June 2006 Sky from the Keeble Observatory
How big is the solar system? To get a sense of its scale, let’s compare to some
familiar objects. Imagine, first, that Earth was the size of a basketball. Our Moon
would then be roughly the size of a baseball some 30 feet away. The Sun is 100 times
the size of the Earth-basketball and 400 times farther than the Moon, so think of
a giant (100’ diameter) hot air balloon two and a quarter miles away. Let’s “re-scale”
so we don’t have to walk so far – let the basketball represent the Sun, then Earth
becomes a 3 mm ball bearing a mere 120 feet away, with the Moon less than a millimeter
in diameter a little over 3½ inches from the Earth-bearing. Jupiter would
be a ping-pong ball 600 feet from the basketball Sun; ringed Saturn a little smaller
but twice as far away, its rings represented by a miniature CD. Let’s call the distance
from Sun to Earth an Astronomical Unit (1 AU). Jupiter lies at about 5.2 AU, Saturn
at 10 AU. Far Pluto is almost 40 AU – almost a mile on this scale. The distant Oort
Cloud, reservoir of long period comets is a thousand times farther.
Humankind’s most distant ambassadors to the cosmos are approaching one of the physical
boundaries of the Solar System, the so called “termination shock” where the solar
wind plows into the interstellar medium. The solar wind is a stream of fast moving
ions, which effectively tries to “blow a bubble” in interstellar space. When the
solar wind slows to supersonic speeds it creates a standing shock wave. Voyager
1 and Voyager 2, both launched in 1977 and which long ago returned images and data
from Jupiter, Saturn, Uranus, and Neptune (only Voyager 2 for these last two planets)
have been moving outward towards the termination shock ever since. In 2003, we told
you about Voyager 1 encountering the termination shock, some 85 AU from the Sun.
Now Voyager 2 is returning data suggesting that it, too, has approached that boundary
at about 76 AU – suggesting that the “heliosphere” is not spherical, but bulges
outward in the “northern” hemisphere and is compressed inward to the south. Voyager
1 is currently about 8.7 billion miles from the Sun, traveling at 3.6 AU per year.
Voyager 2 is 6.5 billion miles out, traveling at a leisurely 3.3 AU per year.
Lunar phases for June: First Quarter on the 3rd, at 7:06 pm; Full Moon at 2:03 pm,
on the 11th; Last Quarter on the 18th, at 10:08 am; New Moon on the 25th, at 12:05
Predawn planet watchers will have to settle for Venus, which will be brilliant about
20 degrees above the eastern horizon. It will drift a bit to the north as the month
Evening planet watchers will have more choices. Jupiter begins at dusk about 25
degrees above the southeast horizon. It sets several hours after midnight. To the
west at sunset, the first planets to emerge from twilight will be Mercury, Mars,
and Saturn. Mercury begins the month low (about 15 degrees from the horizon) but
moves higher as we get later in the month, reaching greatest eastern elongation
on the 20th. Saturn and Mars are higher, beginning the month several degrees apart,
but rapidly closing to about half a degree separation on the 17th. Binoculars will
also show the open Beehive Cluster in the same field of view.
Our overhead view at midmonth, about two hours after sunset, finds little in the
way of bright stars directly overhead. However, let’s take a tour of the brighter
stars and constellations closer to the horizon. To the north, we can find the rather
faint Ursa Minor (Little Dipper). The familiar shape is as thought he dipper is
hanging from its bowl (or standing precariously on its handle), with the end of
the handle at Polaris, the “north star.” Just to the northwest, we find the brighter
and more familiar Big Dipper of Ursa Major, apparently hanging from its handle.
Turning to the west, we find the familiar sickle of Leo, with its brightest star
Regulus. To the southwest, we find Spica in the constellation Virgo – virtually
the only bright star in that asterism, familiar in name only because it’s one of
the constellations of the zodiac. Jupiter is much brighter and a bit to the left.
That bright red star to the south-southeast is Antares (“against Mars”) in Scorpio.
Very low to the southeast lies the constellation Sagittarius, marking the direction
towards the center of our Galaxy. If you get a rare clear and haze free night, binoculars
will show this to be a very rich field, with many bright clusters. To the east and
northeast we find the so-called “summer triangle.” Bright Vega is high to the east-northeast,
with Deneb in Cygnus below and a bit to the left. To the right, more nearly due
east, is Altair. This grouping will climb higher into the evening sky as summer
Speaking of summer, the solstice will take place on the 21st, when the Sun reaches
its furthest northern point at 8:26 am. This marks the beginning of “astronomical
summer” even though the daylight hours will begin to get shorter.
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.