StarWatch: Moravian College Astronomy
StarWatch for the greater Lehigh Valley

APRIL  2000


188   APRIL 2, 2000:     Planets Congregate with Moon
Back in September of 1999 Jupiter and Saturn reappeared in the late evening sky. That was the beginning of a fall and a winter dominated by the titans of the solar system. Time has passed, and the Earth has traveled nearly halfway around the sun. During this interval, our orbital motion and the slow eastward progression of these planets has carried them from the eastern horizon to the western horizon; they are now visible about 30 minutes after sunset. Jupiter has also gotten much closer to Saturn, and Mars has been catching up to both worlds. Shortly, all three planets will be too close to the sun to be easily seen. During this week they can be found within seven degrees of each other, about one and one half fists (15 degrees) above the western horizon at 8:20 p.m. The dance of the planets, however, is still not complete. By Thursday the moon will be positioned just to the left of Saturn, as a paper thin crescent. If the skies are very clear, look for earthshine, reflected sunlight from the Earth reflected back to us by the moon. The portion of the moon which is not directly in sunlight will appear to be dimly illuminated, revealing the entire lunar orb. Binoculars will easily show this ashen light, even if the skies are not radiantly clear. If you own wide-angle binoculars, youíll be able to catch all four objects within the same field of view--spectacular. Check out the on-line map at the web address below. And donít forget National Astronomy Day on Saturday, April 8th. The Lehigh Valley Amateur Astronomical Society, Inc., will have an evening public star party at their South Mountain facility off East Rock Road in Allentown. Phone 610-797-3476 for more information.

Planets visible

189   APRIL 9, 2000:     Aurora Spectacular
Last Thursdayís close proximity of the waxing crescent moon to Saturn and Jupiter in the early evening sky was billed as one of the seasonís celestial highlights. Instead, local media sources across Southeastern PA were bombarded with countless calls after 8 p.m. describing a vertical shaft of red light emanating around the moon. No one really seemed to notice the congregation of planets nearby. The show-stopper turned out to be a rather impressive display of the aurora borealis, known to most of us as the northern lights. The spectacle was attributable to heightened solar activity which created an outburst of plasma (protons and electrons) which headed our way. The plasma cloud was traveling at about one million miles per hour and interacted with Earthís magnetic envelope on Thursday evening, sending a huge electric current into our atmosphere. The electricity caused the nitrogen and oxygen in the air to ionize or glow and was viewed as the shaft of red-violet light around the moon. Nitrogen produces blues and reds, while the oxygen creates hues of red and green. In the case of oxygen, the reds are produced at altitudes of 120 miles and greater, while the greens are nearer to the bottoms of the displays, about 75 miles above the Earth. Since the display was seen near the horizon, our line of sight probably gave us a better glimpse of the top of the discharge, so the color red predominated. Blue light also has more difficulty penetrating the atmosphere. The sun is near peak activity and should remain that way for the next several years. So donít be surprised if nature keeps up these sporadic celestial fireworks right into the next millennium.

Great Aurora of 11-8-1991
The great auroral display of November 8, 1991 was witnessed in a very unlikely location, New Mexico. This photo was taken looking south at the auroral cap which was easily visible from Mexico. Photography by Gary A. Becker...

190   APRIL 16, 2000:     Full Moons of Spring
If you have been watching the moon over the last several nights, you have undoubtedly noticed how it is bearing down upon the eastern horizon. Tomorrow, it is near the bright star Spica of Virgo, the Virgin. The moon is full Tuesday evening. The steep angle that the moon approaches the horizon on successive nights in spring is in sharp contrast to autumn. Then the moon, as it nears its full phase, approaches the horizon much more obliquely. This sideways track towards the horizon in the fall creates a condition in which successive moonrises occur at nearly the same time of the evening around the date of the full moon. The moonís path intersects the horizon at such a shallow angle that its orbital motion does not allow it to get very far beneath the horizon in 24 hours. The Earthís rotation swiftly brings it back to the horizon around the same time for several evenings. The best example of this phenomenon is called the Harvest Moon. In the spring, conditions are just the opposite Now the moonís orbital path intersects the horizon at a steep angle near its full phase. From one day to the next, the moonís orbital motion carries it much farther beneath the horizon. Earthís rotation requires more time to return the moon to a rising position. The period of time between successive moonrises in the spring is about an hour rather than the 30 minute time difference of the fall. Check out the diagram highlighting these circumstances in this weekís StarWatch section at the web address given below.

Moonrise time differences

191   APRIL 23, 2000:     The Back of Beyond
One of Americaís best kept treasures for stargazing is Chaco Culture National Historical Park, located about 150 miles to the northeast of sunny Albuquerque, New Mexico. Chacoís secret to remaining less traveled could be a result of the 20 miles of washboard road that one must traverse to get into the canyon. Or perhaps it is the lack of amenities for the weary traveler. Thereís no gas station or food franchise within 27 miles of the park headquarters. And when it rains that ribbon of clay connecting the park to humanity can turn into a narrow but long mud wresting rink if any passing coyote cared to watch. Blistering hot by summer day, chilly at night, Chaco is not for the faint-hearted. Chaco Culture is the largest archaeological site in the United States. It was the ceremonial hub of a thriving culture of Anasazi Indians 1000 years ago. They erected large masonry structures and were keen observers of the heavens. And understandably so, for when the sun goes down and the blackness of night canopies the canyon, the stars look so close that you can almost reach out and touch them. Itís an amateur astronomerís fantasy land. If you would like to vicariously journey into the canyon via the Internet, come join seven astronomy students from Allen and Dieruff High Schools as they stalk the stars from Chaco Canyon starting this Saturday. The goal of the group is to transmit pictures and informational updates about every two days. But donít hold your breath. Chaco is at the end of a very long phone line which often suffers disruptions. Power outages are frequent too. Look for the "back of beyond" at the web site below.

Pueblo Bonito
Pueblo Bonito at Chaco Culture National Historical Park was constructed between 850 AD and 1140 AD. It remained the largest building in the United States until 1889. Photo by Gary A. Becker...

192   APRIL 30, 2000:     Daylight Conjunction
Lots of people have been looking forward to the great planetary conjunction that will be happening this week. Indeed, all five naked eye planets, Mercury, Venus, Mars, Jupiter, and Saturn will be within 27 degrees of each other after Tuesday. The moon is also part of the action, passing in front of the planetary lineup on Wednesday and Thursday. It is the first in a series of two such events this month. On May 17, the planetary grouping will be the tightest, just under 20 degrees, but the moon will be nearly full and, therefore, almost opposite to the sun. The event this week would be an incredibly wonderful spectacle if we could simply block out the sun. Unfortunately a total solar eclipse is not in the offering when the moon is new on Thursday. So this great assemblage of the planets will go unseen. Will there be any noticeable tidal effects? The answer is an emphatic no! The moon and sun cause the tides on Earth. Tidal forces which represent a difference in the gravitational attraction between two different positions on an object vary directly with the mass of the attracting body and as the inverse cube of the distance. Even though the sun has 27 million times more mass than the moon, the moon affects the tides more. The reason is the sunís distance. The sun is 400 times farther away than the moon. Four hundred cubed is 64 million, making the sun about 40 percent as effective as the moon in producing the tides. If the sun is no match for the moon, how could any of the other planets make any appreciable contribution to the tides? A diagram showing the positions of the planets for May 4 can be found at the web site below.

May 2000 planetary conjunction
The conjunction of the five naked eye planets, the sun, and the moon will take place during the day and be invisible to the eye. The map is drawn for May 4, 2000

April Star Map

April Moon Phase Calendar