Most people have seen the five brightest naked-eye planets, yet
there is a sixth planet that can be spied without optical aid—the planet Uranus. Of course, you'll have to
know exactly where to look for it.
Fortunately, it is currently well placed for viewing in our
evening sky and with the bright Moon now out of the way this week; it will be a
good time to look for it.
Barely visible to the unaided eye on very dark, clear nights,
the planet Uranus—currently shining at magnitude +5.7, which is right on
the verge of what's
visible with the naked eye under dark skies—is now visible during the
evening hours among the stars of Aquarius, the Water Carrier. It recently
passed through opposition to the Sun and is now visible for practically the
entire night.
It is best to study the accompanying map
first, then scan that region with binoculars.
Using a magnification of 150-power with a telescope of at least
three-inch aperture, you should be able to resolve it into a tiny, pale-green
featureless disk. Uranus, which lies nearly 2 billion miles from the Sun,
has a diameter of 31,764 miles and according to flyby magnetic data from
Voyager 2 in 1986, has a rotation period of 17.4 hours.
At last count, Uranus has 27 moons, all
in orbits lying in the planet's equator in which there is also a complex of
eleven narrow, nearly opaque rings,
which were discovered in 1978.
Uranus likely has a rocky core, surrounded by a liquid mantle
of water, methane, and ammonia, encased in an atmosphere of hydrogen, helium,
methane and small amounts of acetylene and hydrocarbons. A bizarre feature is
how far over Uranus is tipped. Its north pole lies 98 degrees from being
directly up and down to its orbit plane. Thus, its seasons are extreme:
when the Sun rises at its north pole, it stays up for 42 Earth years; then it
sets and the north pole is in darkness for 42 Earth
years.
Sir William Herschel discovered Uranus on March 13, 1781, noting that it was moving slowly
through the constellation Gemini. Initially, however, Herschel thought he
had discovered a new comet. Eventually it was determined that Herschel's “comet”
was in fact, a new planet.
For a while, it actually bore Herschel's name. Herschel
himself proposed the name Georgium Sidus—“The Star of George,” after his
benefactor, the King of England. However, the custom for a mythological
name ultimately prevailed and the new planet was finally christened
Uranus.
Prior to Uranus' discovery, the outermost planet was considered
to be Saturn, named for the ancient god of time and destiny, but Uranus was the
father of Saturn and considered the most ancient deity of all.
It probably was for all for the best. After all, if
Herschel's naming request had been granted, just think of how we might have
listed the planets in order from the Sun:
Mercury, Venus, Earth, Mars, Jupiter, Saturn and . . . George?
Joe Rao
serves as an instructor and guest lecturer at New York's Hayden Planetarium. He writes
about astronomy for The New York Times and other publications, and he is also
an on-camera meteorologist for News
12 Westchester, New York.
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DEFINITIONS
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1 AU, or astronomical unit, is the
distance from the Sun to Earth, or about 93 million miles.
Magnitude is the standard by which
astronomers measure the apparent brightness of objects that appear in the
sky. The lower the number, the brighter the object. The brightest stars in
the sky are categorized as zero or first magnitude. Negative magnitudes are
reserved for the most brilliant objects: the brightest star is Sirius (-1.4);
the full Moon is -12.7; the Sun is -26.7. The faintest stars visible under
dark skies are around +6.
Degrees measure apparent sizes of objects
or distances in the sky, as seen from our vantage point. The Moon is one-half
degree in width. The width of your fist held at arm's length is about 10
degrees. The distance from the horizon to the overhead point (called the
zenith) is equal to 90 degrees.
Declination is the angular distance measured
in degrees, of a celestial body north or south of the celestial equator. If,
for an example, a certain star is said to have a declination of +20 degrees,
it is located 20 degrees north of the celestial equator. Declination is to a
celestial globe as latitude is to a terrestrial globe.
Arc seconds are sometimes used to define the
measurement of a sky object's angular diameter. One degree is equal to 60 arc
minutes. One arc minute is equal to 60 arc seconds. The Moon appears (on
average), one half-degree across, or 30 arc minutes, or 1800 arc seconds. If
the disk of Mars is 20 arc seconds across, we can also say that it is 1/90
the apparent width of the Moon (since 1800 divided by 20 equals 90).
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