SOUTHERN ASTRONOMERS and AUSTRALIAN ASTRONOMY
ELEMENTARY ASTRONOMY FOR SERVICE USE PART 1
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.
THE CELESTIAL SPHERE
[*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 SUN, MOON AND PLANETS
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.
STAR POSITIONS AND SIDEREAL TIME
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 Earth’s 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 Earth’s equator, and N the north celestial pole,
which is in the zenith at the Earth’s
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.
TIME
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.
STANDARD TIME
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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