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
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.