By
: A E Roy and D Clarce
From
The Book “ Astronomy Principles and Practice”
1.1
Introduction
The
etymology of the word ‘Astronomy’ implies that it was the
discipline involved in ‘the arranging of the stars’. Today we
might say that astronomy is our attempt to study and understand
celestial phenomena, part of the never-ending urge to discover order
in nature. We do not know who were the first astronomers—what we do
know is that the science of astronomy was well advanced in parts of
Europe by the middle of the third millennium BC and that the Chinese
people had astronomical schools as early as 2000 BC. In all ages,
from the burgeoning of man’s intelligence, there have been people
fascinated by the heavens and their changing aspect and these people,
as far as their cultural environment has allowed them, have tried to
formulate cosmologies. We are no different today.
Nowadays,
the word ‘Astrophysics’ is also used to describe the study of the
celestial bodies. In fact, many astronomers use both terms quite
generally and it is not infrequent to find Departments of Astronomy
and Astrophysics within educational establishments. The question may
well be asked ‘What is the difference between Astronomy and
Astrophysics?’ Very loosely, Astronomy might be defined as the
subject of the ‘where and when’ related to the description of a
celestial body with th ‘why and how’ being covered more by
Astrophysics. Rather than trying to provide a hard and fast rule for
the terminology, we will simply use Astronomy to cover all aspects of
the description of the skies and the Universe.
If
our current theories of the Universe are nearer the truth, it is
probably not that our intelligence has increased in the past six
millennia. It is more likely that the main factor has been the
discovery and development of the ‘scientific method’, which has
led to our present civilization based on the flood of technological
advantages provided by this method. This has enabled scientists in
far greater numbers than ever before to devote their lives to the
study of the heavens, assisted by telescopes, computers, space
vehicles and a multitude of other equipment. Their attempts to
interpret and understand the wealth of new information provided by
these new instruments have been aided by allied sciences such as
physics, chemistry, geology, mathematics and so on.
We
must remember, however, that for more than nine-tenths of the last
five thousand years ofour study of the heavens, we have had to rely
on the unaided eye. The Mediterranean people whoset the
constellations in the sky, the Babylonians, Egyptians and Greeks, the
Arabian astronomers who flourished during the Dark Ages of Post-Roman
Europe, the Chinese, the Mayan and other early American astronomers,
all built their theories of the Universe on naked eye observations.
And so we begin by following in their footsteps and seeing what they
saw as they observed over a few minutes (see section 1.2), over a few
hours (see section 1.3), over a month (see section 1.4) or over at
least a year (see section 1.5). In this way, we will find it easier
to understand why their cosmological theories were formulated in
their particular ways.
1.2
Instantaneous phenomena
1.2.1
Day
During
the day a variety of phenomena may be seen. In a particular direction
lies the Sun, so bright it is impossible (and dangerous) to look
directly at it. In general, the sky background is blue. The Moon may
also be visible, having a distinct shape though certainly not
circular. If the Sun has just set or if dawn is not far away, there
is sufficient daylight to see clearly. We call this condition
twilight.
On
the horizon opposite to the twilight glow, a dark purple band is
sometimes seen. This area corresponds to a zone on the sky which is
cut off from the direct sunlight by the Earth and is receiving very
little light by scattering from the atoms and molecules in the
atmosphere. It corresponds, in fact, to the shadow of the Earth in
the sky. Its presence tells us of the extreme purity and low humidity
of the local atmosphere. Needless to say, it is very rarely seen in
Britain.
To
the ancients, clouds, wind, rain, hail and other atmospheric
phenomena were inadequately distinguished from what we term celestial
events. Our civilization includes them in meteorology, a science
quite distinct from astronomy, so that we need not consider them
further, except to remark that astronomers’ observations have,
until recently, been dependent entirely upon good weather conditions
being available. With the development of radio telescopes and the
fact that other equipment can be placed in artificial satellites and
operated above the Earth’s atmosphere, this dependence is no longer
complete.
1.2.2
Night
If
seeing conditions are favourable, a view of the night sky provides a
far wider variety of celestial phenomena. If the Moon is visible, its
brightness will dominate that of all other objects. Its shape will be
crescent or gibbous or even circular. At the last condition, its
apparent diameter is very close to that of the Sun. To anyone with
reasonable eyesight, its surface will not be evenly bright. Areas
darker than their surroundings will be noticed, so that the fancy of
primitive man could see a ‘Man in the Moon’, a ‘Beautiful Lady’
or a ‘Rabbit’, sketched out by these features.
In
addition to the Moon, some two to three thousand tiny, twinkling
points of light—the stars—are seen, ranging in brightness from
ones easily visible just after sunset to ones just visible when the
Moon is below the horizon and the sky background is darkest. Careful
comparison of one bright star with another shows that stars have
different colours; for example, in the star pattern of Orion, one of
the many constellations, Betelgeuse is a red star in contrast to the
blue of Rigel. The apparent distribution of stars across the vault of
heaven seems random.
With
the eyes becoming accustomed to the darkness, a faint band of light,
the MilkyWay, catches the observer’s attention. Modern astronomers,
with the aid of telescopes, know that this luminous region stretching
from horizon to horizon across the sky in a great circle is made up
of a myriad of stars too faint to be resolved with the naked eye. To
the ancient observer, its presence inspired all kinds of
speculations, none of them verifiable.
One
or two of the tiny points of light may draw a closer scrutiny. They
shine steadily, in contrast to the twinkling of the stars and they
are among the brightest of the star-like objects. There must be some
reason why they are different. If our observer is going to watch for
a few hours, attention will be returned to these objects.
1.3
A few hours
1.3.1
Day
The
heavens are never static. The slowly-moving shadow cast by an upright
rod or a boulder or tree reveals the Sun’s movement across the sky.
If observation is kept up throughout the day, the Sun is seen to rise
above the eastern horizon, climb up the sky in a circle inclined at
some angle to the plane defined by the horizon and culminate, i.e.
reach a maximum altitude above the line joining the north to the
south points, then descend in a mirror image of its forenoon path to
set on the western horizon. If the Moon is seen during that day, it
will appear to imitate the Sun’s behaviour in rising and setting.
1.3.2
Night
As
darkness falls, the first stars become visible above the eastern
horizon. With the ending of twilight the fainter stars can be seen
and, as the hours pass, the stellar groups rise from the eastern
horizon, reach their maximum altitude like the Sun, then set or
become dim and invisible as daylight returns. The impression of being
on a flat plane surmounted by a dark revolving bowl to which the
stars are attached is strong, especially when it is seen that there
are many stars in a particular region of the sky that revolve, never
rising, never setting, about a hub or pivot. These stars are said to
be circumpolar. It is then clear that those other stars that rise and
set do so simply because their circular paths about this pole are so
big that they intersect the horizon.
The
Moon also revolves across this upturned bowl. Although theMoon
appears to have an angular motion across the sky similar to that of
the stars, careful observation over a few hours reveals that it moves
slightly eastwards relative to the star background.
Occasionally
a bright object, called a meteor, shoots across the sky in a second,
looking like a fast-moving or ‘falling star’. It may be too that
faintly luminous sheets are seen, hanging down the bowl of the
heavens like great curtains. These are the aurorae (W 1.1).
If
our observer is watching at any time after October 4, 1957, it is
quite likely that one or more faint specks of light will be seen to
cross the sky, taking a few minutes to do so, their presence giving
reminder that man-made satellites are now in orbit about the Earth.
Indeed, one of the latest satellites— the International Space
Station (W 1.2) —is exceedingly bright—as bright as the brightest
planet Venus— and bears testament to the continual development of
manned orbiting laboratories.
1.4
A month
The
month is the next period of any significance to our watcher. During
this time, the ideas about the heavens and their movements change. It
will be noted that after a few nights the first group of stars seen
above the eastern horizon just after sunset is markedly higher at
first sight, with other groups under it becoming the first stars to
appear. Indeed, after a month, the first group is about thirty
degrees above the eastern horizon when the first stars are seen after
sunset. It is then apparent that the Sun must shift its position
against the stellar background as time passes. The rate is slow
(about one degree per day—or about two apparent solar diameters)
compared with its daily, or diurnal, movement about the Earth.
The
Sun is not the only object to move independently of the stellar
patterns. A few nights’ observations of the Moon’s position
against the stars (its sidereal position) show that it too moves but
at a much faster rate, about thirteen degrees per day, so that it is
seen to make one complete revolution of the stellar background in
twenty-seven and one-third days, returning to the same constellation
it occupied at the beginning of the month. In addition, its shape
changes. From a thin crescent, like a reversed ‘C’, seen in the
west just after sunset, it progresses to the phase we call first
quarter about seven days later. At this phase, the Moon’s
terminator is seen to be almost a straight line. Fourteen days after
new moon, it is full and at its brightest, appearing at its highest
in the sky about midnight. Seven days later it has dwindled to third
quarter and rises before the Sun, a pale thin crescent once more, a
mirror image of its phase just after new moon. Twenty-nine and
one-half days after new moon, it is new once more.
It
was a fairly easy matter for the ancients to ascertain that theMoon
was nearer the Earth than the stars. Frequently the Moon was seen to
blot out a star, occulting it until it reappeared at the other edge
of the Moon’s disc. And occasionally the Moon was eclipsed, the
Earth progressively blocking off the sunlight until the satellite’s
brightness had diminished to a dull, coppery hue. An even more
alarming, but rarer, occurrence took place at times during daylight:
the Moon revealed its unseen presence near the Sun by eclipsing the
solar disc, turning day into night, causing birds to seek their nests
and creating superstitious fear in the mind of primitive man.
The
observer who studies the night sky for a month or so also discovers
something new about the one or two star-like objects noted that do
not twinkle. Careful marking of their positions with respect to
neighbouring stars shows that they too are moving against the stellar
background. There does not seem to be much system, however, about
these movements. In the course of a month, one may move in the
direction the Moon travels in, while a second object, in another part
of the sky, may move in the opposite direction. Indeed, towards the
end of this month’s observing sessions, either object may cease to
move, seem almost to change its mind and begin to retrace its steps
on the celestial sphere. These wanderers, or planets (‘planet’ is
a Greek word meaning ‘wanderer’), are obviously of a different
nature from that of the fixed, twinkling stars.
1.5
A year
A
year’s patient observing, by day and night, provides the watcher
with new concepts. For example, the Sun’s daily behaviour, moving
easterly bit by bit, is linked to the seasonal changes.
Each
day, for most observers, the Sun rises, increases altitude until it
culminates on the meridian at apparent noon, then falls down the sky
until it sets on the western horizon. We have seen that this progress
can be studied by noting the changes in direction and length of the
shadow cast by a vertical rod stuck in the ground (see figure 1.1).
As
the days pass, the minimum daily length of shadow (at apparent noon)
is seen to change, becoming longest during winter and shortest during
summer. This behaviour is also linked with changes in the rising and
setting directions of the Sun. Six months after the Sun has risen
between north and east and setting between north and west, it is
rising between south and east and setting between south and west.
Another six months has to pass before the solar cycle is completed,
with the Sun once more rising between north and east and setting
between north and west.
All
this could be explained by supposing that the Sun not only revolved
with the stars on the celestial sphere about the Earth in one day
(its diurnal movement) but that it also moved much more slowly along
the path among the stars on the celestial sphere, making one
revolution in one year, returning to its original position with
respect to the stars in that period of time. We have already seen
that the observer who notes over a month what group of stars is first
visible above the eastern horizon after sunset will have already come
to the conclusion that the Sun moves relative to the stars. Now it is
seen that there is a regular secular progression right round the
stellar background and that when the Sun has returned to its original
stellar position, the seasonal cycle is also completed.
The
Sun’s stellar route was called the ecliptic by the ancients. The
groups of stars intersected by this path were called the houses of
the Zodiac. The ecliptic is found to be a great circle inclined at
about 231/2 degrees to the equator, the great circle on the sky
corresponding to the projection of the Earth’s equator,
intersecting it at two points, the vernal and autumnal equinoxes, 180
degrees apart.
It
was quite natural, then, for the ancients to worship the Sun. Not
only did it provide light and warmth by day against the evils of the
night but, in addition, its yearly progression was intimately linked
to the seasons and so also to seed time and harvest. It was,
therefore, necessary to keep track of progress to use it as a clock
and a calendar. To this end, the science of sundial-making began,
ramifying from simple obelisks that throw shadows on a fan of lines
radiating from their bases, to extremely ingenious and complicated
erections in stone and metal. Up to the 19th century, these
constructions rivalled most pocket-watches in accuracy as
timekeepers.
For
calendrical purposes, lines of standing stones could be set up,
pointing to the midsummer, midwinter and equinoctial rising and
setting points of the Sun. In the British Isles, there still remain
hundreds of such solar observatories, witnesses to our forefathers’
preoccupation with the Sun-god.
The
observer who watches the night sky throughout a year counts about
thirteen revolutions of the stellar background by theMoon in that
time. Over that period of time, it is not apparent that any simple
relationship exists between the sidereal period of revolution of the
Moon, the period of its phases and the year (the time it takes the
Sun to perform one complete circuit of the ecliptic). That knowledge
comes after much more extended observation, certainly measured in
decades.
It
would be noticed, however, that the Moon’s sidereal path is very
little inclined to the ecliptic (about five degrees) and if records
were kept of the points of the ecliptic crossed by the Moon, it might
be realized that these points were slipping westwards at a rate of
about twenty degrees per year (see figure 1.2).
More
information, too, would be acquired about the star-like objects that
do not twinkle and which have been found in the course of a month to
have a slow movement with respect to the stellar background. These
planets, like the Moon, would never be seen more than a few degrees
from the plane of the ecliptic, yet month after month they would
journey through constellation after constellation. In the case of one
or two, their pathswould include narrowloops, though only one loop
would be observed for each of these planets in the course of the
year.
The
year’s observations would not add much to the observer’s
knowledge of the stars, except to confirm that their positions and
brightnesses relative to each other did not alter and that each star,
unlike the Sun, had its own fixed rising and setting direction,
unless it was circumpolar. It is possible, however, that in a year,
the extra-careful watcher might have cause to wonder if the
conclusions about stars were without exception for, by regular
comparison of the brightness of one star with respect to that of
neighbouring ones, it might be discovered that a few stars were
variable in brightness. This was certainly known to the Arabian
astronomers of the Middle Ages. The appearance of a nova might even
be observed, i.e. a star appearing in a position where one had not
been previously noted. This occurrence might well lead to doubt about
the knowledge of the now familiar constellations—in any event it
could bring about the decision to make a star map for future use if
the phenomenon happened again. It is also possible that in the course
of a year the observer might see a comet, a star-like object with a
long luminous tail. The development of the tail and the movement of
the comet head could be detected from night to night.
Our
observer by now must have come to tentative conclusions concerning
the heavenly phenomena studied and noted. The interpretations, and
the use made of the world-picture, will be constrained by the culture
of the time. A man of Neolithic times and a Greek of Athens’ golden
era would develop entirely different cosmologies from identical
observations. And a hunter or farmer has different needs,
astronomically speaking, from a sailor.
Selanjutnya>>
Selanjutnya>>
Postingan
Tidak ada komentar:
Posting Komentar