A Beginner's Guide to the Night Sky
Four hundred years ago, Galileo began studying the night sky with the early telescope, beginning a scientific revolution. Prior to that, watching the night skies had been with the naked eye, without magnification. Stars, the moon, comets, meteors, and the five planets that the ancients knew as the "wanderers" can all be observed in this manner. One may be fortunate enough to live in a rural area, or perhaps in an urban neighborhood devoid of significant light pollution, as is my good fortune. Viewing the sky from a park or waterfront, or rural area outside the city can limit this problem that adversely affects nighttime observations in most urban areas.
What follows is a brief survey of what is seen in the night sky without magnification with a little time and patient effort.
One quickly notices the constantly changing moon. The full moon rises about local sunset. Then each evening, it rises later, as it wanes to last quarter and new moon (when it is not visible). Then the first crescent is seen in the east around sunrise, waxing over two weeks to full moon again. A full cycle takes about 29.5 days, about a week for each quarter. This is the lunar month, the basis for the first calendars. Cycles of the waxing and waning moon have influenced agricultural planting and harvesting schedules for millennia. Most urban residents have lost sensitivity to this ever-changing natural cycle that dominates the night skies.
A lunar eclipse occurs when the Earth is aligned between the sun and the moon. The full moon changes from a bright yellow-white orb to a dull, blood red or orange. An eclipse is partial or complete, lasting a few minutes to several hours. From any given location, this is visible every few years. The next full lunar eclipse visible from eastern North America occurs December 21, 2010.
The Stars and Constellations
In the night sky, there are thousands of visible stars. Since ancient times, man has connected the dots of stars to form shapes recognized as animals or mythological creatures. These are the constellations, an important foundation for mythology, astronomy, and astrology. In the middle latitudes, most constellations change seasonally, while some are visible year round.
The outer ladle of the easily seen Big Dipper (Ursa Major) points to the handle of the Little Dipper (Ursa Minor). This star is Polaris, or the North Star. One's latitude and the angle at which it is seen are the same. The other stars and constellations appear to rotate around Polaris during the night.
Besides the signs of the Zodiac, and the Dippers, other well-known northern hemisphere constellations include Orion, Cassiopeia, and Cygnus (the Swan).
The ancients recognized that certain "stars" were different. They moved in their relationship to the other stars, rather than appearing stationary. Much brighter than stars, they disappeared for days or months at a time. These were the five "wanderers," or planets.
Venus and Mercury are visible only briefly before sunrise or after sunset. Their orbits pass between Earth and the Sun, and they are known as the inner planets. They are commonly called morning or evening stars. Venus, white to blue, is often the brightest object in the sky, while Mercury is much dimmer and less often seen. A conjunction of Venus and Mercury in early April 2010 made Mercury much easier to observe than usual.
In contrast, the outer planets of Mars, Jupiter, and Saturn might be seen most of the night during much of the year. Mars usually has a reddish-orange tinge. These outer planets are often are not visible for months at a time.
Comets and Meteors
The night sky has other objects besides stars and planets that are more irregular and unusual. Comets wander though the solar system in a large elliptical orbit around the sun. One orbit may take decades or centuries. They usually have a tail of cosmic dust that becomes brighter as it comes closer to the sun.
Meteors are small fragments of asteroids or remnants from a comet's tail. While meteors may be seen individually any time of year, they are more common as meteor showers with sometimes a dozen or more meteors seen per hour. These happen seasonally as the earth travels through the path of a comet's tail, with small fragments burning up in the earth's atmosphere. Meteors can emit a bright streak of light across 5 to 50 degrees arc of the night sky in white, orange, green, or blue. The Perseid Meteor Shower around August 12 is one of the most reliable for seeing large numbers of meteors.
Using such references as the Old Farmer's Almanac, or Stardate and, plan your viewing ahead. These sources provide information on rising and setting time of the moon and various planets, phases of the moon, and conjunctions of planets. Meteor showers and lunar eclipses are also listed in such references.
If the weather permits, take a look at the night sky during a conjunction (when planets appear in close proximity to each other, the moon, or bright stars or constellation). Gradually learn to recognize the major constellations of the season. Plan a camping trip in the country during the Perseid Meteor Shower or a lunar eclipse.
Even in the city, one can often see the phases of the moon, and some of the planets or brighter constellations. Light pollution, seasonal cold, and cloudy weather are all problems at times. But there is little that can rival making an expedition into a rural area away from the city lights. The grandeur of thousands of glittering lights seen in a clear night sky can instill a sense of awe like few other activities.
The Old Farmer's Almanac, 2010