Celestial Lights, Sept 1, 2006
By Phyllis Pitluga
Our Solar System: Eight planets + three dwarf planets
The International Astronomical Union has a new definition for what constitutes a planet: a planet must orbit a star, be nearly round in orbit and have cleared the neighborhood around its orbit. In other words, nature decides what gets included. Gravity is the force that determines what a celestial body orbits, the shape of a celestial body and what it can attract around it (moons, rings). By this definition, our Solar System now has eight round worlds orbiting our star, the Sun. In addition, Pluto has been reassigned to the category Dwarf Planet.
To see for yourself why this happened, study the chart below. The numbers have been rounded for simplicity and easy comparison. DIAMeters are in miles, DISTances in Astronomical Units with Earth as 1 (which equals 93 million miles), ORBITs are in years, INCLination is how tipped the orbit is in degrees, and DISCovery is the date that telescopic worlds were first detected.
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The three Dwarf Planets, in capital letters above, are Ceres, Pluto and Xena. (As a name, “Xena” has not been officially approved yet. Thus, its official designation is 2003 UB313.) Ceres orbits between Mars and Jupiter and is the biggest of the countless asteroids and is big enough to have solidified as a round world. |
Xena is larger than Pluto and twice its distance. It orbits out where there are countless icy worlds. All three have highly inclined orbits to the plane of the Solar System (10-, 17- and 45-degree tilts). By adding the stipulation that a planet must have the gravity to clear its neighborhood of other competing bodies, Pluto got demoted. This is because Pluto’s very non-round orbit brings Pluto as close to the Sun as Neptune’s orbit—though Pluto passes high above Neptune’s pathway around the Sun.
More Dwarf Planets will be named and announced in the coming months and years. Currently, there are a dozen more candidates on the list of potential Dwarf Planets. This list keeps changing as the shapes and orbits become better known and as new objects are found using the huge telescopes available today.
| NAME | |
DIAM | |
DIST | |
ORBIT | |
INCL | |
DISC | |
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| Mercury | |
3000 | 1/3rd | 1/4th | 7.0 | ||||||
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| Venus | |
7500 | 2/3rds | 2/3rds | 3 | ||||||
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| Earth | |
7900 | 1 | 1 | 0 | ||||||
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| Mars | |
4200 | 1.5 | 1.8 | 2 | ||||||
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| CERES | 591 | 3 | 4.6 | 10 | 1801 | ||||||
| Jupiter | 88,000 | 5 | 12 | 1 | |||||||
| Saturn | 74,000 | 10 | 30 | 2 | |||||||
| Uranus | 32,000 | 20 | 84 | 1 | 1781 | ||||||
| Neptune | 31,000 | 30 | 165 | 2 | 1848 | ||||||
| PLUTO |
1475 | 40 | 251 | 17 | 1930 | ||||||
| XENA | |
1850 | 95 | 500 | 45 | 2003 |
Sky Calendar, September 2006
By following the Moon as the biggest and brightest “pointer” in the sky, during the month, you can identify different planets and bright stars. On following nights you can relocate them but without the Moon—the Moon moves about 25 times its own diameter from one night to the next. The Moon is much closer than the planets of our Solar System, and the stars are even farther. So, when the Moon appears close to a celestial light they are truly separated by millions, billions or trillions of miles.
September 5, Tuesday: Very bright Venus passes in the foreground above the bright and more distant star Regulus. They are in the east dawn sky.
September 7, Thursday: Full Moon rises at sunset; the September Full Moon is the Harvest Moon—called this because its bright light allows farmers to continue to harvest into the night; this Full Moon is especially notable this year because eight hours later, the Moon is closest to Earth in its monthly journey around our planet; this means that the high tides will be extraordinarily high on this day. The Moon also is skimming through the Earth’s shadow, creating a partial lunar eclipse, but it won’t be visible in the Americas. For the next two weeks, the Moon rises about 50 minutes later each night.
September 12, Tuesday: The Waning Gibbous Moon passes above the Pleiades Star Cluster.
September 14, Thursday: Last Quarter Moon rises after midnight.
September 18, Monday: Moon passes above Saturn.
September 21–October 7: Zodiacal Light will be visible in the dawn sky away from other bright lights; this cone of light is from myriad grains of meteoric material in the plane of our Solar System; each grain is like a teeny moonlet reflecting the Sun’s light. Together they create the Zodiacal Light; this time of year the cone of light is angled up nearly perpendicular so it stands up from the glow of dawn.
September 22, Friday: New Moon (= no Moon because the dark side faces Earth) and the Autumnal Equinox, the day when the Sun is crossing the Celestial Equator from north to south (the Celestial Equator is an imaginary extension of the Earth’s equator out into space). As we orbit the Sun on our tipped planet, the Northern Hemisphere is tipped toward the Sun in June, placing the Sun above the Celestial Equator; in December, we are tipped away, placing the Sun below. At the equinoxes in March and September, the Sun is on the Celestial Equator.
September 23, Saturday: Crescent Moon passes below Mercury in the evening sky.
September 24, Sunday: Crescent Moon passes just below the star Spica, the brightest star in the constellation of Virgo.
September 27, Wednesday: the planet Mercury stands above the star Spica in the west after sunset.
September 28, Thursday: Crescent Moon passes just below the star Antares, the brightest star in the constellation of Scorpious.
September 30, Saturday: First Quarter Moon (halfway across