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.
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.
Factors Affecting Stargazing
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.
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.
STARGAZING AND MOON PHASES
MOON PHASE EVENING VIEWING MORNING VIEWING New good good 1st Quarter poor good Full poor poor 3rd Quarter good poor
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
Suggested Star Party Check-List
* red flashlight
* white flashlight
* planisphere/star maps
* paper and pencil
* reclining lawn chair
* cell phone
* insect repellent (seasonal)
* warm clothing (seasonal)
Star Party Etiquette
- Lights: Dark adapted eyes are important when stargazing, so use RED flashlights, not white flashlights (except in the case of an emergency, of course). Don't shine light -- even red light -- in people's eyes or face.
- Telescope viewing: Except when being used for a special project (such as astrophotography), telescopes are there for viewing so it is usually OK to move from scope to scope to look through all of them if you wish. They will probably be showing different things. Don't mob the scopes -- get in line to look through them. Try not to bump or move the scope. If you don't see anything, say so -- the object may have moved out of the FOV, the scope may have been bumped, or you might need some tips on how to look into an eyepiece.
- Asking questions and talking: It's quite OK to talk and ask questions. Just try not to get so loud others can't hear. (Loud music is NOT welcome at star parties.)
- Refreshments, etc.: It's OK to bring food and drinks (and sharing is OK, too!). Finally, it's a good idea to go the bathroom before going to a star party -- there usually are no bathrooms there.
Last updated: February 28, 2002