JULY 1998
JULY STAR MAP |
INDEX
- 097
JULY 5, 1998: Summer Stars
- The night sky this week is dominated by the moon, which is full just after
noontime on Thursday. The moon will certainly "look" full to the casual observer
for several days before and after this time which makes the week a bust for
anything except the brightest sky objects. Tonight and tomorrow, watch as the
moon passes above the bright star Antares in the constellation of Scorpius,
the Scorpion. Their separation will be about 10 degrees or one clenched fist
held at arm’s length. I will return next week to this gem of the summer sky.
In the early evening, to the right of Antares by about four to five fists
twinkles Spica, the brightest star of Virgo, the Virgin. By 10:30 p.m. they
are both at the same elevation, so finding them should be easy. Viewing them
with binoculars will show the distinct color difference between them. Antares
is a red supergiant on its way to stellar demise, while Spica is a normal
hydrogen-burning, blue-white star. Now turn towards the east. You’ll see
the three principal stars of the Great Summer Triangle rising triumphantly.
The brightest star highest up will be Vega of Lyra, the Harp. The faintest,
Deneb of Cygnus, the Swan, is to the left, while Altair is to the right. The
size of the triangle will look absolutely huge, so don’t be surprised if it
takes up nearly your full field of vision. Arching across the sky, through
the Great Summer Triangle to the left of Antares flows the "river" of the
Milky Way invisible this week because of moonlight.
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- 098
JULY 12, 1998: Scorpius, the Scorpion
- One of the most beautiful constellations of the season is Scorpius, the
scorpion, one of the twelve zodiacal constellations which the sun moves through
during the course of the year. From our latitude much of its splendor is
masked by its low altitude as it hugs the horizon in the south right after
darkness. Its brightest star, Antares, should be an easy target, even from
urban locations. It twinkles with a reddish hue. Indeed its location, color,
and brightness gave this star its name. Antares means the "rival of Ares."
Ares, the Greek god of war, the planet Mars to most of us, can pass very close
to this star. When it does, the similarity in appearance can be uncanny. The
rest of the constellation is far more inconspicuous, more for a dark sky site.
This is not a result of the stars being particularly faint, but because the
Earth’s atmosphere, coupled with dust and moisture, causes the diminution of
a star’s brightness when near the horizon. For suburban and country observers
away from any direct illumination, you’ll notice a delicate line of stars
moving down and to the left of Antares, then hooking back to the right
near the horizon. You’re looking at the body and tail of the Scorpion.
Three fairly bright stars to the right and slightly above Antares completes
the head of the Scorpion, noted in classical mythologies for having killed
the greatest hunter of them all, Orion.
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- 099
JULY 19, 1998: Observing the Milky Way
- Without moonlight to bother us this week, we are presented with an
opportunity to view our own Milky Way galaxy now gaining prominence in the
evening sky. Indeed all the stars that we see scattered across the heavens
are part of its giant pinwheel-shaped structure. The Milky Way may contain
as many as 2-6 hundred billion stars, and most astronomers agree our visible
galaxy is only the tip of the iceberg. More than 90% of the Milky Way is
composed of matter that is not shining. Some of this dark matter, in the
form of giant clouds of interstellar dust shroud most of the galaxy from
our view. When we gaze from the countryside along the plane of our galaxy,
the sky glows with a soft iridescent light, along a wide band of the
firmament. Currently about 11 pm, the Milky Way spatters the sky from
due south, then up across the Great Summer Triangle, high in the east,
then arcs downward through the rising "W" of Cassiopeia in the northeast.
By using binoculars, observers unable to escape the lights of the city can
also glimpse the Milky Way. Look high in the east before midnight. As you
move your binoculars back and forth, you’ll notice an increase in the star
count which reveals the location of where the Milky Way is secretly hiding.
View from the country and the beautiful starscapes of our galaxy will begin
to tantalize you with the insignificance of our place in the vast universe
which surrounds us.
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- 100
JULY 26, 1998: Aristarchus and the Sun's Distance
- The moon debuts this week as a waxing crescent, reaching first quarter phase
by mid-afternoon Friday. Look for the moon’s terminator, the region between
light and dark, to appear as a straight line on Friday evening. The Greek
astronomer Aristarchus of Samos (about 310-230 BC) used the straight terminator
to determine a relative distance of the Sun from the Earth. Aristarchus knew
that when the moon’s terminator was straight, the Earth-Moon-Sun angle was
exactly 90 degrees. He argued that if the Sun was relatively close to Earth,
the period of time between the first and third quarter moon would be a longer
interval than from the third to first quarter. Aristarchus also assumed that
the Moon’s orbital path was a circle, and that it revolved at a constant speed.
By measuring the exact times of the quarter moon and knowing the moon’s orbital
period, Aristarchus could proportion the number of degrees which the moon had
traversed during each interval and thus make his geometric sketch. Aristarchus
felt that the period between the third and first quarter moon was about one day
shorter than between the first and third quarter phases. This resulted in an
answer which put the sun about 20 times more distant than the Moon. In
reality the true solar distance is approximately 20 times greater than what
Aristarchus calculated, but it was still a brilliant quantitative beginning.
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