Thursday, March 10, 2011

Wonders of the Night Sky with the Naked Eye


Prelude

Recently I happened to go through the June 2009 issue of NCERT’s quarterly journal, School Science and chanced to see an article titled, “Viewing the Night Sky with the Naked Eye”, attributed to me as the author.  I was taken aback since I had not sent such an article for publication to any journal recently.  Then I noticed the caption at the footer: “Reproduced from School Science, Vol. 19, No. 4, December 1981”.  I had totally forgotten about its first publication so far back in time.  The editor found it fit to reprint it in its entirety, and completely untouched, as part of the special issue of the journal on the International Year of Astronomy 2009 (IYA09).   After reading it, I thought it fit to include it in this blog post, but only after some essential editing and updating.  So, here it is, with the sky charts left as they were despite some obvious spelling errors.  I wish the editor had asked me to take another look at it and update it before deciding to print it on his own.

Introduction

One of the greatest hobbies and pastimes one can indulge in, and entirely free of cost as an added incentive, is watching the wonders of the night sky with the naked eye, especially if one gets the chance to do so on clear dark nights far off from any disturbing lights nearby.  I have been doing this ever since 1965 when my interest in night sky objects was aroused by the sight of the great comet Ikeya-Seki.  Incidentally, comets that get visibly close to the Sun as well as the Earth and develop a characteristic tail are among the most spectacular sights for the human eye to behold.  However, they show up very infrequently and the number of them that I have been able to see with my own eyes since 1965 can be counted with the fingers of my two hands.
 
There is a widespread popular misconception that a telescope is necessary for any worthwhile observation of celestial objects in the night sky. Nothing can be farther from the truth.  It must be remembered that, till Galileo turned his first crude telescope to the skies, the human eye was the only means of observing and studying celestial phenomena. These naked eye observations and measurements extended back several thousand years through many civilizations and reached their pinnacle of refinement and precision in the hands of the great Danish astronomer, Tycho Brahe, in the sixteenth century. They later paved the way for Kepler's discovery of the laws of planetary motion followed by Newton's great universal laws of motion and gravitation.

The amateur sky gazer can make a wide range of fascinating observations of the night sky with nothing more than his bare eyes, even from 'light polluted' urban areas. He needs no special skills or training other than the ability to read simple star charts, a skill which he can teach himself.  If he is lucky to find himself in a place far removed from the disturbing effects of street and city lights, with a dark and clear sky above, he can see one of the most glorious spectacles of Nature – an incredibly beautiful, dazzling, star-studded sky.  I have experienced this on numerous occasions when I found myself far removed from urban locations with no lights of any kind around me, not even moonlight.  This is the setting which should transform any observer from an armchair student of Astronomy to a 'living' amateur astronomer intent on discovering for himself the wonders of the night sky.

In the words of Robert Burnham, the great sky watcher, 'The appeal of astronomy is both intellectual and aesthetic; it combines the thrill of exploration and discovery, the fun of sight-seeing, and the sheer pleasure of first hand acquaintance with incredibly wonderful and beautiful things."

The late Prof G T Narayana Rao, who lived in my neighborhood in Mysore, was a well known writer on popular science and other topics in Kannada and widely known for his untiring efforts to popularize night sky watching as a hobby among lay people in particular.  Apart from his writings on the topic, he used to engage groups of enthusiastic viewers in an enthralling live demonstration and description of the night sky objects whenever he had an opportunity to do so.   His commentaries used to be recorded and played out on radio stations to synchronize with the appearance of the night sky at the time of the broadcast.  This way they reached out to large and widely distributed audiences right on their roof tops or backyards and had become very popular.  His efforts were directed as much at developing a much needed scientific temper among the viewers as in sharing the excitement of viewing the wonders of the night sky.  Though he used names, characters and stories drawn from Indian mythology extensively in his descriptions, he used to brusquely dismiss any suggestions that the stellar objects influenced human lives in any manner and had only contempt for the practitioners of Astrology and other pseudo sciences.  I had known him very well and he was associated with me as a resource person in some of my Astronomy education programmes aimed at teachers and students.  It is with great fondness and gratitude that I remember him here.

In this article I shall make a brief survey of the wonders of the night sky and present tips on how to see some of them with the naked eye.  Towards this end, the four figures presented later should be of considerable help to the reader. 

Stars and Constellations

The first thing that strikes the casual sky-gazer is the large number of stars of different brightness and color distributed quite chaotically in the sky. Ancient civilizations tried to put some order in the heavens by conceiving certain arrangements or groupings of stars which they associated freely with mythological stories and characters. Most of these groupings or constellations as they are called have been preserved even to this day despite their highly fanciful associations. Indeed it requires a very fertile imagination to “see" such characters as Orion the hunter, Pegasus the winged horse, Draco the dragon or Sagittarius the archer in the night sky. Astronomers today recognize 88 constellations with well defined boundaries. Some of these constellations can be identified quite easily since they contain a number of bright stars grouped into clearly recognizable patterns. A few examples are Ursa Major, Orion, Cassiopeia, Scorpius, Acrux (Southern Cross) and Cygnus. Others such as Hercules, Centaurus, Cepheus and Capricornus are not as easy to identify. The majority of constellations are rather difficult to identify for beginners.

A knowledge of the constellations helps to pinpoint celestial objects of special interest such as star clusters, nebulae, novae, galaxies and variable stars for further observation, especially with telescopes. In the olden days it had a more practical use since man could orient himself in an unknown territory or navigate the seas by following the major constellations or bright stars as 'landmarks'. Long distance spaceflights of the future will also have to rely on stars for navigation in space.

Stars and constellations have a regular apparent daily motion which can be easily followed even over a short period of time. This is actually due to the rotation of the earth about its axis once in 24 hours. Also, because of the earth's motion round the Sun once in 365 days, the constellations show another regular apparent motion, changing with the seasons. This can be easily followed by noting the location of a particular star or constellation in the sky at the same time of day over a period of several weeks or months.


The annual apparent path of the Sun in the background of the fixed stars is called the ecliptic and its plane cuts the celestial equator at an angle of 23.5 degrees. Since all planets and the Moon have their orbits in nearly the same plane, they are to be found in the sky within a belt of about 7 degrees wide on either side of the ecliptic. This is called the zodiacal belt and the 12 prominent constellations on this belt are called the zodiacal constellations.  Actually, there is a 13th constellation on this belt, called Ophiucus (lying between Scorpius and Sagittarius), but this is conveniently ignored by those who blindly associate these constellations and planets with human affairs against all evidence to the contrary. 

The earth's axis of rotation is pointing very nearly in the direction of the star Polaris, the North Pole star. Therefore, this star is always seen at nearly the same point in the sky at all times. It is easy to locate the pole star. Look directly northward at an angle equal to the latitude of your place above the horizon. The brightest star you see in this direction is the pole star. There is no other star of comparable brightness to confuse you. Observe it for a few hours to assure yourself that its position is unchanging.

The night sky presents stars of widely differing brightness as seen by the eye. On a very clear dark night, away from all disturbing lights, the human eye can see as many as 3,000 stars - from the brightest to the faintest. Astronomers indicate the apparent visual brightness of a star by a unit called the stellar magnitude following a logarithmic scale of variation. On this scale the limit of visibility to the normal unaided eye in ideal viewing conditions is magnitude 6.

A fifth magnitude star is about 2.5 times as bright as a sixth magnitude star, a fourth magnitude star about 2.5 times as bright as a fifth magnitude one, and so on down the scale. The pole star is of magnitude 2. Negative magnitudes are assigned to the brightest four stars in the sky as well as some planets.


Another striking characteristic of a star is its color, which is dependent on its surface temperature. White and blue stars such as Sirius and Spica are the hottest, with temperatures ranging from 10,000°C to 50,000°C. Yellow stars (our Sun belongs to this category) are 'cooler' with surface temperatures of about 6,000°C. Red stars such as Betelgeuse and Antares are the coolest with temperatures about 2,000°C.

Fig 1 (Click to enlarge)

Fig 2 (Click to enlarge)

Fig 3 (Click to enlarge)

Fig 4 (Click to enlarge)


You may begin a systematic observation of stars and constellations by first collecting simple star charts easily downloadable from the Internet or available in some publications. Most elementary texts in Astronomy also give such charts for different seasons. Choose a convenient open space as far away from disturbing lights as possible and begin to identify the brightest stars in a constellation from the appropriate chart for the time and day of the year. Hold the chart over your head with your right hand side towards the west. A small flash light will come in very handy for looking at the charts at frequent intervals.

Start looking for the most prominent constellations. In doing so, look for simple geometrical patterns such as straight lines, triangles, polygons, curves, etc., in the constellation maps and then try to identify the corresponding patterns in the sky. In the winter evening sky, some of the easily identifiable constellations are Orion, Cassiopeia, Auriga, Taurus, Leo, Pegassus, Canis Major and Gemini. In the summer evening sky, you can easily pick out Ursa Major, Scorpius, Sagittarius, Cygnus, Lyra, Corona Borealis and the Southern Cross. Figures 1-4 show these and other constellations with the names or designations (in Greek letters) of their brightest stars as well as the season and the time of the day they are best seen. The lines and curves joining the stars are drawn only for convenience in identification and serve no other purpose.

The thirteen zodiacal constellations are of special interest since it is only in these regions of the sky one has to look for the planets. These are Aries, Taurus, Gemini, Cancer, Leo, Virgo, Libra, Scorpius, Ophiucus, Sagittarius, Capricornus, Aquarius and Pisces. Some of these are easily identifiable through the presence of some bright stars in recognizable patterns. Scorpius is easily the most spectacular zodiacal constellation, with a shape true to the name and a large number of bright stars including the bright red giant Antares. Figures 1-4 also show the zodiacal constellations skirting the ecliptic.

Planets

The planets of the solar system constitute a distinct class of celestial objects characterized principally by their irregular motion in the heavens as viewed from the earth which is itself such an object. The five planets - Mercury, Venus, Mars, Jupiter and Saturn – which have been the subject of the most intense observational study and analysis from time immemorial – can all be easily identified by any systematic observer.

Venus is the brightest object in the sky after the Moon and hence the easiest to identify in the western sky after sunset or in the eastern sky before sunrise. It is popularly known as the 'evening star' or 'morning star', depending upon when it is visible. Jupiter is brighter than the brightest star and hence again very easy to identify. Saturn appears to the eye like a bright star of magnitude about -0.4 (about as bright as Arcturus) and even a cursory knowledge of the zodiacal constellations will suffice for its identification.

Mars is a fascinating object for the sky watcher. Because of large variations in its distance from the earth we see a corresponding variation in its brightness (about - 2.8 magnitude at its brightest; about 2.0 at its faintest) over a period of 25 months. Further, it has a characteristic red color which distinguishes it clearly from other planets. At its brightest, it is as bright as Jupiter. 

Mercury is generally a difficult planet to locate since it is always close to the Sun. The best time to look for it is when it reaches maximum elongation (i.e., maximum angular distance from the Sun) sometime after sunset or before sunrise. This information can be obtained from data of planetary positions such as the website http://www.heavens-above.com/.  If you locate this rather elusive planet, you will have done better than the great Copernicus who spent a lifetime without seeing it!

Planet Uranus discovered by Sir William Herschel in 1781 is near the limit of the naked eye visibility and requires a special effort and experience to view with the unaided eye. However, the slightest optical aid such as a small prismatic binocular or a small spotting telescope will bring it to clear view provided one knows exactly where to look for it. Neptune and Pluto (the latter no longer regarded as a planet) are invisible to the unaided eye.

As pointed out earlier, the most distinctive feature of the planets is their irregular motion in the sky. This is easy to follow, especially if the planet in question is close to some bright stars. Repeated observations of it with respect to the fixed pattern of the nearby stars over a period of a few days or weeks show an unmistakable change in its relative position. One can even plot the approximate position of the planet at convenient intervals of time on a chart of the zodiacal constellations. If this is done systematically and regularly, you will discover for yourself the fascinating retrograde motion during which the planet appears to reverse its direction of general motion from west to east with respect to the constellations. Such a motion for Mars, Jupiter and Saturn can be detected when the planet is nearly opposite to the Sun in the celestial sphere.

Moon

This is the most conspicuous object in the night sky and everyone is familiar with its changing phases, rising and setting. However, not everyone will have noticed that when the thin crescent moon is observed carefully, one can also see the rest of the lunar disc faintly illuminated. This fascinating phenomenon is due to the reflection of earth light.

Because of its large disc in the sky, it is not very unusual for the Moon to blot out a bright star or even a planet in its daily path in the zodiacal belt. This phenomenon, with the object appearing to go behind the Moon and emerging on the other side sometime later, is called an occultation and can sometimes be observed with the naked eye. However, advance knowledge of such phenomena will be necessary.

Eclipses

Most people are familiar with the eclipses of the Moon and the Sun, partial or total. Lunar eclipses are best observed with the naked eye while solar eclipses might prove disastrous to the eye if any attempt is made to view them without safe filters.

A total solar eclipse, during which the Sun is completely obscured by the lunar disc for a short period, is one of nature's most fantastic spectacles. What a great pity that millions of people in this country were misled from watching one seen in the northern parts of the country on 22 July 2009!  It is perfectly safe to watch this kind of eclipse with the bare eyes during the brief period of totality and (note carefully!) only during this period of totality. Considering its rarity, a total solar eclipse is a once-in-a-life-time experience for most people, but what an experience!

Stars and planets are not the only objects of major interest in the night sky. A wide variety of other celestial objects and phenomena are well within the reach of the naked eye. Some of them are described here.

Other Naked Eye Objects

(i) The Milky Way: On a dark clear moonless night, far away from disturbing lights, the sky watcher can easily see large irregular milky white patches of diffused light in the sky, especially in the summer evening sky. This is the Milky Way, the galaxy of which we form a part. A powerful telescope will resolve this hazy patch into hundreds of thousands of individual stars. The Milky Way passes through the constellations Andromeda, Cassiopeia, Perseus, Auriga, Taurus and Canis Major in the winter skies and through Cygnus, Aquila, Scutum, Sagittarius and Scorpius in the summer skies.  Parts of the Milky Way in Sagittarius and Scutum are so dazzlingly bright that they can often be mistaken for clouds in the sky.

(ii) Star Clusters: Several beautiful clusters of stars can be seen with naked eye. There are two basic types – open clusters and globular clusters.

The Pleiades (popularly known as the Seven Sisters) is the best known open star cluster (see Fig 1). The average eye can see a small group of six bright stars in this cluster while very keen eyes may see a few more (A low power telescope shows over a hundred). When the constellation Cancer is nearly overhead, one can, on a very dark night, see a large cluster of faint stars known as Praesepe or the Beehive (see Fig 2). The open cluster known as M7 in Scorpius [see Fig 3) can also be spotted easily. The famous double cluster in Perseus (see Fig 4) is another object that can be seen with the naked eye. This is best seen when on the meridian.

Globular clusters are aggregates of hundreds of thousands of stars appearing as a diffused disc through low power telescope. To the naked eye the largest among them appears as a tiny diffuse speck of light.  ω (Omega) Centauri (see Fig 3) is the best known globular cluster and can be easily located in the southern sky. The famous M13 in the constellation Hercules is another. A third, 47Tucanae can be best seen only from the southern part of the country.

(iii) Variable Stars: Thousands of stars in the night sky show regular or irregular variations in their brightness. While most of these require telescopes or binoculars to be observed (of course by comparison with neighboring stars of comparable constant magnitude), a few can actually be followed easily with the naked eye. The most famous of these is Algol (Beta Persei) in the constellation Perseus (see Fig 1). Known even to the ancients as the winking eye of Medusa, its brightness varies regularly between magnitudes 2.1 and 3.4 with a period of 2.87 days and can be easily followed with the naked eye in the constellation Lyra (see Fig 3). Beta [β] Lyrae is a variable of period 12.92 days. Its visual magnitude varies between 3.4 and 4.3 and can be observed easily by comparison with its near neighbor Gamma [ϒ] Lyrae whose magnitude is constant at 4.2. Delta [δ] Cephei in the constellation Cepheus (Fig 4) is another famous variable star with a period of 5.37 days. Its variation between magnitudes 3.6 and 4.3 can be followed by comparison with the nearby stars Zeta [ζ] (3.6), Epsilon [ε] (4.2) and Nu [ν] (4.5).

Among the few long period variables which can be followed with the naked eye, the most famous is Mira, the 'wonderful' in the constellation Cetus (see Fig 1). Its brightness varies generally between 3.4 and 9.3 with a period of 331 days. When at its maximum it is one of the brightest stars in the constellation. At minimum it is even beyond the reach of binoculars. A similar long period variable is Chi [χ] Cygni (407 days), also easy to follow (Fig 3). Scores of variable stars can be followed even with a small prismatic binocular. Observing irregular tong period variable stars can be a particularly useful and fascinating hobby for the serious amateur.

(iv) Close Star Pairs: While most true double and multiple stars require a telescope to be resolved into their components, the human eye can still see many closely spaced star pairs, some of them true binaries. The Alcor-Mizar pair in Ursa Major (see Fig 3) has been a test of good sight for centuries (Mizar itself is a double when seen through a telescope). Slightly easier to see is Epsilon [ε] Lyrae which is actually a 'double=double' through the telescope. The list of such pairs is large and the observer is invited to let his gaze wander in the sky and discover them for himself.

(v) Nebulae: These are diffuse interstellar gaseous material from which stars eventually form due to gravitational accretion and binding.  As such, they are the birth place of stars.  Emission nebulae glow from the ultraviolet light of stars within them. The Orion nebula is a spectacular sight through a telescope but can be just seen with the naked eye as a tiny bright patch in the position shown in Figure 1. The Lagoon nebula is Sagittarius (see Fig 3) can be more easily located with the naked eye. It looks like a small elongated patch of light. This constellation has two other nebulae which a keen eye can make out in ideal viewing conditions.

(vi) Andromeda Galaxy: The most remarkable thing about this is that it is the most distant celestial object visible to the naked eye - at a mind boggling distance of 2,200,000 light years away.  A well trained eye can make out a tiny nebulous object corresponding to the central core of this great 'island universe' at the position shown in Fig 1. However, very good viewing conditions are necessary.

Till the early part of this century this was believed to be a nebula. Then its extragalactic nature was established with a reliable estimate of its enormous distance from us. It is a companion galaxy to our own Milky Way with an estimated 100 billion stars in it. Its structure is evident from any photograph taken with giant optical telescopes.

(vii) Meteors: Most people may have been fascinated by an occasional 'shooting star' in the dark sky. At certain periods of the year ‘meteor showers' can be observed. They consist of single meteor streaks coming in quick succession (generally at irregular intervals, averaging one every few minutes) from a particular region (constellation) of the sky. The Perseid showers in August and the Leonids in November are two best known examples. They are best seen when most active. Several hours of watching and counting can be exhilarating.

(viii) Comets: Comets with their awesome, long blazing tails visible to the naked eye are rather rare visitors from outer space. When they show up, they are among nature's most spectacular sights, especially because of their relatively rapid motion in the sky. Rarely are they seen for more than a few days.  The best known periodic comet, Halley, was a great disappointment visually when it last appeared in 1986.

(ix) The Magellanic Clouds: These have almost the same appearance as the small patches of the Milky Way and are best seen from the southernmost part of the country. These extra-galactic objects are enormous star clouds, actually miniature galaxies in themselves.

(x) Artificial Earth Satellites: Since the beginning of the Space age in 1957, the USA, the USSR and several other countries including India have put into earth orbit thousands of artificial satellites, large and small, for various purposes. While watching celestial objects, one encounters these satellites occasionally. They are characterized by their rapid motion in the sky and can sometime be seen moving from horizon to horizon. Many of them show variations in brightness due to their rotation which causes changes in the amount of sunlight reflected towards the ground. A systematic watching of the dark sky early in the evening or late in the morning should show quite a few of these man-made objects in space.

Accurate information about the path and visibility of the more prominent earth satellites such as the International Space Station (ISS) and the Hubble Space Telescope (HST) from any given location can be obtained from the highly informative website mentioned earlier, viz., http://www.heavens-above.com/.   Because of its large size and proximity to the earth, the ISS is easily the brightest satellite one can see and at times can be seen to move from horizon to horizon for several minutes at a stretch.  Information about the visibility of the spectacular Iridium flares can also be obtained from the same site.  These flares (caused by the reflection of sunlight directly on to the location of the viewer from a solar panel of any particular Iridium communication satellite) last just a few seconds, but the light can be as bright as a -8 magnitude celestial object – about a thousand times as bright as planet Venus.

Conclusion

The night sky opens out a wide variety of fascinating objects for viewing with the bare eyes at no expense and with no specialized skills. This is the basic preparation one needs before embarking on an even more exciting exploration of the night sky with binoculars and telescopes. Urban dwellers should not get discouraged easily; they should look out for suitable locations as far away from disturbing lights as possible and indulge in the hobby whenever possible.  The wonders of the night sky are within easy reach of anyone curious enough to turn his searching eyes towards them.

2 comments:

A V G Rao said...

Useful to a beginner.

Blogger said...

There's a chance you qualify for a new government solar energy rebate program.
Determine if you qualify now!