ASTRONOMICAL NOTES

Evening Star, Issue 21634, 1 February 1934, Page 15

ASTRONOMICAL NOTES

THE SKIES IN FEBRUARY, 1934 [Written by A. G. C. Crust, M.Sc'., for the ‘ Evening Star.’] POSITIONS oTTHE STARS Local Sidereal time, 6h, latitude 46deg S. The positions of the stars as described last month continue to hold good in February, four minutes earlier each day—e.g., at 10 p-m. on February 1, 9 p.m. on February 16, and 8 p.m. on March 3. The Moon will pass south of Regulus on the evening of February 1, being situated E.N.E., lldeg, at 10 p.m. Our satellite wilt pass the planet Jupiter on the morning of the 6th, and will reach the last quarter phase on the evening of the 7th. Having passed the planet Saturn on the morning of the 14th,- the Moon will be new about midday, causing a total eclipse of the Sun on a narrow track extending from Borneo almost to Alaska. New Zealand, however, will be far outside the zone of partial eclipse. Having passed the Sun, the Moon will pass the planets Ma rs and Mercury on the afternoon and evening of the 15th, being 25deg north of Fomalhaut in the early evening. First quarter will occur on the evening of the 21st, the Moon’s position at 8.40 p.m. being N.N.W., 13deg, and on the forenoon of the: 22nd she will pass lOdeg north of Aldebaran. Early on the 26th the Moon will pass 4.4 deg south of Pollux, and at 8.20 p.m. will be placed N.E. by N., 16deg. No planets are visible at 6h, S.T., this month. _ Mercury should be visible very early in the evening, however, passing very close to Mars on the forenoon of the 9th, and reaching greatest elongation, 18deg, east of the Sun on February 18. Mercury will be stationary in the evening sky on February 24. Venus will pass out of the : evening sky, being in inferior conjunction with the Sun on the sth, and stationary in the morning sky on the 24th; ; Mars is drawing near to' the Sun, and must be looked for very early in the evening, while Jupiter will be stationary in the morning sky on the 7th, and Saturn will be in conjunction with the Sun on the Bth. EXPLANATION OF THE NOTES In the notes we endeavour, in various ways, to provide a ready guide to the principal heavenly bodies, and to help our readers to recognise their positions and movements. It should not be necessary to point out that astronomy cannot be learned solely by reading books and notes. On thefcontrary, just as the naturalist must learn his sciences in the field, the astronomer really makes a beginning og his study ouly when he goes out and seeks to identify the actual stars in the night sky, and it is our purpose to encourage our readers in every way to take this initial step. It is true that astronomy may be approached by way of mathematics, but it must be conceded that those who do this show a strong tendency to remain mathematicians rather than to become “ allround ” astronomers. It has been very truly said that a knowledge of the world is the foundation of success.; we might, witli good reason, add that a knowledge of the heavens is the foundation of a knowledge of the world. ’Probably, in spite of all the teaching of modern schools, the astronomers are by far the best able to realise that this world is a globe for this fact is vividly impressed upon them by their study of the stars. It is true that most people believe that the world is globular, just as they believe in so-called historical facts, many of which cannot now he rigorously proved. If an interest in greater things and wider spaces can be said to improve the mind, here astronomy has the greatest claim to recommendation. We do not admire the person whose whole interest is devoted to himself; he is said to be introspective, and is likely to become very selfish. It is far better to bo interested in other people, even one’s immediate neighbours, while we certainly expect intelligent people to take an interest in other cities and other nations. Now, when our interest embraces, in some degree, the whole world, astronomy offers to show us something- of 'bur Sun, on which we are so dependent, of our sister planets, and of those distant Suns, the starts. Astronomy thus challenges the world, just as all the sciences and arts challenge the individual, to find its place in the heavens just as the individual is to,find his place in the world. There are several possible ways of identifying the stars, so that one can go out and feel as much at home in the starry heavens as in any familiar district of the earth. The . ancients made a picture book of the skies, using the stars to outline the forms of animals and men. One drawback to this method, especially in the Southern Hemisphere, is that, the constellations often appear upside down or in other unusual positions, while another difficulty,, is that various alterations made over long periods of time have left some asterisms with very little resemblance tq the animal whose name they bear. However, it is interesting to be able to trace some of these ancient forms in the sky, especially as these pictures explain so many of the nafnes given to the stars by the Arabs and others.

One of the sources of confusion to those who would like to know the stars is simply the rotation of the earth, if the earth could stop spinning for a considerable period, then the stars would remain fixed'in the sky until the earth resumed her rotation again. Needless to say, such behaviour on the part of the earth would be very inconvenient in many ways, but it would certainly give ample time to familiarise us with the positions of the stars, in relation to well-known hills, etc. As we cannot stop the earth, however, we may adopt another plan, which is quite as effective. The earth rotates on its axis once in twenty-four, hours, and at the same time revolves around the Sun once in a year of a little over 365 days. Now, the day according to the stars is shorter than the day according to the Sun. A good analogy is provided in a ballroom, with a central lamp to represent tile Sun, a waltzing couple to represent the Earth, and doors, stage, windows, etc., to represent the distant constellations. As the couple progress round the floor it will be realised that each complete turn, say, relative to the stage, is shorter than the complete turn relative to the central light; in fact, if fifty turns are niade in a complete circuit of the room, the couple will have faced and turned away from the jamp just forty-nine times. Similarly. there are 365 whole days in the year according to the Sun, but 366 whole.days by the stars. To return to the ballroom example, the waltznng couple see the central light, as it were, gradually travel round the opposite side of the room, passing now the doorway, then the window, the stage. *tc., and if it is a dazzling light, it will obscure these portions of the room in.doing so. Exactly in the same way, the Sun obscures the constellations in front of which it appears to pass, and so on in February, for instance, the observer might look in vain from evening tijl sunrise for Aciuarius. where the Sun is now situated. The Sun’s progress around the sky is very gr»-

dual, and causes the star to appear in any given set of positions four minutes earlier every mean solar day. We have adopted a plan for stating eleven ol these positions, as if the earth rotated once in a year by a series of eleven sharp jerks. Sidereal time is the time measured by the stars, and, 'being the most regular and convenient kind ot time supplied by Nature, is used by astronomers as the basis Irom which mean solar time is calculated. Ihe starry skies are divided into hours ot sidereal time, just as the Earth’s surface may be divided for longitude. When sidereal time is thus, as it were, attached to the stars, it is called Right Ascension, and the measurement as made eastward from the (Southern Hemisphere) autumn equinox, in the constellation Pisces in the present age. Now the astronomer actually obtains his fundamental “ local sidereal time ” by observing exactly the Right Ascension of a star crossing the : meridian, if the times are required only to the nearest minute this observation is a very,simple matter. Thus, when.a star of R.A. 6h is on the meridian, the local sidereal timne is 6h, this kind ol time being always the same ns the Right Ascension of the stars op the meridian. : Our predictions of the positions of the stars have been worked , out by spherical trigonometrv for the eleven epochs as followS.'-D, 6h for January and February,' Bh' for March, lOh for April, 12h for May, Ith for June, 16h for July, 18h for August, 20h for September, 24h for October, 2h for November, and 4h for December, the epochs being chosen so as to fall m the evening hours-each month. _ Thus, 6h S.T. occurs about midnight in summer time at the beginning of January, about 11 p.m. in mid-January, 10 p.m. at the end of the month, 9 p.m. about February 15, and 8 p.m. at the beginning of March. The predictions give the tre bearing, of the objects, stated, according to the , compass, N.N.E,. etc., and the elevation in degrees, while a short paragraph gives a description of the sky as seen from a definite well-known site in the city. If readers will, go to the site mentioned and Observe the stars every fine!night for a month at the times indicated in the ‘Positions,’ they will find that the stars are constantly in the same places, relatively to buildings, streets,, etc;, Thus the rotation of the earth hat been eliminated, .with its attendant difficulties for the learner. ' The star positions as described may he checked by the observer in various ways. For instance, the position of the Moon at the current sidereal time is usually given three times a month, the mean time of each epoch being given, and cases of two stars appearing in the same direction or at the same elevation are usually pointed out. The positions of planets at the current sidereal time are also given, if they are visible. Such a method of giving stellar positions is fairly suitable for enabling ns to identify the bright stars, but to make this knowledge more secure, it is really necessary to become acquainted with the fainter stars. The apparent relative brightness of the stars, and in the case of bright ones their colour also, are a great help towards.their identificatipn. It would be as difficult to remember thousands cf stars as it is to remember thousands of people, and just as unnecessary, for in either case only the more prominent ones, from a practical point of view, heed be considered.

For those who are sufficiently interested in the stars to take tlie trouble, we have devised a system of zones for mapping the stars. This system-differs technically from most other systems, in, that the maps are not projections. Nevertheless, they will provide those who make them with very faithful representations of _ the stars down to the fourth magnitude, and add very considerably to the map-makers’, knowledge of the heavens. So far we have been describing the “ equatorial zone,” extending 30deg on either side of the celestial equator, which may be plotted on squared paper as a zone 60 squares widb and 360 squares long. The positions of stars are stated simply by such figures a 5.97,36, for example. This means the 97th square from the lefthand side of the zone, and the 3Gth from the bottom, and all such position figures are to be interpreted similarly. As many of our, readers should realise by now" the 90th square coincides with the 6th hour of Right Ascension, so. that at the. epoch given for this month, the squares around the 90th appear in the northern sky. The limits of the zone, as they appear in the sky from our latitude, are at elevations 14deg and 74deg, on the meridian to the north of us. As an example of the usefulness of the zone maps we may mention the three bright (second magnitude) stars of Orion’s Belt. They lie midway betwen the bright stars Rigel and Betelgeuse, as the maps show, and consequently help us to identify those two stars wherever they may appear in the sky. In previous explanations of the notes we have dealt more fully with the planets. In this article we shall explain some of the particulars given in connection with the stars. All stars are immensely distant in comparison with the planets If we tried to draw a map of the stars, showing their distances to tlie correct scale, we might begin to draw, in the Octagon in Dunedin. a body as small as a grain of sand to represent the Sun, with a circle Tin in radius around it, for the orbit of the Earth, which would be quite a microscopic speck on this scale of distances. Now all the planets except Pluto would be comprised within a circle of 30in radius, and for all of them, also for the periodic comets with their long elliptical orbits, there would be ample rom in the Octagon. On the other hand, Alpha Centaun, the nearest stellar system, could be placed no nearer to the Octagon than Macandrew’s Bay, and Sirius might be . represented by a sand grain in Papamu Inlet. ‘With a sheet of paper actually as large, as Australia, we might hope to make a fairly complete map of the visible stars in this way, for largo portions of the heavens. Astronomers refer to the parallax of a star, which is the radius of the small circle which a star appears to trace out on the background of the sky, as the Earth makes her annual journey around her orbit. The earliest deterinitiations of the distances of the stars were, in fact, visual measurements ot these tiny parallactic circles, but now photography is used for measurements of this kind, and stellar distances may bo found also by employing totally different methods —e.g., the proper motions of the stars, their spectra, the orbits of binary stars, and the periods of variable stars. The parallax, ot course, is not a distance, but an angle, of which the distance may be stated (in parsecs) as the reciprocal. Perhaps the most interesting unit for stellar distances is, however, the “ light year.” This is the distance traversed by a ray of light in the course of one year; it is about six billion miles, and it exceeds the distance of the Sun from the Earth, the astronomical unit,” (92.900.000 miles), in nearly the ratio of the English mile to the inch. The diameter of the Sun is 864,000 miles, that of the Earth 7,900 miles. Curiously enough, the diameter of the Sun is very nearly 108 times that of the Earth, and its distance from the Earth nearly 108 times its own diameter. Remembrance of this curious relationship should help our readers greatly to understand the dimensions of the stellar systems we are describing.