The Education Master 1994 (4th Edition)/EDUCATIONS_MASTER_4TH_EDITION.bin/files/educnomy/ace1/aspect.dat

Text File | 1986-11-16 | 8KB | 157 lines

SUN
Hazards:  There are hazards in observing the sun, and effective safety
precautions must be taken.

Sunspots:  Successive 11-year peaks of sunspot activity follow long-term trends
that can in extreme cases result in prolonged periods of very low activity.

Aurorae:  For an observer at the ground, the shifting patterns of the aurora
over the night sky reflect the changes in the magnetic and electric fields
along the paths of electrons streaming toward earth.  A faint auroral display
may not exceed the brightness threshold of color perception for the eye; it
will be sensed as white.  Most aurorae appear green or blue-green with
occasional faint patches of pink or red.
$
MERCURY
Of the five planets visible to the unaided eye, Mercury is by far the most
difficult to observe and is seldom conveniently located for either unaided eye
or telescopic observation.

Binoculars are of great assistance in searching for the planet about 40 minutes
to an hour after sunset, or before sunrise during the periods when it is
visible.  Mercury's true color is almost pure white, but absorption from
Earth's atmosphere within 15o of the horizon, where Mercury is usually best
seen, usually imparts a yellow or ochre hue to the planet.

Telescopic observers will find the rapidly-changing phases of Mercury of
interest.  The planet appears to zip from gibbous to crescent phase in about
three weeks during each of its elongations.

Mercury's phases have been detected with telescopes of 75 mm aperture or less,
but generally a 100-mm or larger telescope is required to distinguish them.  In
larger instruments under conditions of excellent seeing (usually when Mercury
is viewed in the daytime), dusky features have been glimpsed by experienced
observers.
$
VENUS
Venus is the brightest natural celestial object in the nighttime sky apart from
the moon and, whenever visible, is readily recognized.

When Venus is about a 20% crescent, even rigidly-held, good-quality binoculars
can be used to distinguish that the planet is not spherical or a point source.
A 60-mm refractor should be capable of revealing all but the gibbous and full
phases of Venus.  Experienced observers prefer to observe Venus during the
daytime, and indeed the planet is bright enough to be seen with the unaided eye
if one knows where to look.

Venus appears to most observer to be featureless no matter what type of
telescope is used or what the planet's phase.  However, over the past century
some observers using medium- or large-size telescopes have reported dusky,
patchy markings usually described as slightly less brilliant than the dazzling
white of the rest of the planet.

When Venus is less than 10% illuminated, the cusps (the points at the ends of
the crescent) can sometimes be seen to extend into the night side of the
planet.  When Venus is a thin sliver of a crescent, the extended cusps may be
seen to ring the entire planet.
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EARTH
Moon (Luna)

Libration is the shifting, or rather apparent shifting, of the visible disc of
the moon.  Sometimes the observer sees features farther around the eastern or
the western limb (libration in longitude), or the northern or southern limb
(libration in latitude).

The moon often passes between earth and a star, an event called an occultation.
During an occultation a star suddenly disappears as the east limb of the moon
crosses the line between the star and observer.  The star reappears from behind
the west limb some time later.  Because the moon moves through an angle about
equal to its own diameter every hour, the longest time for an occultation is
about an hour.  Since observing occultations is rather easy, amateur
astronomers are encouraged to try this activity.
$
MARS
In many ways Mars is the most interesting planet to observe with the unaided
eye.  It moves rapidly among the stars--its motion can usually be detected
after an interval of less than a week--and it varies in brightness over a far
greater range than any other planet.  Mars may be distinguished by its orange-
red color, a hue that originates with rust-colored dust that covers much of the
planet.

Telescopically, Mars is usually a disappointingly-small, featureless, ochre
disc except within a few months of opposition, when its distance from the earth
is then near minimum.  Such close approaches occur at intervals of 15 to 17
years.  At a perihelion opposition much detail on the planet can be
distinguished with telescopes of 100 mm aperture or greater.  At oppositions
other than when Mars is at perihelion, the disc is correspondingly smaller.
$
JUPITER
The equatorial region of Jupiter's clouds rotates five minutes faster than the
rest of the planet.  This means that there are basically two rotational systems
from the viewpoint of week-to-week telescopic observation.

The Great Red Spot, a salmon-colored oval vortex, and the changing cloud
structures that stripe the planet can be easily observed in small telescopes
because the apparent size of the visible surface of Jupiter is far greater than
that of any other planet.  Occasionally the Red Spot loses its prominence,
becoming difficult to detect in smaller telescopes, only to return to its
normal state a few years later.

The smallest of telescopes will reveal Jupiter's four moons, each of which is
equal to or larger than earth's satellite.  A 150-mm telescope reveals the size
differences as well as color variations among the moons.  When the Galilean
satellites transit the disc of Jupiter, they are seldom visible in telescopes
under 100 mm and are best seen near the planet's limb when entering or leaving
the disc.  Tracking a satellite  transit completely across Jupiter is a
challenging observation.  On the occasions when one of the moons casts its
shadow on the disc of the planet, the thin black shadow of one of the moons can
be particularly evident if it is cast on one of the bright zones of Jupiter.
According to some observers this phenomenon is evident in a good 60-mm
refractor.
$
SATURN
Saturn is the telescopic showpiece of the night sky--the chilling beauty of the
small pale orb floating in a field of velvet.  In telescopes less than 100-mm
aperture, probably no features will ever be seen on the surface of the planet
other than the shadow cast by the rings.  As the size of the telescope is
increased, the pale equatorial region, a dusky equatorial band, and the darker
polar regions become evident.  Seldom in telescopes less than 200-mm aperture
do more than one or two belts come into view.

From earth only the three most prominent components of the rings--known simply
as rings A, B, and C--can be distinguished visually.  Cassini's Division, a gap
between rings A and B, is visible in small telescopes when the ring system is
well included to our view.  Ring C, also known as the crepe ring, is seen only
with difficulty in small telescopes.  From year to year the rings of Saturn
take on different appearances from maximum inclination to edge-on.

Titan, the largest satellite, is easily seen in any telescope about 5 ring
diameters from the planet at elongation.  Telescopes over 60-mm aperture should
reveal Rhea less than 2 ring-diameters from Saturn.  When brightest, Iapetus is
located about 12 ring-diameters west of its parent planet. Tethys and Dione may
be glimpsed in a 150-mm telescope; the other moons require larger apertures.
$
URANUS
Uranus can be seen with the unaided eye under a clear, dark sky.  It can be
easily seen with binoculars, and a telescope will reveal its small, greenish,
featureless disc.

Uranus has at least fifteen satellites, none of which can be detected in small-
or moderate-sized telescopes.
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NEPTUNE
Telescopically, the planet appears as a very small, featureless, bluish-green
disc.

Neptune's large moon Triton can be seen by an experienced observer using a 300-
mm telescope.
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PLUTO
Pluto is a difficult target in telescopes below 250 mm.
$
Compiled from OBSERVER'S HANDBOOK 1987, pp. 57-58, 106-118, 28, 90.

Copyright 1984, 1985, 1986 AstroSoft, Inc.  Excerpts from copyrighted material
are included by permission of The Royal Astronomical Society of Canada.