Astronomy - Department of Physics

469 Pages·2006·5.9 MB·English

Chapter 1
Naked eye observations
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 the
‘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 of
our study of the heavens, we have had to rely on the unaided eye. The Mediterranean people who
set 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.
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4 Naked eye observations
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 Milky Way, 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
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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 the Moon 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
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bowl of the heavens like great curtains. These are the aurorae .
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—
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the International Space Station —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 the Moon 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
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Figure 1.1. The change in length of a shadow according to the time of day and the time of year.
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
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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
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at about 23 degrees to the equator, the great circle on the sky corresponding to the projection of the
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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 the Moon 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 paths would include narrow loops, 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
8 Naked eye observations
◦
Figure 1.2. The Moon’s sidereal path crosses the ecliptic twice each month at an angle of about 5 . For successive
◦
lunations the crossing points move westward, covering about 20 over a year. The Constellation of Leo is shown
to give an indication of the scale of the movement.
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.
Chapter 2
Ancient world models
First theories were necessarily simple. The Earth was a flat plane with rivers, hills, seas and land, fixed,
eternal. The heavenly bodies revolved, passing from east to west. But if the land continued indefinitely,
how could the Sun that set in the west be the same Sun that rose in the east the next morning? Perhaps,
the Babylonians reasoned, the Earth was flat but finite with a circle of ocean beyond which a ring of
mountains supported the heavens, the firmament. Then, if doors were provided in the base of this great
solid half-sphere on the eastern and western sides, the celestial bodies would be able to slip through the
western doors on setting and be transported in some miraculous way to the east to reappear as ordained.
The Babylonians were skilled astronomers though their world-picture was na¨ıve. They observed
the positions of the Sun, Moon, planets and stars for many centuries with great accuracy. They
found that they could predict eclipses. Their observations were motivated by their belief that the
future of human beings could be predicted from celestial configurations and events such as eclipses
or the appearance of comets. Because of this, kings kept court astrologers and the wealthy paid for
horoscopes. This belief in astrology, found in all nations, should have withered away with alchemy
and the search for the philosopher’s stone but even today there are many who set great faith in this
pseudo-science. It is perhaps needless to say that modern astronomy demonstrates how ludicrous such
beliefs are.
The Egyptians, astronomers almost as skilled as the Babylonians, had equally simple world-
pictures. They noticed that the yearly inundation of the Nile valley coincided with the days when
the star Sirius could be seen best in the morning twilight. This linking of celestial and earthly
events spurred on their development of astrology and brought religion into the picture. The Sun-god
descended at night, passing beneath the Earth to visit the dead.
Farming people were more interested in the solar cycle since it was linked with seed time and
harvest. Seafaring peoples like the Phoenicians and the Minoans used the rising and setting directions
of the stars as navigational aids. It may well have been as an aid to memory that the stars were grouped
in constellations, embodying myths current at that time.
As is to be expected, the ancient Chinese civilizations produced schools of astronomy and
cosmological theories. Serious Chinese astronomy probably began prior to 2000 BC although details
of events in that era are largely legendary. The story of the two Chinese astronomers, Ho and Hi,
executed for failing to predict an eclipse of the Sun in 2137 BC is possibly apocryphal and may refer
to two astronomical colleges of a much later date destroyed in civil strife. Reliable historical details
begin about 1000 BC. A farming people required a calendar and so the lengths of month and year were
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quickly ascertained. A year of 365 days was certainly used by 350 BC.
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By that date, the Chinese constellation figures, 122 in number and quite different from those
handed down to us by the Greeks, had been mapped out, the Sun’s path—the ecliptic—being divided
into 12 regions. The size of a region was not only connected with the heavenly arc inhabited by the
Sun each month but also with the yearly journey of the planet Jupiter. The other planetary motions
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