Paul Derrick's Stargazer

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Stargazing Information

This section contains some basic stargazing information which can be accessed by reading and scrolling, or by jumping to a particular section by clicking on the desired section listed below.

Magnitudes
Angular Distance
Factors Affecting Stargazing
Sky Maps and Planispheres
Red Flashlights
Comfort Issues (and Star Party Check-list)
Star Party Etiquette


Magnitudes

As even casual stargazers know, some stars are brighter than others. Many are so faint they are barely detectable while some are so bright they can be seen from light-polluted cities. One star in particular--our Sun--is blindingly bright.

To make sense of these variations, astronomers use a measurement scale called visual magnitudes. It's a reverse scale where the brighter the object, the lower the magnitude number.

In 120 BCE, long before the invention of the telescope, the Greek astronomer Hipparchus devised the first scale by which he ranked stars into six levels of brightness. (Our Sun wasn't recognized as a star then.) The brightest stars he considered "first magnitude." Those barely visible were "sixth magnitude." The rest were equally ranked in between. Although imprecise and cumbersome, his scale has been used ever since.

Modern astronomers have made the scale more precise, and have expanded it to measure the full range of brightnesses, from our Sun to objects fainter than the Greeks could imagine. The scale includes fractions to account for subtle differences in brightness. For example, a star's visual magnitude might be +3.6. The scale also now dips into negative numbers for very bright objects and goes well beyond +6 for objects fainter than the naked eye can see.

The difference in brightness between magnitude values is two and one-half times. For example, Virgo's Spica with a magnitude of +1 is two and one-half times brighter than Polaris (the North Star) with a magnitude of +2.

The apparent brightness of stars -- how bright they appear from Earth -- depends primarily upon two factors: luminosity (their absolute brightness, i.e., the actual amount of light they give off) and distance. Our Sun, a star of average brightness, appears so bright because of its nearness. Its visual magnitude is measured at -27.

By contrast, Orion's supergiant Rigel is 50,000 times brighter than our sun, but at a distance of 900 light years, its visual magnitude is 0. Even at that distance it is still the 7th brightest star in our night sky. Viewed from Rigel's distance, our Sun wouldn't even be visible to the naked eye.

Polaris, the North Star, at 650 light years away, is magnitude +2. Venus, which only reflects sunlight but is much nearer than the stars, appears much brighter with a magnitude of -4.

From urban areas where light pollution is a problem, the naked eye can only see objects down to 3rd magnitude or so whereas from rural areas, objects of 5th and even 6th magnitude can be seen. Thus, naked-eye urban stargazers see a few as dozens, and at most a few hundred, stars, whereas the rural stargazer can see upwards of a couple of thousand.

Typical 7X50 binoculars can reveal objects down to about 8th magnitude, and even fainter under excellent viewing conditions. The Hubble Space Telescope has taken photos of incredibly faint objects at magnitude +30, which, it has been said, is comparable to seeing a candle flame several thousand miles away.


Angular Distance

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Degrees, Arcminutes and Arcseconds.

Beginning stargazers are often frustrated trying to tell a friend how far an object appears to be from another. For example, how far is Polaris (the North Star) from the Big Dipper? Usual measures of distance like inches, feet, or miles are useless. Is Polaris several inches from the Big Dipper, or maybe several feet? Without a common frame of reference these measures are meaningless. Even astronomical measures of distance such as light years don't help when discussing apparent sky distances.

Fortunately, there's an easy-to-learn method available. When talking about the apparent distances, and sizes, of objects as viewed from Earth, astronomers use angular distances -- degrees and fractions thereof.

The entire sky, being half of a hemisphere, spans 180 degrees. A couple of new terms will help us here. The point exactly straight up in the sky is called the zenith. It is 90 degrees from the horizon in all directions. An imaginary line drawn from due north on the horizon, up through the zenith and down to due south on the horizon is called the meridian. Thus, the meridian spans 180 degrees, as does the distance from any point on the horizon, across the "top" of the sky and to the corresponding point on the opposite horizon.

Using degrees, we can measure the distance between any two points in the sky, such as the distance from one end of the Big Dipper to other, which is about 25 degrees. (The use of approximate rather than exact values is adequate for most amateur stargazing, including measures of distance, size and magnitude.) Degrees are especially useful measures when dealing on the scale of naked-eye viewing. For example, a given constellation may be some 10 by 20 degrees in size, or star A is 15 degrees from star B.

Degrees are also useful measures for binocular viewing. A typical pair of binoculars, such a 7X50s, shows about a 7-degree field of view (FOV). So, for example, if star A is near the center of your field of view and star B is near one edge, the two stars would be about 3 or so degrees apart.

But when we get down to the smaller sizes and distances, especially in telescopes (but also in binoculars), we have to measure in fractions of a degree. For example, the full Moon is about 1/2 degree in diameter, and even the largest lunar crater would be but a small fraction of a degree. So rather than use fractions of degrees, a geometric convention is used which is similar to the way we divide segments of time smaller than an hour. Degrees are divided into minutes of arc and seconds of arc: 60 arcminutes make one degree, and 60 arcseconds make one arcminute. So rather than saying a particular lunar crater is 1/60 (or .0167) degree in diameter, we can say its diameter is 1 arcminute, often written as 1'. And the diameter of an even smaller crater might be expressed as 1 arcsecond (1") rather than 1/3600 or .00028 degrees.

Angular Distances with Binoculars and Telescopes.

You will find it useful to know the FOV of your binoculars if you don't already know. If you have zoom binoculars, your FOV will decrease as you zoom in on an object. Therefore, you will want to determine the FOV range -- from zoomed all the way out (widest) to all the way in (narrowest).

Telescopes present a different situation. Most owners have two or more eyepieces, yielding different magnification powers, and therefore different FOVs. Generally, the higher the power, the narrower the FOV -- and the variations can be considerable. In smaller scopes at lower powers the FOV is likely to be a few degrees. In sophisticated amateur scopes at high powers, the FOV can be a few arcminutes or less. You'll need to experiment to learn the FOV of each of your eyepieces with your scope.

Hand Measures.

Maybe you're thinking: "This is all great, but how can I measure degrees without special equipment?" Fortunately, there's a handy (pun intended) method for gauging degrees in the sky. You have all the equipment you need at the end of your arm -- your hand and fingers. While they don't give precise measurements, they are close enough for casual stargazing.

Hold your hand out at arm's length in the direction you are viewing and close one eye. At this distance, your index finger is about 1 degree across, twice the diameter of a full Moon. (Most folks think of the Moon as larger and are surprised to find their finger can cover it.) A fist is about 10 degrees across, and a wide open handspan from thumb tip to little finger tip about 20 degrees.

This method works for adults and children, regardless of hand size. People with larger hands generally have longer arms, making their hand appear about the same size as one belonging to a person with a smaller hand and shorter arm.

Try it out. The bowl of the Big Dipper is about 10o wide -- about one fist-width. The Little Dipper is about 20 degrees in length -- about one hand-span. Measure Polaris' distance above your northern horizon. Its altitude in degrees will equal the latitude from which you are viewing. For example, in central Texas, near latitude 30 degrees N, Polaris is about 30 degrees, or 3 fists, above the horizon.

Altitude and Azimuth.

This is a good place to introduce altitude and azimuth, both of which are measured in degrees of angular distance. Altitude refers to angular distance from the horizon where 0 degrees is at the horizon up to 90 degrees straight overhead. (Negative numbers are used to express distances below the horizon.) Azimuth refers to angular distance around the horizon, and is measured from north and moving around the horizon eastward. North is 0 degrees, east is 90 degrees, south 180 degrees and west 270 degrees. Thus if, at a given time, an object is located at an altitude of 44 degrees and an azimuth of 132 degrees, it will be about half way up in the southeastern sky.


Factors Affecting Stargazing

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Several factors effect one's ability to see fainter objects when stargazing. Three common ones are failure to dark-adapt one's eyes, light pollution, and moon light. Any of these can cause serious problems, but fortunately all are factors over which we can have some control.

Dark Adaptation.

The easiest factor to control is that of letting your eyes become accustomed to the dark, called dark-adaption. This is accomplished by not exposing your eyes to lights for 20-30 minutes prior to your viewing session. Your pupils will open wide, letting in more light, and chemical changes will occur, making your eyes more light sensitive. It will be like having a stellar dimmer switch enabling you to increase the brightness of every object in the sky.

While it takes eyes a while to dark-adapt, they un-adapt very quickly -- pupils close more quickly than they open. The headlights of a passing auto or a few seconds looking at a star map with a white flashlight is all it takes to spoil viewing for several minutes. For this reason white lights are not used or welcomed at star parties.

This doesn't mean we can't use any light when stargazing. We still need to see things like star maps, planispheres, eyepieces and the like. Fortunately, the use of soft red light maintains relatively good dark adaptation. (See Red Flashlights.)

If you stargaze from your yard, it's a good idea to plan ahead so you won't keep running back into your lighted house.

Light pollution.

Light pollution has become a serious problem for stargazing, and in most places, it is getting worse. Still it is a factor over which we can have some control. Ideally, we could do all our observing from remote locations far from civilization, but rarely is this possible. Nevertheless, unless you live in one of the mega-cities, just driving 15-25 miles into the country can make a big difference. From there, viewing is generally much better in all directions except back toward the city. Since much of the sky's action takes place in the southern half of the sky, it's good to try to find a viewing location south of your city.

There are things even an urban backyard stargazer can do to improve viewing like finding a spot shielded from direct street lights, porch lights, passing cars, and the like. Also, keep in mind that views of the Moon and planets are generally much less affected by light pollution. So if your home viewing site is seriously light polluted, use it for lunar and planetary viewing and save those deep sky beauties for your excursions into the country.

This is a good place to plug the International Dark-Sky Association (see Links page), an organization dedicated to combating light pollution in constructive ways. Much polluting light is wasted light shining directly into the sky rather than on the intended earthly objects. Reducing light pollution would save energy and money, as well as preserve the beauty and wonder of the night sky for future generations. Stargazers, arise against light pollution and join IDA!

Moonlight.

The Moon, when it is up, floods the sky with light. With anything more than a thin crescent Moon above the horizon, it is difficult to see anything more than the major planets and brightest stars. The Moon even obscures meteor showers. Having the Moon out is essentially like viewing from a heavily light polluted location. So to have some control over this factor, you'll need to understand the Moon's phases.

The Moon orbits Earth every 29 1/2 days, or about every month (moonth), so each of the four phases lasts about one week. The following chart summarizes good and poor viewing times related to the Moon's phases.

STARGAZING AND MOON PHASES

MOON PHASE EVENING VIEWING MORNING VIEWING
New good good
1st Quarter poor good
Full poor poor
3rd Quarter good poor

During new Moon the Moon can't even be seen as it is too close to the Sun. As the Sun goes down, the Moon goes with it, leaving dark skies all night long.

A few days after new Moon, the situation begins to deteriorate for evening stargazing. Each night the waxing crescent Moon grows larger and stays in the evening sky longer. In about a week, the Moon will have moved a quarter of the way around Earth, producing the 1st quarter Moon -- when the right half of the surface facing Earth is illuminated. During this phase, the Moon rises around noon, it high in the sky in the evening and sets around midnight. 1st quarter Moon, therefore, is bad for evening stargazing but good for morning viewing.

After 1st quarter, the waxing gibbous Moon becomes even larger and stays in the sky longer each night. After about a week, it reach full. It is now on the opposite side of Earth from the Sun and fully illuminated -- the worst possible time for stargazing. When full, the Moon is at its largest, rises as the Sun is setting, stays in the sky all night, and doesn't set until sunrise. This might be a good time of the month to read about stargazing as you won't see much in the sky.

Finally, after full Moon, the situation gets better each night for evening viewing. The waning gibbous Moon rises nearly an hour later each night and gets smaller. By 3rd quarter, the Moon is three-quarters of the way around on its monthly journey and the left half of the surface facing Earth is illuminated. It doesn't rise until around midnight -- great for evening viewing, but not for morning. Following 3rd quarter, the waning crescent Moon continues to get smaller and rise later each day until it once again aligns with the Sun in the next new Moon.

A brief note: During most months, the new Moon doesn't exactly align with the Sun -- it passes slightly above or below the Sun. An exact alignment produces a solar eclipse.

Another note: To avoid interfering moonlight, astronomy clubs usually hold their star parties between the 3rd quarter and new Moon.

Averted Vision.

One final trick employed for seeing very faint objects utilizes "averted vision." The anatomy of the human eye is such that the cones in the center of the retina are not as sensitive to faint light as the rods which surround the center. Therefore, this trick entails focusing one's eyes slightly to one side (either side will do) of the faint object while continuing to concentrate attention on the object itself. The image of the faint object then focuses on the more sensitive rods, rather than the central cones, and actually appears slightly brighter than when looked at directly.

One can practice this trick during the day. Focus on any object, then shift your attention, but not your eyes, to another object a little to one side of the first object. While still focused on the first object, notice that you can see and describe the second object without looking directly at it. You are seeing the second object with averted vision. Try it.


Sky Maps and Planispheres

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Just as terrestrial maps help travelers find their way around Earth, sky maps help stargazers find their way as they "travel" around the night sky. And star maps are not just for the inexperienced. There are simply too many objects for even veteran observers to memorize. So star maps are basic for all stargazers.

And just as terrestrial maps range widely in sophistication and detail, so do sky maps. The simplest show no more than a few dozen 1st and 2nd magnitude stars and only major constellations. They can help with a general orientation to the night sky but that's about it. At the other end, star maps show many thousands -- indeed, millions -- of stars and other deep sky objects, very precisely plotted in right ascension and declination (like longitude and latitude in the sky). They can be used to find just about any object binoculars or amateur telescopes can see.

More detailed sky maps come as booklets or larger charts and are called sky atlases. Separate maps cover different parts of the sky, just as a U.S. travel atlas might have separate maps for each state.

While star atlases come in many levels, a couple are quite popular. A "mag 6" sky atlas, which shows objects down to 6th magnitude, covers most of what can be seen with naked eyes. A "mag 8" atlas, with objects down to 8th magnitude, shows most objects within the range of typical binoculars. Serious telescope users who want to find even fainter objects are apt to go with computer-based sky maps rather than, or in addition to, paper maps.

A special kind of star map, called a planisphere (a.k.a., star wheel), is a flat device consisting of two attached pieces. The bottom piece shows all the principal stars and constellations visible throughout the year from a given latitude zone on Earth. The top piece is a movable overlay which one sets to reveal that portion of sky visible at a given time and date. With minimum instruction they're simple to use, and can show the night sky any time of night and any night of the year. Planispheres are quite inexpensive and every stargazer should have one.

One limitation of star maps and planispheres is that they can show only the "fixed" objects -- they don't show planets and the Moon which move from night to night. (They do, however, usually trace out the ecliptic -- the path of the Sun, planets and Moon through the sky.)

A final note on star maps and planispheres: they look backwards at first glance. When held with N (north) up, E is to left (west) and W is to the right east), which seems wrong. But when held overhead (they are sky maps!), with N pointed north, the E and W will then be properly oriented to east and west, respectively.


Red Flashlights

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White light is taboo when stargazing. Yet we must be able to see our star maps, locate eyepieces, take notes, find the pencil we dropped on the ground and the like. Since red light enables us to see while preserving our dark adaptation, a red flashlight is an essential part of stargazing equipment.

Fortunately, they can be made very inexpensively. Any flashlight can be converted by covering the light with red cellophane (available in most hobby shops) or painting the glass with a red marker.

Even red light, if it is too bright, will affect dark adaption, so it's best to keep the light subtle. And please don't shine even a red flashlight in someone else's face.

A final note: even though white light is taboo when stargazing, it's a good idea to have a white flashlight in your bag. It might come in handy in emergencies, and you might wish to use it in your end-of-session wrap-up to make sure you've not left anything (assuming, of course, that others are not still viewing).


Comfort Issues

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Finally there are some "creature comfort" issues which need to be addressed. Many relate to personal needs and preferences so I'll simply identify them for your consideration. (Below is a summary check-list.)

At some point, you'll almost certainly want to take notes so paper and pencil/pen should be a part of your astronomical equipment. And, of course, you'll want to have some kind of table and chair. You might find it convenient to have a small red-light lamp so you don't have to hold your red flashlight all the time.

You might find you'll also want a reclining lawn chair, especially for meteor showers and binocular viewing. You'd be surprised how tired your neck can get when looking up while standing.

Seasonal (and regional) items include insect repellent and warm clothing. It's no fun stargazing while slapping mosquitoes and scratching itches. If the weather is at all cool, take more clothing than you think you'll need. As the night progresses, the temperature usually drops. And unless you're an unusually hyperactive person, stargazing is a rather sedentary activity that keeps the body's metabolism rate low. After a while 50 degrees can feel like it's freezing, and it's hard to have fun when your hands and feet are aching with cold and your body is shivering. So it's wise to keep several layers of additional clothing handy, including a cap, gloves and even extra socks.

If your observing session is a lengthy one, you're likely to want drinks and snacks. In cold weather, coffee, hot tea or hot chocolate can be almost as welcomed as a good observing friend.

Unless you're observing from your back yard, having a cell phone with you can come in handy. Your family might feel better knowing they can contact you if they need to, and you never know when your car battery may decide to give out, especially if you're using its power for a light, telescope drive or other accessories for several hours.

And speaking of cell phones, a related issue involves safety. Finding dark viewing sites is often a real challenge, and at one time or another, many of us have driven into a rural area and set up beside a little-used road. While I've done this more than a few times, it's really not a good idea (and I rarely do it any more). Aside from the loss of dark adaptation from an occasional passing car, it invites an accident or worse.

Furthermore, your presence can be alarming to others who don't know what you're doing. Back in 1986, while leading a small group out on a public road behind a rural cemetery to see Halley's Comet, our dark adaptation was spoiled big time by the bright spotlight of a sheriff's deputy who had gotten a report of "suspicious activity." Although we were doing nothing illegal or even noisy, in retrospect I think our unannounced presence was inconsiderate of the neighbors who feared we were up to no good.

If you're an urban dweller, as most of us are, here are a couple of ideas for finding alternative, and safer, view sites. If there's an astronomy club in your area (check the Internet), contact them to see where they do their viewing. You might even find some kindred spirits and want to join. If there's not, perhaps you have friends or relatives who live beyond the city and would allow you to use their yard. Who knows -- you might even get them interested in stargazing.

Suggested Star Party Check-List

* binoculars/telescope
* red flashlight
* white flashlight
* planisphere/star maps
* paper and pencil
* reclining lawn chair
* cell phone
* insect repellent (seasonal)
* warm clothing (seasonal)
* drinks/snacks


Star Party Etiquette

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Last  updated:  February 28, 2002

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