July 2 and with a bright star and Saturn after sunset in mid-December. But its most remarkable conjunction will be an ultraclose encounter with Mars on August 5; I'll describe that event in the section on Mars.
Venus will spend much of 1989 on the far side of the sun from us. At the beginning of the year, it will be low in the east as morning twilight strengthens, and will pass extremely close to three planets — Uranus, Saturn, and Neptune — in succession (of these, however, only Saturn, which Venus will go by on January 16, will be plainly visible to the naked eye). Venus will reach superior conjunction (position opposite the sun from Earth) on April 4 — interestingly, the same day that Mercury also reaches superior conjunction. After this, Venus will play its role as the evening star, getting gradually higher in the sky throughout the summer and early fall. It'll form a close but low conjunction with Jupiter on May 22, and in July will pass near Mars and the star Regulus. Finally, on November 8, the planet will reach its greatest elongation from the sun. At this time viewers with telescopes will get a view of the planet in a half-moon-like phase. The final month of 1989 will find Venus blazing brilliantly in the evening sky, looking ever larger in telescopes, and setting dramatically sooner after the sun each night. Don't miss the fine moon-Venus pairings after sunset on July 4, October 3 and November 2.
Earth seems holiday-oriented this year, at least in terms of its orbit. Our home world will reach perihelion (the closest point in its orbit to the sun) on New Year's Day, 1989, and will be at aphelion (farthest from the sun) on the Fourth of July.
Mars was as close to Earth last autumn as it has been in a generation, but during the first months of 1989 it will be left far behind by our own speedier planet. In January, Mars will cross constellations from Pisces to Aries and will shine as bright as many of the brightest winter stars. By March, it will have dimmed but will still form a dramatic conjunction with Jupiter in the west after nightfall on the 11th. The red planet will also pair up with Venus low in the west on July 11 and 12.
The real high point for Mars, though, will come after sunset on August 5, when it will be a participant in the closest fully visible conjunction of planets taking place before the year 2020. From our vantage point on Earth, the distance between Mars and Mercury will appear to be less than 1/30th of the moon's diameter. The event will be low enough in twilight to make binoculars and very clear skies desirable for viewing (the naked eye will perceive the two planets as a single point of light). To add icing to this cosmic cake, the star Regulus will be only about a degree away. In fact, three nights earlier, on August 2, Mars, Mercury and Regulus will be joined by the crescent moon, forming a marvelously tight quadruple gathering.
Jupiter will be very bright in Taurus in the south on January nights. Gradually, this slow giant will be overtaken by much dimmer Mars, leading to a fine March 11 conjunction. Jupiter will have faded by May but will still be bright enough to be seen meeting dramatically with Venus at dusk on the 22nd. Shortly afterward it will pass behind the sun and out of view until late June, when it will appear in the dawn sky in time — a few days later, on July 2 — to give us a glimpse of the planet outshining a bright Mercury low in the northeast.
In late July, Jupiter will pass from Taurus into Gemini, where it will brighten alone, rising ever earlier in the night, until it reaches opposition on December 27. Opposition occurs when an outer planet reaches the point in its orbit where it is on the opposite side of Earth from the sun. When this happens, the planet rises at sunset and is visible all night long until it sets at sunrise. As a result, this is also the time when the planet appears brightest and biggest in telescopes. Even though it's roughly half a billion miles away, Jupiter — our largest planet — usually reveals more detail than any other planet when seen through a telescope (or even good binoculars). So the December 27 event should provide excellent viewing.
Saturn will be low in the dawn sky when Venus passes near it on January 16, and over the next six months will rise ever earlier — in the middle of the night by April, at the end of evening twilight by June. Finally, on July 2, it will reach opposition, rising at sunset and shining brilliantly throughout the evening. The planet's famous rings will still be tilted, as they were last year, about 27° downward from edge-on, revealing their broad, reflective upper side. But Saturn will also reach the most southerly point in its 29 1/2-year-long orbit in 1989, making it appear for U.S. observers quite low in the southern sky in the constellation Sagittarius.
Advanced viewers with telescopes will want to pay special attention to Saturn in 1989, to witness a series of rare sights on March 3, June 24 and November 12, when Saturn will be in conjunction with dim Neptune. In fact, the two will be relatively near each other (and near several naked-eye stars in Sagittarius) all year. From July through December, Saturn will be up and ready to view as night falls.
To make finding Neptune easier, consult a detailed chart from an astronomy magazine (such as Sky and Telescope or Astronomy) or a star almanac (such as Guy Ottewell's Astronomical Calendar, available for $12 postpaid from Astronomical Workshop, Furman University, Greenville, SC 29613). One other special sight in store for astronomy enthusiasts with telescopes will occur on the night of July 3, when Saturn makes an exceedingly rare passage centrally in front of the naked-eye star 28 Sagittarii.
Uranus will be in Sagittarius in 1989, only a few degrees west (left) of Saturn and Neptune. On January 12, a good, powerful telescope might reveal its conjunction with Venus in the dawn sky; look for a dim, green speck close to a slightly brighter star and just half a degree south of Venus. By summer Uranus will be rising at sunset and shining its brightest — bright enough to glimpse with the naked eye in excellent country skies if you know exactly where to look.
This year, however, naked-eye observers wishing to spot Uranus around the time of its opposition (June 24) will have a problem: The aforementioned slightly brighter star will be too close to Uranus for the two to be separated without optical aid. The two objects will form an especially close pair on July 14 and 15. Uranus will remain visible in the evening sky until November, when it will become obscured by twilight.
Pluto in 1989 will arrive at what for sky watchers is the most favorable position of its entire 248.5-year-long orbit: perihelion, the planet's closest point in space to the sun. It'll still be no closer than 2,766,000,000 miles, but that's nearer to the sun (and to Earth) than Neptune—something that can be said for only about 20 years (in this case, from 1979 to 1999) out of each of Pluto's near-quarter-of-a-millennium cycles. Besides, when Pluto was last at aphelion (farthest out), in 1866, it was 4,566,000,000 miles out, a distance it takes light over six and a half years to traverse.
Unfortunately, even this year, when it's at its brightest, the planet will be hard to see. Pluto is much smaller than Neptune and much dimmer. A telescope with a six- to 10-inch diameter main mirror, or lens, is necessary — as are extremely detailed star charts.
Neptune should this August become the eighth of nine planets in our solar system to be photographed and studied by a passing spacecraft (only Pluto will remain unvisited). Voyager 2, launched over a decade ago, has already given us spectacular photographic views of Jupiter, Saturn and Uranus, and now will make its closest pass of all by Neptune — the last world the spacecraft will visit before heading on its way out of our solar system.
For earthbound viewers, observations of Neptune call for a small telescope, or at least very good binoculars, and the help of charts from an advanced astronomy almanac such as the Astronomical Calendar. Neptune will be at opposition on July 2, just 10 hours after Saturn.