StarWatch: Moravian College Astronomy
StarWatch for the greater Lehigh Valley



180   FEBRUARY 6, 2000:     Winter's Forgotten
There are 88 constellations which grace our skies. More than half of them would be visible from our latitude if the heavens were free from light pollution, and our imaginations permitted us to see them. Ancient skywatchers didnít play fairly. Actually for modern astronomers the precise locations of the constellation boundaries was more important than the star figures themselves. How these stars were connected to make the pictures is still up for grabs. Every astronomy educator seems to envision his or her own way of navigating through the heavens. So when the constellations of Monoceros, the Unicorn, Lepus, the Hare, and Eridanus, the River are mentioned, forget about the pictures and just concentrate on the basic locations of these figures. Happily, they are all positioned around one of the most easily seen star figures of the sky, Orion, the Hunter. Orion is due south at 8:00 p.m. See the star map associated with this article at the web address below. Monoceros is the blank space to Orionís left bordered by Sirius, the Dog Star below, and Procyon, of the Little Dog, above. Sirius is the brightest star of the nighttime sky. Directly beneath brilliant, blue-white Rigel, the right leg of the Hunter, lies Lepus who is hiding from the dogs and Orion. Orion is preoccupied with fighting the bull, and the dogs are looking for food from the Hunter. Starting just below and to the right of Rigel lies Eridanus, which stretches over 60 degrees down and to the right of Rigel and eventually below the horizon. Dozens of faint stars outline its path. Only one of these stars, Zauak would have the slightest chance of being visible from the Valley at the low altitude of this constellation.

Winter's Forgotten

181   FEBRUARY 13, 2000:     Messenger God Visible
The planets dominate the southwest this week, and added to the three that have been visible for months, Jupiter, Saturn, and Mars, comes the Messenger God, Mercury. Seeing it can be as illusive as finding the pot of gold beneath a rainbow. Thatís because Mercuryís solar distance can vary from 28.5 to 43.3 million miles, so it is never far away from the sunís luminescence. On Monday Mercury reaches its greatest elongation (angle) away from the sun, 17 hours before it is closest to the sun. Mercury will be only 18.2 degrees away from our daystar. Another frustrating aspect of Mercuryís visibility is the steepness of the angle at which Earthís orbital plane and Mercuryís orbital plane are tilted to the horizon. Mercury can get as far as 28 degrees from the sun, but still be too close to the horizon after sunset to be glimpsed. However, this week the positives win the field. When Mercury is closer to the sun, it appears brighter. It reaches its closest distance to the sun on Tuesday, so Mercury will be brighter than usual. The big advantage, however, is that Mercury is almost vertically placed over the sun at sunset. Like the Titanic, Mercuryís slide toward the western horizon and oblivion will take some time, giving us a decent window in which to observe the planet in a relatively dark sky. Look just a tad south of west about 6 p.m. and Mercury will be about one fist above the horizon. Make sure your west is unobstructed, and if you have binoculars use them to enhance the view. Mercury sets by 7:05 p.m. Donít confuse Mercury with Jupiter high in the southwest. Jupiter will outshine Mercury by nearly five times. See the on-line map at the web address below. Much success!

Mercury visible

182   FEBRUARY 20, 2000:     Bonus Day
There is a quiet little celebration that is going to occur next week on the 29 of February. It is a very special leap year day, for it happens only once every 400 years. In fact, it has only occurred twice since the inception of the Gregorian calendar in 1582. Previously in the Julian calendar, any year divisible by 4 was considered a leap year, making the seasonal or Tropical Year equal to 365.2500 days. This was not a bad call for its time, 45 BC, when the Julian calendar was introduced. But the true time it takes the Earth to orbit the sun with respect to the seasons is 365.2422 days. The difference of 0.0078 day or 11 minutes, 14 seconds meant that every year we were overcorrecting the Earthís orbital position by that amount of time. By 1582 this little error had accumulated to nearly 13 days. The date that the sun crossed the Vernal Equinox, called the first moment of spring, had slipped from March 25 to March 11. The reform initiated by Pope Gregory XIII in 1582 dropped 10 days from the calendar. The day following October 4, 1582 became October 15, 1582. This brought the date of the Vernal Equinox back to March 21, established at the Council of Nicaea in 325 AD. There were riots in Catholic Europe because people felt that they had been cheated out of 10 days from their lives. Then for a centurial year to be considered a leap year, it had to be evenly divisible by 400. So the years 1600 and 2000 would leap, but 1700, 1800, and 1900 would not. The calendar was now accurate to one day in 2500 years. So thatís the reason for the little celebration about this yearís leap year. Party hearty; for the next time it happens, weíll all be dead.

184   FEBRUARY 27, 2000:     Sun Pillars
The sun sets or rises on a winterís day, and there above the solar disk is a shaft of light emanating straight up into the sky almost like a searchlight. You are witnessing a sun pillar. Sun pillars are not an ultra rare optical phenomena, but they certainly can be striking, taking one by surprise when they do appear. The moon or artificial lights can also create pillars. They are caused by ice crystals which are in the shape of a pencil falling through the atmosphere with their long axes horizontal to the ground. An unsharpened pencil perfectly duplicates the hexagonal shape of these columnar crystals. Since the pillar retains the same color as the sun, the light striking the crystal is being reflected back to the observer and not refracted or broken apart into its component colors as would be perceived in a halo. Even though the pencil crystals are falling horizontally to the ground, their axes can be oriented in any direction. This actually intensifies the reflected beam in the direction above or below the sun and is the reason why the phenomenon appears in the form of a pillar rather than a diffuse glow. Sun pillars are low altitude phenomena. If the sun is at an altitude of 20 degrees or greater, they become diffuse, losing their vertical structure. But when the sun is within six degrees above or below the horizon, their vertical components are accentuated, and they can be extremely memorable. This article was the result of an e-mail inquiry by Ted Mead of Pittsford, NY and a photo which he took this past Christmas morning of a spectacular sun pillar. The photo can be found by going to the web address given below and navigating to this weekís StarWatch column.

Christmas Sun Pillar
Ted Mead's December 25, 1999 sun pillar photo taken before sunrise from Pittsford, New York was like an added present under the tree. Pittsford is located in the NY Snowbelt about 12 miles to the SE of Rochester.

February Star Map

February Moon Phase Calendar