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By convention we think of north as "up" and south as "down." We tend to view the Earth's North Pole as being "above" the ecliptic (Earth's orbital plane) and the South Pole as being "below" it. But since Venus' North Pole points "down" while its South Pole points "up," Venus throws a monkey wrench into this accustomed way of looking at things. Because the north pole of a planet -- by definition -- belongs to the hemisphere that rotates counter-clockwise, Venus' North Pole points south of the ecliptic.
However, after consulting a number of astronomers, I'm not so sure there's a firm concensus on the definition of "north pole." Apparently, some feel a planet's north pole is the one that points north of the ecliptic, regardless of the planet's rotational direction. (See note.)
The term "north pole" notwithstanding, we can still visualize Venus -- the "upside down" planet -- in space. From the perspective of the Earth's Northern Hemisphere, all the planets (including Venus) revolve counter-clockwise around the Sun. Unlike Earth and most of the planets, however, Venus rotates in retrograde, or in a direction opposite to its revolution. Venus' "upper" hemisphere rotates clockwise whereas its "lower" hemisphere rotates counter-clockwise, which is the reverse of planet Earth.
It's commonly believed that Venus was once an "upright" planet like Earth, with its "upper" hemisphere initially rotating counter-clockwise. Theoretically, a colliding asteroid or some mighty force caused Venus to tumble over at some stage in its storied past. More recently, astronomers suspect Venus' heavy atmosphere might have something to do with it.
Be that as it may, how would we in the Northern Hemisphere be affected if Earth -- like Venus -- flip-flopped 180 degrees? All other things being equal, we'd be "down under," since Polaris, the north star, would be at zenith over the Antarctic Pole. Surprisingly, the Sun would still rise and set in the "same" direction -- in the respect that the Sun would first rise over the Atlantic Coast and set last upon the Pacific Coast.
We'd see a marked difference at night, however. We'd never see the North Star anymore and the Big Dipper only seasonally. The Southern Cross would be seen all year long.
But that's hardly the end of it. Stars would rise and set four minutes LATER (not earlier) with each passing day. Heliacal risings would occur at DUSK, and heliacal settings at DAWN. (Heliacal refers to a star's first annual appearance -- or disappearance -- in the twilight, the star being lost in the Sun's glare for a portion of the year.) In addition, the waxing quarter Moon would appear in the MORNING sky, while the waning quarter Moon would appear in the EVENING sky.
Now "upside down," Earth rotates from east to west, no longer west to east. Since the Sun in this scenario rises in the west and sets in the east, this means -- in the view of some astronomers -- that the Atlantic would become the WEST COAST, and the Pacific would become the EAST COAST. Other astronomers say it's all a matter of definition.
Wow! Such a topsy-turvy world might be best left to the Venusians!
copywrite 2004 by Bruce McClure
NOTE: Years after having written this article, I read the following on page 299 of More Mathematical Astronomy Morsels by Jean Meeus: "In 1970 the International Astronomical Union (IAU) decided that the rotational pole of a planet or satellite that lies on the north side of the invariable plane of the solar system shall be called north. (This plane is close to that of the ecliptic.) We regret this decision and would prefer that the north pole be the one above which an observer sees rotation in the direct (counterclockwise) sense. Thus, for instance, Venus' north pole would be south of the ecliptic. Our definition does not depend on a particular reference frame, eliminates negative rotation rates and simplifies the mathematics."
"Upside Down" World was orginally published in the North American Skies Newsletter.