The Time of Twilights
When we plan a stargazing session we obviously need to especially consider when the sky will be dark. Between sunset and maximum darkness we have 3 categories of twilight. Horizon in these cases is 90 degrees from zenith (directly overhead) and not the mountains to our west.
Civil Twilight is the time between sunset and the point where the sun is 6 degrees below the horizon. This is the period most of us consider to be twilight. At our latitude, Civil Twilight usually ends about 30 minutes after sunset and is referred to as Civil Dusk. When reasonable, this is the period we use for setting up our telescopes with some natural light still available.
Nautical Twilight is the time between the Civil Dusk (the sun is 6 degrees below the horizon) and the point where the sun is 12 degrees below the horizon. It is called “nautical” twilight because during this time most of the brighter, navigation stars become visible. Nautical Twilight ends about 1 hour after sunset with Nautical Dusk.
Finally we have Astronomical Twilight that starts with Nautical Dusk (the sun is 12 degrees below the horizon) and ends when the sun is 18 degrees below the horizon with Astronomical Dusk. At this point the sun is no longer effectively illuminating the atmosphere above us, or about 1.5 hours after sunset.
Typically we can start viewing the brightest objects such as the planets and brightest stars during Nautical Twilight. We have to be later into Nautical Twilight to begin viewing dimmer objects such as open star clusters. The dimmest objects like globular star clusters and nebulae generally need Astronomical Twilight to really be appreciated.
A convenient web site that provides the twilight times along with other related information is at Sunrise/Sunset Calendars for Colorado.
What constitutes a “good” viewing night?
Of course, there is no exact answer to the question. However, there are some things that will help you see the heavens more clearly. And some simple checks you can make. If you want to see, photograph, or just explore the sky, there are some nights that are better than others. That is especially true if you are looking for faint objects, or if you would like to do some photography.
First, it should be dark. That means that the moon, especially if it is nearly full, should not be up. Many of us have to tolerate street lights, signs or fearful neighbors who feel better with the yard brightly lit. Those things are bad, but if the moon is shining also, almost everything else can be wiped out.
There are three technical terms astronomers use to describe the quality of conditions in the night sky. These are darkness, seeing and transparency. Darkness is just as it sounds. For instance, the moon or city lights degrade darkness. Seeing refers to how still the air is from the ground to the top of the atmosphere. Turbulent air makes for degraded seeing. Finally there is transparency. This refers to how clear the air is. Smoke and haze are some of the causes of degraded transparency.
AVAS is listed with a web site called the Clear Sky Chart. Our summer meeting site at Stone Bridge has been registered. The site not only goes into much greater detail about these terms, but also provides a chart of what the conditions are predicted to be at Stone Bridge. You can visit the Stone Bridge Clear Sky Chart at http://cleardarksky.com/c/StnBrCOkey.html?1 or by clicking =>Stone Bridge Clear Sky Chart.
There are a couple quick checks one can use to evaluate conditions:
First, the stars should not be twinkling very much. Stars low in the sky may twinkle, but those higher up generally should not. If wind or rising warm air are stirring the atmosphere, the images will “dance”, and you will not be able to see close star pairs, any detail of a planet or diffuse objects.
Second, the two stars that make the inside of the bowl of the Little Dipper, which are dimmer than 4’th magnitude, should be visible. Another clue is the nucleus of the Andromeda Galaxy, should be visible if someone shows you where to look (it is also 4’th magnitude, but is more difficult to see, since it is a diffuse patch, not a sharp point).
And What to do if There is Haze, Light Pollution, and Such?
There are still things to do unless it is clouded over. The bright planets, namely Venus, Jupiter and Saturn are worth looking at, if you can see them at all. Venus is always a bit of a washout since it is completely shrouded in clouds. But, what you can see is that it has phases — a thin crescent when it is close to the earth, an almost full disk when it nears opposition (i.e. the other side of the sun), and everything in between. Jupiter and Saturn put on a good show almost any time you can see them.
And the moon is well worth looking at, as well. This is especially true several days before or after full moon. The craters near the terminator (i.e. the shadow line) are thrown into sharp relief by the sun which is close to the local horizon.
A good many people stick a dark filter into the eyepiece of their scope when they are looking at the moon. It enhances the contrast, especially when the moon is nearly full. Even without a filter there is no danger of actually harming your eye. The actual surface brightness of the moon is about the same as that of old worn pavement with the sun shining on it.