Keeble Observatory
June 2004 Sky from the Keeble Observatory
We will be unable to view the June 8th "transit of Venus"
from the Keeble Observatory - the event will be over before the
rising Sun clears our obscured eastern horizon. You will, however,
be able to view it from several global web sites: The
Exploratorium, in San Francisco will carry links to the Penteli
Observatory, near Athens, Greece. NASA
will offer links to several observatories around the world. The
European Southern Observatory
will have live coverage, as well.
May weather was not kind to comet watchers in the Center of the
Universe. Haze on those evenings when it wasn't actually cloudy,
and clouds and rain on other days, meant that it was unlikely that
anyone actually saw comets NEAT and LINEAR. June weather, typically,
will not be any improvement, and the comets are fading rapidly as
their orbits carry them away from the Sun. There will be others
in the future, hopefully more fortuitously positioned. Bright comets,
like Hale-Bopp of several years ago, are roughly once per decade.
Comets have long fascinated humankind. Regular meteor showers and
the stately motion of the planets through the zodiac could be anticipated.
But, what to make of these "hairy stars" which persist
for months, seemingly at random? (Our word "comet" comes
from the Latin "coma" - which means hair!) Aristotle assumed
they were atmospheric phenomena, describing them in his treatise
on weather. Tycho Brahe used geometry to show that they were in
the realm of the planets (but not the stars). It was once church
doctrine that these represented harbingers of evil - firebrands
hurled by an angry God to warn sinful humankind. Indeed, Halley's
Comet was high above the Battle of Hastings, which was certainly
bad luck for the English. It might have been considered good luck
for the Norman victors!
Tycho got it right, of course. Comets are part of our solar system,
just as much as are the planets, asteroids, and meteoroids. Indeed,
meteor showers are residue from comets, which provides us a clue
to their origin and makeup.
Some 5 billion years ago, comets were among the first large aggregate
objects to condense out of the cloud of gas and dust which formed
our solar system. Far from the growing heat source at the center,
which was to become our Sun, they contain dust and frozen ices of
water, methane, and ammonia. We believe these represent the oldest
undisturbed remnants of the original pre-solar cloud. Pristine material
from the time the Earth and other planets first formed. Fred Whipple
dubbed them "dirty snowballs" and the description is apt.
There are two major reservoirs of comets left from these early times.
The so-called Kuiper Belt lies beyond the orbit of distant Neptune
and lies in the plane of the ecliptic, roughly from 30 to 500 AU.
(1 AU - "astronomical unit" is the average distance between
Earth and Sun, about 150 million kilometers, or 93 million miles.)
Pluto, and recently-discovered Sedna are among the largest denizens
of this region ... yes, Pluto is probably just a large comet! Far
beyond the Kuiper Belt lies the huge spherical shell called the
Oort Cloud - stretching from 10,000 to perhaps 100,000 AU.
A comet's orbit may be disturbed by a collision, or by gravitational
perturbations from passing stars or clouds. The comet then falls
toward the inner solar system. As sunlight warms the comet, the
volatile ices vaporize and carry with them dust and rocks from the
nucleus of the comet. These gasses and dust particles are pushed
away from the Sun by the pressure of sunlight and the streaming
solar wind to form the tails of the comet. Ultraviolet light ionizes
the gas and makes it glow. As the comet sweeps through its orbit
the dust from its tail is strewn along its orbit, leaving the debris
which makes up periodic meteor showers.
Lunar phases for June: Full Moon on the 3rd, at 12:20 am; Last
Quarter on the 9th, at 4:03 pm; New Moon on the 17th, at 4:27 pm;
First Quarter on the 25th, at 3:08 pm. Summer solstice, when the
Sun is highest above the equator, will occur at 8:58 pm on the 20th.
This is sometimes called the "longest day of the year,"
but it's 24 hours just like any other day! In fact, the solstice
event will take place after sunset on the 20th! We will experience
over 14 hours of sunlight on the 20th and 21st, however.
Evening planet watching in June is largely Jupiter watching. After
sunset, Jupiter emerges from twilight high to the southwest, about
50 degrees off the horizon. It sets about 1:00 am. Saturn and Mars
are above the horizon at sunset, but low to the northwest and probably
lost in the horizon clutter and haze. Mornings are not promising
for planet watching, either. Mercury and Venus have returned to
the pre-dawn sky, but will be very low (< 10 degrees) on the
northeast horizon at sunrise.
An hour or so after sunset at mid-month, the overhead view is essentially
out of the plane of the Galaxy, which is nearly coincident with
the horizon at 8:30 pm. Castor and Pollux are to the west, settling
towards the horizon. High above the southern horizon, almost at
zenith, is bright Arcturus, in the constellation Bootes. To the
west, just below and to the right of Jupiter, you'll find Regulus,
the heart of the Lion in the constellation Leo. The days around
the new Moon are a good time to hope for clear skies ... maybe a
cold front will sweep away the haze enough so that you can use Jupiter
to find some deeper objects with your binoculars. Within 4 degrees
to the right and slightly above Jupiter you may find several galaxies
from the Messier catalog - M105, M95, and M96 are all in Leo. About
6 degrees above Jupiter is another, known as M65. These objects
were all catalogued by Charles Messier to avoid confusing them with
comets. We'll say more about the Messier Catalog next month.
Vega is to the ENE about 37 degrees off the horizon and appearing
higher each night. It will be prominent in the late summer and autumn
skies. Near Vega, use binoculars to find the Ring Nebula. Below
Vega rises the constellation Cygnus. This marks the plane of the
Galaxy, and the general direction towards which our Sun in moving
in its orbit about the distant center of the Milky Way.
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 2004
George Spagna