Our Spectacular Night Sky
By Judith Frey
Throughout human history until the last 100 years, the spectacular night sky was part of everyone's experience. We are fortunate that the Snowmass-Capitol Creek valley has some of the darkest skies in Colorado. In the Denver area, I encountered school children who had never seen the stars!
Successful stargazing depends on acclimating one's eyes to total darkness so that very faint objects can be seen with the naked eye. Ambient light prevents one's eyes from adjusting to the dark. It is my hope that all homeowners will shield outside lights so that they shine downward, where the light is needed, and not upward or in all directions. The first-hand experience of the universe that we can have in this valley is increasingly rare in our age.
The Pitkin County Land Use Code LUC 7.20.140 provides specific guidance on exterior lighting:
"Briefly, exterior lights must be shielded and/or directed so that the bulb is not visible from the property line."
I would also add that it is not necessary--or advisable from a stargazer's perspective--to illuminate the night sky! The Land Use Code goes into more detail and provides excellent examples of shielded lighting.
The Night Sky Through the Seasons
On New Year's Eve at midnight, the brightest star in the sky, Sirius, reaches its highest point in the heavens. Called the star's midnight culmination, this event happens once every year. Sirius is the bright star below and to the left of the constellation Orion.
Welcome back, Orion--the large constellation dominating the high southern sky in the winter months. The middle "star" in Orion's "sword" appears fuzzy to the naked eye. This is actually the Orion Nebula, an interstellar cloud of gas that is a major star-forming region--the closest one to the earth at 1,270 light-years away. With a small telescope you can see four newly-formed stars that are blowing the gas away from them into space. It's amazing what lies just beyond our sight in the night sky!
The Milky Way--our own galaxy--stretches across the night sky in summer. In winter, it is very much fainter because we are looking outward through the ends of the spiral arms where the stars are fewer and farther between. In summer, we are looking inward toward the center of the galaxy where the arms are much closer together and the stars are denser. To the left of the constellation Scorpius, very low in the southern summer sky, about halfway above the curving tail, is the rotational center of our galaxy.
In August and September, with dark enough skies, you can see the Andromeda Galaxy fairly low in the eastern sky. Just 2.5 million light-years away, it is our closest galactic neighbor (except for the dwarf galaxies that orbit our own Milky Way--most notably the Magellanic Clouds, visible to the naked eye in the southern hemisphere). The Andromeda Galaxy is fainter than individual stars--you need at least 15 minutes in total darkness for your eyes to adjust enough to see it. So—you are straining to make out a faint blob of light that you can barely see, ho-hum! But when you realize what you're looking at--a completely different star system, larger than the Milky Way, that you can view with your own eyes--then it becomes thrilling. With binoculars or even a small telescope, you can make out the spiral arms of the galaxy.
Yearly Solar Events
The latest sunrise of the year occurs on January 4, at 7:28 a.m. The days have been lengthening almost imperceptibly since the solstice on December 21, but from now on the daylight is increasing in the morning as well as the evening.
The spring (vernal) equinox is March 20 at 3:36 a.m. The sun is directly over the equator at noon, and it rises due east and sets due west. The length of the day has been increasing more and more rapidly since the winter solstice, to a rate of change of about 5 minutes per day.
June 20 is the summer solstice, the longest day of the year. At about 9:32 p.m., the north pole is tilted the closest to the sun. The sun’s angle relative to earth’s equator changes so gradually around the solstices that the difference is almost imperceptible for about ten days. This means that the sun rises and sets, respectively, in the same places on the horizon during that period. The word solstice is derived from the Latin solstitium, a combination of sol, “the sun,” and sistere, “to make stand, stand still.”
June 27 is the day of the latest sunset of the year, at 8:37 p.m., after this the long summer twilight begins to shorten very gradually.
On July 5, the earth reaches its aphelion, the farthest point from the sun, at 4:27 p.m. At that point, the sun and earth are separated by some 94.5 million miles--while the average distance is around 93 million miles. Perihelion--the earth's minimum distance from the sun at 91.3 million miles--occurs on Saturday, January 2, at 7:00 a.m.
The fall (autumnal) equinox is September 22 at 1:20 p.m., now the days and nights are nearly equal in length. The rate of change is very rapid now; the days are getting shorter faster and the light is dimming noticeably.
The year’s earliest sunset is on Dec. 7, at 4:43 p.m. Every year I raise a glass and offer a toast to the setting sun, for soon there will be more light in the early evening!
The winter solstice, the first day of winter, is Dec. 21 at 10:50 a.m. At this time, the south pole is tilted closest to the sun. This night is the longest of the year and the day is the shortest, with about 9 hours and 24 minutes of daylight in Colorado.