At first, sky observation can be frustrating. It may take minutes, hours & even days in some cases to understand the basics of sky observation. But this must not deter you from making a beginning. With the availability of information at the click of a mouse, everythings getting simpler these days. So lets get started!
There are a few things you might want to know before you begin. These sections are a "must know" for any astronomer.
To understand the glossary of astronomy terminology used, see section on Astronomy Jargon.
What does the term "magnitude" mean?
The Stellar Magnitude SystemBy Alan MacRobert Most ways of counting and measuring things work logically. When the thing you're measuring increases, the number gets bigger. When you gain weight, the scale doesn't tell you a smaller number of kilograms or pounds. But things are not so sensible in astronomy, at least not when it comes to the brightness of stars. Star magnitudes do count backward, the result of an ancient fluke that seemed like a good idea at the time. Since then the history of the magnitude scale is, like so much else in astronomy, the history of increasing scientific precision being built on an ungainly historical foundation that was too deeply rooted for anyone to bulldoze it and start fresh. The story begins around 129 BC, when the Greek astronomer Hipparchus produced the first well-known star catalog. Hipparchus ranked his stars in a simple way. He called the brightest ones "of the first magnitude," simply meaning "the biggest." Stars not so bright he called "of the second magnitude," second biggest. The faintest stars he could see he called "of the sixth magnitude." This system was copied by Claudius Ptolemy in his own list of stars around AD 140. Sometimes Ptolemy added the words "greater" or "smaller" to distinguish between stars within a magnitude class. Ptolemy's works remained the basic astronomy texts for the next 1,400 years, so everyone used the system of first to sixth magnitudes. It worked just fine. Galileo forced the first change. On turning his newly made telescopes to the sky, Galileo discovered that stars existed that were fainter than Ptolemy's sixth magnitude. "Indeed, with the glass you will detect below stars of the sixth magnitude such a crowd of others that escape natural sight that it is hardly believable," he exulted in his 1610 tract, Sidereus Nuncius. "The largest of these...we may designate as of the seventh magnitude...." Thus did a new term enter the astronomical language, and the magnitude scale became open-ended. Now there could be no turning back. As telescopes got bigger and better, astronomers kept adding more magnitudes to the bottom of the scale. Today a pair of 50-millimeter binoculars will show stars of about 9th magnitude, a 6-inch amateur telescope will reach to 13th, and the Hubble Space Telescope has seen objects as faint as 30th magnitude. By the middle of the 19th century astronomers realized there was a pressing need to define the entire magnitude scale, both telescopic and naked eye, more precisely than by eyeball judgment. They had already determined that a 1st-magnitude star shines with about 100 times the light of a 6th-magnitude star. Accordingly, in 1856 the Oxford astronomer Norman R. Pogson proposed that a difference of five magnitudes be defined as a brightness ratio of exactly 100 to 1. This convenient rule was quickly adopted. One magnitude thus corresponds to a brightness difference of exactly the fifth root of 100, or very close to 2.512 -- a value known as the Pogson ratio.
The Meaning of Magnitudes
This difference in magnitude... ...means this ratio in brightness
20 100,000,000 to 1
- 0 1 to 1
- 0.1 1.1 to 1
- 0.2 1.2 to 1
- 0.3 1.3 to 1
- 0.4 1.4 to 1
- 0.5 1.6 to 1
- 0.6 1.7 to 1
- 0.7 1.9 to 1
- 0.8 2.1 to 1
- 0.9 2.3 to 1
- 1.0 2.5 to 1
- 1.5 4.0 to 1
- 2 6.3 to 1
- 2.5 10 to 1
- 3 16 to 1
- 4 40 to 1
- 5 100 to 1
- 6 251 to 1
- 7.5 1,000 to 1
- 10 10,000 to 1
- 15 1,000,000 to 1
DETAILS ABOUT MAGNITUDE
By the late 19th century astronomers were using photography to record the sky and measure star brightness, and a new problem cropped up. Some stars having the same brightness to the eye showed different brightness on film, and vice versa. Compared to the eye, photographic emulsions were more sensitive to blue light and less so to red light.
Accordingly, two separate scales were devised. Visual magnitude, or mvis, described how a star looked to the eye. Photographic magnitude, or mpg, referred to star images on blue-sensitive black-and-white film. These are now abbreviated mv and mp.
This complication turned out to be a blessing in disguise. The difference between photographic and visual magnitudes was a convenient measure of a star's color. The difference between the two kinds of magnitude was named the "color index." Its value is increasingly positive for yellow, orange, and red stars, and negative for blue ones.
But different photographic emulsions have different spectral responses! And people's eyes differ too. For one thing, your eye lenses turn yellow with age; old people see the world through yellow filters (S&T: September 1991, page 254). Magnitude systems designed for different wavelength ranges had to be more firmly grounded than this.
Today, precise magnitudes are specified by what a standard photoelectric photometer sees through standard color filters. Several photometric systems have been devised; the most familiar is called UBV after the three filters most commonly used. U encompasses the near-ultraviolet, B is blue, and V corresponds fairly closely to the old visual magnitude; its wide peak is in the yellow-green band, where the eye is most sensitive.
Color index is now defined as the B magnitude minus the V magnitude. A pure white star has a B-V of about 0.2, our yellow Sun is 0.63, orange-red Betelgeuse is 1.85, and the bluest star believed possible is -0.4, pale blue-white (see "The Truth About Star Colors," S&T: September 1992, page 266).So successful was the UBV system that it was extended redward with R and I filters to define standard red and near-infrared magnitudes. Hence it is sometimes called UBVRI. Infrared astronomers have carried it to still longer wavelengths, picking up alphabetically after I to define the J, K, L, M, N, and Q bands (S&T: June 1995, page 23). These were chosen to match the wavelengths of infrared "windows" in the atmosphere where absorption by water vapor does not entirely block the view. Appearance and Reality
What, then, is an object's real brightness? How much total energy is it sending to us at all wavelengths combined, visible and invisible?
The answer is called the bolometric magnitude, mbol, because total radiation was once measured with a device called a bolometer. The bolometric magnitude has been called the God's-eye view of an object's true luster. Astrophysicists value it as the true measure of energy emission as seen from the location of Earth. The bolometric correction tells how much brighter the bolometric magnitude is than the V magnitude. Its value is always negative, because any star or object emits at least some radiation outside the visual range.
Up to now we've been dealing only with apparent magnitudes -- how bright things look from Earth. We don't know how intrinsically bright an object is until we also take its distance into account. Thus astronomers created the absolute magnitude scale. An object's absolute magnitude is simply how bright it would appear if placed at a standard distance of 10 parsecs (32.6 light-years).
Seen from this distance, the Sun would shine at an unimpressive visual magnitude 4.85. Rigel would blaze at a dazzling -8, nearly as bright as the quarter Moon. The red dwarf Proxima Centauri, the closest star to the solar system, would appear to be magnitude 15.6, the tiniest little glimmer visible in a 16-inch telescope! Knowing absolute magnitudes makes plain how vastly diverse are the objects that we casually lump together under the single word "star."
Absolute magnitudes are always written with a capital M, apparent magnitudes with a lower-case m. Any type of apparent magnitude -- photographic, bolometric, or whatever -- can be converted to absolute.
Lastly, for comets and asteroids a very different "absolute magnitude" is used. It tells how bright they would appear to an observer standing on the Sun if the object were one astronomical unit away.
So, are magnitudes too complicated? Not at all. They're as simple as they can be considering their historical roots and what they have to describe today. Hipparchus would be enchanted.
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What are constellations?Constellations by David Aguilar Constellations are like countries on a wall map. They help narrow down the search for those tiny hard-to-find little cities or deep sky objects you would like to visit. By learning the constellations, you also share in the imagination of the people who created them thousands of years ago. Today there are 88 internationally recognized constellations. From either hemisphere, forty-five to fifty should be visible throughout the year.
Most northern constellation names come from the Greeks and Romans, who had vivid imaginations and no television to watch at night. They depicted the lives of the gods and goddesses, heroes and monsters that made up their legends. European astronomers who gave them mundane names like the Microscope, the Telescope, and the Sextant mostly named the southern constellations during the seventeenth century.
Expanding Your Horizons
Not all the constellations look like what they're supposed to, and there are so many of them, it's tough to keep them all straight.
First, get a good star chart. A revolving star wheel, called a planisphere, is an excellent choice. When you set it for the current time and date, it shows what stars and constellations are visible from your location right then. Monthly star charts that appear in astronomy magazines also work well. Use a flashlight that emits red-colored light to read your star chart. Red light works best because it does not spoil your night vision like white light does. Stay away from porch and street lights too.
The next step is to decide just what constellations you want to tackle. On any given evening, set your sights on mastering no more than four new star figures. Carefully trace them in the sky as you learn them and then go back and review the ones you found earlier. On your next night out, before you push off again into uncharted waters, go over what you memorized the previous night.
Studying the constellations over a period of a few hours also serves as a dramatic reminder that the Earth is spinning in space. Constellations near the equator rise and set while those near the North or South poles always seem to be hanging around in the sky. The circumpolar constellations located near the North Celestial Pole include some very famous star groups such as the Big Dipper, the Little Dipper, and Cassiopeia.
What's Your Sign?
When pointing out constellations to someone else, be prepared for someone to ask the big question. "Can you show me my astrological sign?"
Twelve constellations make up the signs of the Zodiac. The reason these particular star groups were chosen is because they form the "Highway of the Gods." If you point your arm to the east where the Sun or Moon came up and move it across the sky to where it set, you have just traced out the ecliptic, or the pathway where all the major members of our solar system can be found. The early Greeks and Babylonians thought the planets, the Sun, and Moon were gods walking across the sky. They also recognized that the constellations visited by these gods must be very special. That is why these twelve particular constellations were chosen.
Incidentally, there is a lot of confusion when people go out on their birthdays and try to locate their sign in the night sky. When the ancients put this whole thing together they reasoned that the constellations must be at their greatest importance when the King of the Gods, the Sun, was visiting them. So, on your birthday, you will not find your sign in the nighttime sky. It is straight overhead at 12 noon right behind the Sun. Unless you are blessed at that very moment with a total solar eclipse (when some stars are briefly visible in the daytime), you will have to wait six months before your special constellation rolls around to the nighttime sky.
Capturing the Constellations on Film
Putting together your own personal set of constellation photos is fast and easy. All you need is
To create your own set of constellation photos, first set your lens at f/2.8 to prevent stars from looking like footballs around the edges of your photograph. Set your focus at infinity. Then frame the constellation in the camera finder, and open the shutter for about 20 seconds. Exposures longer than 20 seconds will begin to record the rotational movement of the Earth, and the stars will "trail" on the film instead of appearing as nice sharp points. You will be amazed at the sheer number and different colors of stars visible in the photographs that were invisible to your eyes alone.
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What are maps & charts? How do I read them?Using a Naked-Eye Sky Map By Alan MacRobert Lots of people buy a telescope only to discover that they can't find much of anything with it in the sky. Their problem? They haven't learned their way around the stars as seen with the naked eye, and they try to use inadequate maps.
Here are some simple tricks for finding your way that should save you a lot of grief.
First things first. You need maps. To start with, you need a simple all-sky map, for use with the naked eye that shows where to find the brightest stars and constellations as seen at your particular time, date and latitude on Earth.A simple planisphere or "star wheel" can do the trick. You turn a plastic or cardboard dial to set your time and date and get a rough map of your whole sky. The map's edges represent the horizon all around you, as if you were standing in an open field and turning around in a complete circle. Compass directions should be printed around the horizon/edge. The center of the map represents the part of the sky directly overhead. A star that's plotted on the map halfway from the edge to the center, therefore, can be found about halfway up the sky -- halfway from horizontal to overhead.
That's really all there is to it!
Many planispheres are offered for sale, all too many of them poorly designed. Look for one with small, fine, carefully drafted star dots and patterns. These will be easier to match to real stars in the sky. Avoid glow-in-the-dark star maps; the glow paint can't be printed very accurately, so the result is usually a map that looks confusingly different from what it's supposed to represent.
An excellent all-sky map appears in or near the center of every month's issue of Sky & Telescope magazine. It works the same way: the big round edge is the horizon all around you (with compass directions labeled), and the center is the point overhead. This map is drafted for specific times and dates (printed in its upper right corner). This way it avoids the distortion of the southern sky that a planisphere has to be drawn with in order to work for all times and dates.
Many planetarium programs for computers can display and print a customized all-sky map for whatever time, date, latitude, and longitude you specify.
Into the Night
To read the map outdoors, bring along a dim flashlight. The best flashlight for astronomy is red, not white; red light affects your night vision less. You can rubber band a piece of red paper or plastic over the front of the flashlight. This both dims and reddens the light.
Outdoors with your map, start by looking for only the brightest stars plotted on it. The difference between bright and faint stars in the sky is much greater than is represented on paper. In fact, if you live in a populated area where there is much light pollution (artificial skyglow), the faint stars will be completely invisible.
Also, be aware than the constellations on an all-sky map appear much smaller than they do in real life. The star patterns you're hunting in the night are mostly big! Go out often with your map, and use it to learn all the constellations you can. You are establishing the familiar, major landmarks that you'll need when you start using a more detailed map with binoculars or a telescope.E-MAIL me TOP OF PAGE Disclaimer