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