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January 2005 Feature: Saturn at Opposition & Other Tidbits

The Five-Planet Parade

The first two weeks of January offer the best opportunity to see all five visible planets in the same sky till 2016. But you need to be up an hour or so before sunrise to see the lights of Mercury, Venus, Mars, Jupiter and Saturn all strung across the sky. For a map showing the predawn positions of these planets, click here. Mercury and Venus stage themselves in a single binocular field of view throughout late December and early January. If you can't see Mercury with the eyes alone, aim your binoculars at Venus, the brightest of all these planets, to spot Mercury nearby!

Saturn at Opposition

If you're not a morning person, you must at least step outdoors on January evenings to see Saturn, this golden gem adorning the night sky from dusk till dawn. Earth in its orbit passes between the Sun and Saturn on January 13 -- at which time Saturn is said to be at oppostion. At opposition, a planet rises at sunset, climbs to its highest point in the sky at midnight, and sets at sunrise. And at opposition, a planet also comes closest to Earth and shines at its brightest for the year.

But oppositions of Saturn, which come about two weeks later every year, are hardly the same each time around. After this year's opposition, Saturn will dim year by year, and in the years 2009 and 2010 will appear at its dimmest in Saturn's 29.5-year cycle. Saturn won't match the brightness of this year's opposition till the opposition of December 11, 2031. (For a chart giving the dates of Saturn's oppositions for the years 2001 to 2400, click here.)

There are two reasons why this year's opposition is so favorable. First, Saturn is about as close to Earth as the ringed planet can get -- a distance of just over 8 AU. (AU stands for "Astronomical Unit" -- the mean distance between Earth and the Sun -- about 93,000,000 miles or 150,000,000 kilometers.) Second, the rings are almost as wide open as they can get. These rings reflect a lot of sunlight in our direction, making the planet bright to the naked eye, and a wonder to behold through a small telescope. In 2009, Saturn's rings will appear edge-on, featuring the planet at its dimmest. For some idea of how Saturn's appearance changes over the years, click on Saturn Oppositions: 2001-2029.

Occultation of the star Antares

On January 7, the Moon will occult -- or pass in front of Antares, the bright, ruddy star depicting the beating heart of Scorpius the Scorpion. This is the first occultation of Antares since April 4, 1991. January's occultation of Antares represents the first of a series of monthly occultations of Antares that will last for five years, the final occultation to pass on February 7, 2010.

If Antares resided on the ecliptic (the Earth's orbital plane), this series of Antares' occultations would not be nearly so enduring. As an example, the star Regulus of Leo the Lion stands on the ecliptic, and yet its series of monthly occultations by the Moon will not last for even one and a half years. Its series of occultations will begin on January 7, 2007 and conclude on May 12, 2008.

Now, for an explanation -- or perhaps, more precisely, a suggestion of an explanation. The plane of the Moon's orbit around Earth is inclined at about five degrees to the plane of the ecliptic. For half the month, the Moon is north of the ecliptic, and for the other half of the month is south of the ecliptic. Antares, unlike Regulus, lodges close to the southern limit of the Moon's travels south of the ecliptic. Stars that stand rather close to the northern or southern limit of the Moon's orbit north or south of the ecliptic have the longest series of lunar occultations.*

The Moon intersects the ecliptic at points called nodes. An ascending node is when the Moon moves south to north, and a descending node represents the Moon moving north to south. The nodes move 360 degrees westward along the ecliptic in periods of 18.6 years. So expect the series of Antares occultations to repeat themselves in cycles of 18.6 years. The next series of Antares occultations will be from August 25, 2023 to August 27, 2028.

An ecliptical star, such as Regulus, has a much shorter occultation series than a star hovering near the northern or southern regions of the Moon's orbit, yet an occultation series for an ecliptical star happens twice as often. For Regulus (a star close to but not exactly on the ecliptic), the series repeats itself roughly every 9.3 years, the next series taking place from December 18, 2016 to April 24, 2018!

Comet Machholz nears the Bull's-eye

Comet Machholz starts off the year near Aldebaran of Taurus, the star representing the ruddy eye of the Bull. By the end of the first week of January, Comet Machholz will be breathtakingly close to the beautiful Pleiades star cluster! Moreover, astronomers are predicting that this comet will be at its brightest at about this time. Though the brightness of comets is notoriously difficult to predict, it's thought that Comet Machholz will become a naked eye object in a dark country sky, and will almost certainly be a binocular target.

Update: yes, it's already an easy binocular object!! Saw the comet on the night of December 17, after what seemed like weeks of cloudy weather at my home in northern New York. At the end of the first week of January -- when the Moon will be close to new -- Comet Machholz is expected to be about 2.5 times brighter than it is now (December 17). It should be grand! To read more about Comet Machholz and to view some sky charts showing the comet's position amidst the stars, read Comet Machholz in the Evening Sky by Greg Bryant and Alan M. MacRobert, and Newfound Comet Set for Winter Diplay by Joe Rao.

copyright 2004 by Bruce McClure

Occultation Resources: Astronomical Tables of the Sun, Moon and Planets and Mathematical Astronomy Morsels by Jean Meeus
*On page 116 of Jean Meeus' Mathematical Astronomy Morsels, it's reported that stars residing 4 degrees north or south of the ecliptic have 5.9-year durations, and stars residing 4.67 degrees north or south of the ecliptic have 4.9-year durations.
New Year's Special by Steve Daniels

Astronomy & The Da Vinci Code

December 2004 Feature * February 2005 Feature