Keeble Observatory
July 2006 Sky from the Keeble Observatory
July 4th is a traditional time for fireworks displays. The largest
planet in our solar system is getting ready for a July 4th display
of a different kind. Two giant storm systems are going to undergo
a near collision in plain view of terrestrial telescopes.
Half a century after Galileo’s first report that Jupiter
was not a point of light, rather a disk clearly visible in his newly
invented telescope, in 1665 Cassini recorded the first description
of a large red spot on the planet Jupiter. Using their great powers
to describe nature clearly, astronomers have come to refer to this
storm as the Great Red Spot. Located in Jupiter’s southern
hemisphere, the Red Spot is large enough to swallow three Earths,
and the winds driving the clouds have been clocked at over 350 miles
per hour. It’s a high altitude “anticyclone” –
the visible cloud tops (actually all we can see of Jupiter’s
atmosphere – there’s no real surface) circulate counter-clockwise.
In a sense, it’s like looking at the top of a hurricane. (On
Earth, northern hemisphere hurricanes have counterclockwise surface
winds. The high altitude circulation is clockwise. These motions
are reversed in the southern hemisphere.) Deep in the atmosphere
there is a clockwise inrush of gas which has no place to go but
up. As the rising gas cools, clouds form, in this case towering
ten or more kilometers above the surrounding clouds.
Its reddish color has faded and intensified over the centuries,
but the actual chemistry which gives the cloud tops this tint is
not certain. Also uncertain is what keeps this storm from dissipating
– it has, after all, been observed almost continuously for
over three centuries! - since there’s no surface below, there
can’t be a rigidly fixed source for the upwelling gas. The
Red Spot has been observed to swallow smaller storms carried nearby
on counter circulating bands of winds, and some believe this is
where it derives the energy to persist. Some of the puzzle may be
better understood in coming months, because Red has a little brother.
Image courtesy NASA/STSCI
About six years ago, two smaller storms a bit further south of
the Red Spot merged. Ovals A and B combined to form Oval BA –
yet another example of brilliant naming! Though less intense than
the Red Spot, this smaller storm showed white cloud tops thought
to be frozen ammonia. This oval is about the same size as Earth.
In 2005 the storm developed a reddish vortex which spread to color
the entire oval, now dubbed “Red Jr.” Red and its younger
brother will pass one another on the 4th of July, and since Jupiter
rotates every 10 hours or so, many terrestrial observers will have
a chance to see what, if anything happens. Some researches suspect
that the newly reddish color of the smaller spot indicates that
it has intensified, and suggest that an encounter with the larger
storm will weaken it and cause the oval to revert to its original
white color.
Lunar phases for July: First Quarter on the 3rd, at 12:37 pm (coincidentally,
Earth will be at its greatest distance from the Sun – some
152 million kilometers, or 95 million miles); Full Moon at 11:02
pm, on the 10th; Last Quarter on the 17th, at 3:12 pm; New Moon
on the 25th, at 12:31 am.
Venus remains bright in the predawn sky, rising about 2 hours before
the Sun early in the month, but more like an hour and a half by
the end of the month. Look for a brilliant “star” about
20-25 degrees above the eastern horizon. Mercury will return to
the morning sky by the end of the month, shining brightly below
Venus, which will fade a little as it pulls ahead in its orbit and
farther from Earth.
Evening planet watchers will have more choices, at least right
after sunset. Jupiter begins at dusk about 50 degrees above the
southern horizon. To the west at sunset, the first planets to emerge
from twilight will again be Mercury, Mars, and Saturn. Look for
three bright planets strung out every 5 degrees or so to the west
early in the month. Saturn and Mercury will end the month disappearing
into the Sun’s glare, and Mercury will return to morning skies
around the 25th. By the last week, even Mars may be too low at sunset
to pick out of the haze and clutter.
Looking at mid-month toward zenith, about two hours after sunset,
you will see a faint constellation looking like a slightly squashed
square. This is Hercules, and although the constellation has no
truly bright stars, binoculars should allow you to find the great
globular cluster known as M13 just to the west of zenith. The Summer
Triangle is to the east. High and bright (about 70 degrees from
the horizon) is Vega, in Lyra. At about 50 degrees, below and to
the northeast is Deneb, in Cygnus. Altair, in Aquila, is to the
southeast about 40 degree altitude. Back to Cygnus, which lies roughly
parallel to the horizon, you can see how this constellation came
to be known as the Swan. Follow the “body” from bright
Deneb at the tail to Albireo at the head. Albireo is not terribly
bright to the naked, but shows a brilliant pair of orange and blue
stars in a small telescope. Turning back to the west, that bright
star about 50 degrees from the horizon is Arcturus, in Bootes. Below
and to the right of Jupiter is Spica. Turning your gaze to the south
you’ll see bright red Antares, in the Scorpion.
For your own monthly star chart, you can direct your web browser
to http://www.skymaps.com.
You will find extensive descriptions of what's worth looking for,
and you can download and print a single copy for your personal use.
Copyright 2006
George Spagna