This is the fourth edition of a booklet which was originally produced for boy scouts with a supplement to make it more useful for military purposes. The advantage of being able to use astronomical means for finding direction or for getting a rough idea of the time has already been shown in campaigns in the desert and in comparatively uninhabited country, and officers who have taken part in such campaigns seem impressed with its value which is increased by the present-day practice of performing many military movements at night. Rough naked-eye methods of the kind explained here may be of use to sailors who find themselves in a boat with no navigator and more than once a pilot who has been forced down in the desert has walked home with no other guide. If you know the sky you will never in fine weather find yourself in completely unfamiliar surroundings; the part above you will be familiar and friendly and will enable you, to orientate yourself. The contents of this booklet will be easy to grasp if while reading it you look at the sky/or things mentioned and see them for yourself. The best way to learn about the stars is to combine practical observation with reading, for things which may appear somewhat complex in the mere description become quite simple with acquaintance. The reading by itself will tell you very little and learning merely by looking at the sky would be a much slower process. Not everyone will want to use all the material in the booklet and perhaps it is as well to say that it is not necessary to learn all of the constellations in order to use the stars for finding direction; familiarity with the more prominent ones should prove enough.


[*05] If we look up at the sky on a clear night it is easy to imagine that the stars are placed on the inside of a huge sphere and it is convenient to regard them as so placed when considering their motions as they appear to an observer on the Earth. This sphere is called the celestial sphere. An examination of the sky on a few occasions shows that the celestial sphere is in motion relative to the horizon. Stars rise towards the east and set towards the west and at the same time of night different stars are visible at different times of the year. However, the stars do not move relative to one another, and so the star groups always keep the same shape although they change in position. Figure A is drawn to help in the description of this apparent motion of the celestial sphere. The Earth is drawn at the centre of the celestial sphere, the point N representing the north pole and S the south pole of the Earth. The points on the celestial sphere in the same straight line as the poles of the Earth are the celestial poles marked CN and CS. The celestial sphere appears to be in rotation about fixed pivots at the points CN and CS. (These are the points in the sky at the centre of the two polar maps at the end of the booklet.) The equator of the Earth and the celestial equator are also shown in the diagram. Now suppose we are situated. at a point P on the Earth, and consider how the sky will appear to move. The point straight above our heads, the zenith, is marked Z on the diagram. We can see only that part of the sky above our horizon which is represented by shading on the diagram. Expressed in another way, we see the part of the sky within 90° of our zenith. Now consider the movement of the stars as the celestial sphere rotates about CN and CS. If we are in the southern hemisphere the point CS will be above the horizon but CN will not. If there were a star at CS it would, of course, appear to be quite still and stars near this point appear to move in circles round it without ever setting. Further from the pole the stars rise in the eastern half of the horizonwhen they come within 90° of the zenith and set in the western half of the horizon. Such a star is represented by X on the diagram, and its path is shown by a full line above the horizon and a broken line below it. Stars situated near the pole below the horizon will never be seen. The apparent movement of the sky is due to the rotation of the Earth of which we are not directly conscious, so that it is the sky [*6] that appears to move while we remain still. By thinking in terms of our zenith we can concentrate on only one point in the sky and see what happens when we change our location on the Earth. When we are at the equator our zenith is on the celestial equator and the poles are on the horizon. If we are situated at a pole, the corresponding celestial pole will be overhead and all of the stars will merely rotate round us parallel with the horizon and when the sun comes into the half of the sky within 90° of our zenith it will be day continuously. As we go from 0° latitude to 90° latitude the height of the pole above the horizon changes from 0° to 90°. In fact, the elevation of the celestial pole above the horizon is equal to the latitude.


The stars, as was mentioned in the previous section, appear fixed on the celestial sphere, but the Sun, Moon and planets move among the stars and it is easy to see the change of position after the lapse of a few nights. The Sun follows a path across the celestial sphere, called the ecliptic. (The ecliptic is drawn and marked on the star maps at the end of the book and the celestial equator is the line marked 0° running through the centre of the maps.) The Sun moves among the stars in a direction from west to east and so each day comes across the sky a little further east relative to them. It takes a year to perform its journey right round the celestial sphere and return to the same place. The planets move in a rather complex manner among the stars but always keep in the neighbourhood of the ecliptic and within the zodiac, which is the name given to a zone around the sky extending a little on either side of the ecliptic.

The Moon, too, moves in a direction from west to east among the stars and remains in the zodiac. Its motion can be easily seen by noting its position relative to the stars on one evening and observing its position on the next night. It will be appreciably further east. It takes 27½ days to complete its journey round the celestial sphere.


It is necessary to have some permanent way of specifying the position of any star, and since the celestial sphere is in motion (as described in the previous sections) it is easiest to define the position in terms of a system of circles fixed on the sphere itself. Position on the Earth is measured by latitude and longitude. The latitude of a place is its distance from the equator, and in the same way one of the measures defining position in the sky is distance from the celestial equator. The name given to this measure in the sky is declination. If the Earth is thought of as a globe and a series of circles is drawn from pole to pole, then the longitude of a place is the angle, measured where they cross at the pole, between a certain [*7,8] fixed circle and the circle through the place whose position we wish to define. The fixed circle from which the measurement of longitude begins on the Earth is the one through Greenwich Observatory, near London. Similar circles drawn from pole to pole of the celestial sphere serve to define a measure of star position. This measure of star position corresponding with longitude on the Earth is called right ascension. For right ascension the starting point is the point in the sky occupied by the Sun at the March equinox There is no star at this place (called the first point of Aries because it was formerly in that constellation), which is the central point of Map II at the end of the book. If we regard Figure B as a drawing of the Earth, the lines running from the north pole N to the south pole are the meridians of longitude. On the Earths surface the meridian through a place is the line running due north and south through it. On the diagram G represents Greenwich and P the place whose position we wish to define the line AEF is the equator. Then the latitude of the place is the distance PE and the longitude the angle GNP.

Now imagine the same drawing to represent the celestial sphere G now represents the point in the sky straight over Greenwich and P the zenith of the place defined in the last paragraph. Since the celestial sphere is in rotation, these points are constantly changing their position on the celestial sphere. AEF is the celestial equator, which is exactly overhead to observers on the Earths equator, and N the north celestial pole, which is in the zenith at the Earths north pole. The point A is the first point of Aries and X is the position of a star. Then the right ascension of X is the angle ANX, and its declination is FX. If the star is north of the equator its declination is north and is given the sign plus. If it is south, its sign is minus. The circle throughthe celestial poles which passes through the zenith of a place is overhead everywhere along the meridian of the place and, in fact, is also called the meridian. The line NPE on the celestial sphere is the meridian of P. The celestial sphere is in rotation relative to the meridian of a place, so that the line NPE must be regarded as fixed and all of the stars on the celestial sphere will cross our meridian each day. Now consider the celestial sphere at a given instant with the meridian NPE on it. Then from our definition. of right ascension all the points on the line NPE will have the same right ascension equal to the angle ANP, that is, all points on the meridian at a given time have the same right ascension. This gives a convenient way of measuring right ascension. Suppose we have a clock set to show 0 hours every time the circle of zero right ascension crosses its meridian. Then the right ascension of any particular star will be proportional to the time shown by our clock when the star crosses our meridian. If the dial of the clock is divided into 24 hours, it is convenient to refer to the right ascension of the star in hours and minutes. A star of right ascension 10 hours will then be [*9] on the meridian when the clock indicates 10 hours. The time shown by such a clock is called sidereal time (star time). The position of a star on the celestial sphere, then, is measured in so many degrees of north or south declination and so many hours, minutes and seconds of right ascension. For example, the position of the star marked — in Crux (the Southern Cross) on the south polar map is R.A. 12h. 21m., Dec. 6½° S.


The most natural measure of time for ordinary use, the day; isprovided by the movement of the Sun. Each day the Sun reaching its highest point when it is on the meridian (due north when we are south of the tropics). Unfortunately the motion of the Sun in the sky is such that intervals of time between succeeding passages across the meridian are not always exactly equal, so that for convenience the average length is taken, for we could not have clocks going at different rates at different times of the year: This period is a mean day, and is divided into 24 equal parts called hours, and these hours into minutes and seconds. This kind of time is called mean solar time or more shortly mean time. So we have two kinds of time, mean solar time, which is necessary since most human activity is governed by the position of the sun, and sidereal time, which is convenient to astronomers; and also to navigators, or a description of star positions. Just as the Sun passes over the meridian at approximately the same mean time every day, stars will pass the meridian at the same sidereal time every day. There is a simple relation between the two. The Sun, as described in a previous section, is moving eastwards round the celestial sphere, so that each day as it crosses the meridian the stars are a little further west than on the previous day, that is, as far as apparent daily rotation is concerned the stars appear to be gaining on the Sun. Since the Sun makes a complete circuit once in a year, the celestial sphere will appear to gain one whole revolution in a year, and as the Sun crosses the meridian an average of 365¼ times each year the stars rotate 366¾ times, so that the sidereal clock, which has to fit 366¼ days in a year, gains about four minutes a day on the mean time clock, which has to fit only 365½ days in a year.


Remembering that the celestial sphere and all the objects on it appear to move from east to west, it is obvious that the stars, Sun and Moon will all cross our meridian earlier if we shift our position on the Earth further east. If the mean time at any place is governed by the meridian transit of the Sun, the clocks at a certain place, say Sydney, will show a later time at a particular instant: than the clocks at a place further west, such as Melbourne. In fact, Sydney clocks would be 25 minutes ahead of Melbourne [*10] clocks. For many purposes, such as travel and communication, this was found to be inconvenient, so that New South Wales, Victoria, Queensland and Tasmania agreed to set their clocks at the mean time of the 150th meridian, that is exactly ten hours ahead of the mean time at Greenwich in England. The same method of dividing countries up into time zones has been adopted all over the world. In some countries, including Australia at present, the difference from the Greenwich mean time is different at different times in the year, and during the summer eastern Australia keeps time 11 hours ahead of Greenwich mean time, which is sometimes called universal time by astronomers. In England in the summer at present the time kept for ordinary civil purposes is 2 hours ahead of Greenwich mean time. In Table V, at the end of the booklet, are given the differences from Greenwich mean time of the standard times in various countries. For example, New York time is 5 hours behind Greenwich mean time, while in Java the time kept is 7½ hours ahead of Greenwich mean time, so that if it were 10 a.m. in New York it would be 10.30 p.m. in Java. South Australia, the Northern Territory and the Broken Hill area of New South Wales keep time half an hour, and Western Australia 2 hours behind eastern Australian time. Thus if it is 8 a.m. in Perth it is 9.30 a.m. in Central Australia and 10 a.m. in the eastern states. Time is an important factor in all civil and military work, including navigation, and it is best obtained by correcting a clock (or finding and allowing for its error) by time signals from a fixed observatory. In Australia, and most parts of the British Empire, a six dot signal is given by a number of broadcasting stations. This consists of six dots, given at second intervals (preceded sometimes by warning signals at ten-second intervals). The last dot falls exactly on the hour. Signals intended for use by navigators are relayed from four Australian observatories on wavelength 600 metres at certain hours during the day.


Last Update : 13th August 2012

Southern Astronomical Delights © (2012)

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