ASTRONOMY HANDOUTS
1. PRIMITIVE ASTRONOMY AND ASTROLOGY.
Newton’s great law of universal gravitation:
-Concluded that the inclination of the earth’s equator to the plane of her orbit (the obliquity
of the ecliptic) has been diminishing slowly since prehistoric times.
Reasons why we must be tolerant of the crude notions of the ancients
1. Only a few records of very ancient astronomy are extant, and the authenticity of many of
these is open to doubt.
2. It is very difficult to divest ourselves of present knowledge, and to appreciate the originality
of thought required to make the first beginnings.
Heliacal rising
-Star rise of a star occurs annually, or the similar phenomenon of a planet, when it first becomes
visible above the eastern horizon at dawn just before sunrise after a complete orbit of the
earth around the sun.
The theoretical prediction of eclipses of the sun and moon, and of the motions of the planets
among the stars, became later the highest goal in astronomy.
In conclusion, let us bear in mind the limited point of view of the ancients when we try to estimate
their merit. Let us remember that the first astronomy was of two dimensions; the second
astronomy was of three dimensions, but still purely geometrical. Since Kepler’s day we have
had a dynamical astronomy
2. ANCIENT ASTRONOMY—THE CHINESE AND CHALDÆANS.
Bailly (Astronomie Ancienne (1781)
-Drew, from these and other sources, the conclusion that all we know of the astronomical learning
of the Chinese, Indians, Chaldæans, Assyrians, and Egyptians is but the remnant of a far more
complete astronomy of which no trace can be found.
Delambre (Histoire de l’Astronomie Ancienne (1817)
-Ridicules the opinion of Bailly and considers that the progress made by all of these nations is
insignificant.
Yu-Chi made a sphere to represent the motions of the celestial bodies. It is also mentioned, in
the book called Chu-King. It is said that the Emperor Chueni (2513 B.C.) saw five planets in
conjunction the same day that the sun and moon were in conjunction. This is discussed by Father
Martin (MSS. of De Lisle); also by M. Desvignolles (Mem. Acad. Berlin, vol. iii., p. 193), and by M.
Kirsch (ditto, vol. v., p. 19), who both found that Mars, Jupiter, Saturn, and Mercury were all
between the eleventh and eighteenth degrees of Pisces, all visible together in the evening on
February 28th 2446 B.C., while on the same day the sun and moon were in conjunction at 9 a.m.,
and that on March 1st the moon was in conjunction with the other four planets.
Yao
-He gave them further orders. If this account be true, it shows a knowledge that the vault of heaven
is a complete sphere, and that stars are shining at mid-day, although eclipsed by the sun’s
brightness.
3. ANCIENT GREEK ASTRONOMY
Greek astronomy from Herodotus (born 480 B.C.)
-He supposed the earth to be flat, and to float upon water. He determined the ratio of the sun’s
diameter to its orbit, and apparently made out the diameter correctly as half a degree.
-His successors, Anaximander (610-547 B.C.) and Anaximenes (550-475 B.C.), held absurd
notions about the sun, moon, and stars, while Heraclitus (540-500 B.C.) supposed that the stars
were lighted each night like lamps, and the sun each morning. Parmenides supposed the earth
to be a sphere.
Anaxagoras (born 499 B.C.)
-Studied astronomy in Egypt. He explained the return of the sun to the east each morning by its
going under the flat earth in the night. He held that in a solar eclipse the moon hides the sun, and
in a lunar eclipse the moon enters the earth’s shadow—both excellent opinions.
They seem to have been in great difficulty to explain how the earth is supported, just as were
those who invented the myth of Atlas, or the Indians with the tortoise. Thales thought that
the flat earth floated on water. Anaxagoras thought that being flat, it would be buoyed up and
supported on the air like a kite. Democritus thought it remained fixed, like the donkey between
two bundles of hay, because it was equidistant from all parts of the containing sphere, and there
was no reason why it should incline one way rather than another. Empedocles attributed its state
of rest to centrifugal force by the rapid circular movement of the heavens, as water is stationary
in a pail when whirled round by a string. Democritus further supposed that the inclination of the
flat earth to the ecliptic was due to the greater weight of the southern parts owing to the exuberant
vegetation.
Aristarchus (320-250 B.C.)
-Showed that the sun must be at least nineteen times as far off as the moon, which is far short of
the mark. He also found the sun’s diameter, correctly, to be half a degree.
Eratosthenes (276-196 B.C.)
-Measured the inclination to the equator of the sun’s apparent path in the heavens—i.e., he
measured the obliquity of the ecliptic, making it 23° 51’, confirming our knowledge of its
continuous diminution during historical times. He measured an arc of meridian, from Alexandria
to Syene (Assuan), and found the difference of latitude by the length of a shadow at noon, summer
solstice. He deduced the diameter of the earth, 250,000 stadia.
Hipparchus (190-120 B.C.)
He measured the obliquity of the ecliptic and agreed with Eratosthenes. He altered the length of
the tropical year from 365 days, 6 hours to 365 days, 5 hours, 53 minutes—still four minutes too
much. He measured the equation of time and the irregular motion of the sun; and allowed for this
in his calculations by supposing that the center, about which the sun moves uniformly, is situated
a little distance from the fixed earth. He called this point the excentric. The line from the earth to
the “excentric” was called the line of apses. A circle having this centre was called the equant,
and he supposed that a radius drawn to the sun from the excentric passes over equal arcs
on the equant in equal times. He then computed tables for predicting the place of the sun.
In the year 134 B.C. Hipparchus observed a new star. This upset every notion about the
permanence of the fixed stars. He then set to work to catalogue all the principal stars so as to
know if any others appeared or disappeared.
Hipparchus was also the inventor of trigonometry, both plane and spherical. He explained the
method of using eclipses for determining the longitude.
Ptolemy (130 A.D.)
He theorized on the planetary motions and held that the earth is fixed in the centre of the universe.
The positions of the stars indicated the time to plough, and the time to sow.
The acuteness of the early observers enabled them to single out the more important of the
wanderers which we now call planets,
They saw that the star-like objects, Jupiter, Saturn, and Mars, with the more conspicuous Venus,
constituted a class of bodies wholly distinct from the fixed stars among which their movements
lay, and to which they bear such a superficial resemblance.
It would seem that eclipses and other phenomena were observed at Babylon from a very remote
period, while the most ancient records of celestial observations that we possess are to be found
in the Chinese annals.
The study of astronomy, in the sense in which we understand the word, may be said to have
commenced under the reign of the Ptolemies at Alexandria. The most famous name in the science
of this period is that of Hipparchus who lived and worked at Rhodes about the year 160BC.
Hipparchus
-Was a Greek astronomer, geographer, and mathematician regarded as the greatest astronomer
of antiquity and one of the greatest of all time. He is best known for his discovery of the precession
of the equinoxes and contributed significantly to the field of astronomy on every level.
1. Commenced by undertaking, on a small scale, a task exactly similar to that on which modern
astronomers, with all available appliances of meridian circles, and photographic telescopes, are
constantly engaged at the present day.
2. He compiled a catalogue of the principal fixed stars, which is of special value to astronomers,
as being the earliest work of its kind, which has been handed down.
3. He also studied the movements of the sun and the moon, and framed theories to account for
the incessant changes which he saw in progress.
4. He found a much more difficult problem in his attempt to interpret satisfactorily the complicated
movements of the planets.
Hipparchus possessed one of the masterminds of all time was the detection of that remarkable
celestial movement known as the precession of the equinoxes.
The word equinox implies the condition that the night is equal to the day. When the night and day
are equal in spring, the point which the sun occupies on the heavens is termed the vernal
equinox. There is similarly another point in which the sun is situated at the time of the autumnal
equinox.
Hipparchus
He was led to the conclusion that each equinox was moving relatively to the stars, though that
movement was so slow that twenty-five thousand years would necessarily elapse before a
complete circuit of the heavens was accomplished.
Ptolemy
-Ptolemy synthesized Greek knowledge of the known Universe. His work enabled astronomers
to make accurate predictions of planetary positions and solar and lunar eclipses, promoting
acceptance of his view of the cosmos in the Byzantine and Islamic worlds and throughout Europe
for more than 1400 years.
-The doctrines he laid down in his famous book, "The Almagest," prevailed throughout those ages.
No substantial addition was made in all that time to the undoubted truths which this work
contained. No important correction was made of the serious errors with which Ptolemy's theories
were contaminated. The authority of Ptolemy as to all things in the heavens, and as to a good
many things on the earth (for the same illustrious man was also a diligent geographer), was
invariably final.
Ptolemy is, without doubt, the greatest figure in ancient astronomy.
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He gathered up the wisdom of the philosophers who had preceded him.
He incorporated this with the results of his own observations and illumined it with his
theories.
His speculations, even when they were, as we now know, quite erroneous, had such an
astonishing verisimilitude to the actual facts of nature that they commanded universal
assent
Ptolemy commences with laying down the undoubted truth that the shape of the earth is globular.
Ptolemy mentions that travelers who went to the south reported, that, as they did so, the
appearance of the heavens at night underwent a gradual change.
Ursa Major is a constellation in the northern sky, whose associated mythology likely dates back
into prehistory. Its Latin name means "greater bear," referring to and contrasting it with nearby
Ursa Minor, the lesser bear.
If the earth were flat, said this ingenious reasoner, sunset must necessarily take place at the same
instant, no matter in what country the observer may happen to be placed. Ptolemy, however,
proved that the time of sunset did vary greatly as the observer's longitude was altered.
Difference between lunar and solar eclipse
When Ptolemy, therefore, demonstrated that the time of sunset was not the same at various
places, he showed conclusively that the earth was not flat.
By this reasoning he arrives at the fundamental conclusion that the earth is a globular body freely
lying-in space, and surrounded above, below, and on all sides by the glittering stars of heaven.
It may, however, be well imagined that, to one who thought the earth was a flat plain of indefinite
extent, it would be nothing less than an intellectual convulsion for him to be forced to believe that
he stood upon a spherical earth, forming merely a particle relatively to the immense sphere of the
heavens.
What Ptolemy saw in the movements of the stars led him to the conclusion that they were bright
points attached to the inside of a tremendous globe. The movements of this globe which carried
the stars were only compatible with the supposition that the earth occupied its centre.
Ptolemy was an excellent geometer. He knew that the rising and the setting of the sun, the moon,
and the myriad stars, could have been accounted for in a different way.
Two totally distinct methods, each of which would completely explain all the observed
facts of the diurnal movement.
1. One of these suppositions requires that the celestial sphere, bearing with it the stars and other
celestial bodies, turns uniformly around an invisible axis, while the earth remains stationary at the
centre.
2. The other supposition would be, that it is the stupendous celestial sphere which remains
stationary, while the earth at the centre rotates about the same axis as the celestial sphere did
before, but in an opposite direction, and with a uniform velocity which would enable it to complete
one turn in twenty-four hours.
Copernicus
-Was born there on the 19th of February 1473.
-Proposed that the planets have the Sun as the fixed point to which their motions are to be
referred; that Earth is a planet which, besides orbiting the Sun annually, also turns once daily on
its own axis; and that very slow long-term changes in the direction of this axis account for
the precession of the equinoxes. This representation of the heavens is usually called
the heliocentric, or “Sun-centred,” system—derived from the Greek helios, meaning
“Sun.” Copernicus’s theory had important consequences for later thinkers of the
Scientific Revolution, including such major figures as Galileo, Kepler, Descartes, and Newton.
Copernicus probably hit upon his main idea sometime between 1508 and 1514, and during those
years he wrote a manuscript usually called the Commentariolus (“Little Commentary”). However,
the book that contains the final version of his theory, De revolutionibus orbium coelestium libri
vi (“Six Books Concerning the Revolutions of the Heavenly Orbs”), did not appear in print until
1543, the year of his death.
Copernican
heliocentrism is
the
astronomical model developed by Nicolaus Copernicus and
published in 1543. This model positioned the Sun at the center
of
the Universe,
motionless,
with Earth and
the
other planets orbiting around it in circular paths, modified
by epicycles, and at uniform speeds. The Copernican model
displaced the geocentric model of Ptolemy that had prevailed
for centuries, which had placed Earth at the center of the Universe.
-Copernicus having established a theory of the celestial movements which deliberately set aside
the stability of the earth.
Tycho Brahe
-(Born December 14, 1546, Knudstrup, Scania, Denmark—died October 24, 1601, Prague),
Danish astronomer whose work in developing astronomical instruments and in measuring and
fixing the positions of stars paved the way for future discoveries. His observations—the most
accurate possible before the invention of the telescope—included a comprehensive study of
the solar system and accurate positions of more than 777 fixed stars.
-The most picturesque figure in the history of astronomy is undoubtedly that of the famous old
Danish astronomer. Notable for his astronomical genius and for the extraordinary vehemence of
a character which was by no means perfect.
Instruments used
1. His first instrument was, indeed, a very primitive one, consisting of a simple pair of compasses,
which he used in this way. He placed his eye at the hinge, and then opened the legs of the
compass so that one leg pointed to one star and the other leg to the other star. The compass was
then brought down to a divided circle, by which means the number of degrees in the apparent
angular distance of the two stars was determined.
2.His next advance in instrumental equipment was to provide himself with the contrivance known
as the "cross-staff," which he used to observe the stars whenever opportunity offered.
GALILEO
-Galileo sparked the birth of modern astronomy with his observations of the Moon, phases of
Venus, moons around Jupiter, sunspots, and the news that seemingly countless individual stars
make up the Milky Way Galaxy.
- Galileo was born at Pisa, on 18th February 1564.
Galileo pendulum
Galileo's contribution was essentially theoretical: as a young man he
noticed that a pendulum swings at a constant rate (at least, almost
constant for small angles). At the end of his life, he devised a scheme for
using a pendulum to regulate a mechanical clock.
What did Galileo do with his telescope?
In 1609, he learned of the spyglass and began to experiment with telescope-making, grinding
and polishing his own lenses. His telescope allowed him to see with a magnification of eight or
nine times, making it possible to see that the Moon had mountains and that Jupiter had satellites
The last of Galileo's great astronomical discoveries related to the libration of the moon. I think that
the detection of this phenomenon shows his acuteness of observation more remarkably than does
any one of his other achievements with the telescope. It is well known that the moon constantly
keeps the same face turned towards the earth.
Galileo produced this extremely famous set of six watercolours of the
Moon in its various phases "from life", as he observed the Earth's satellite
through a telescope in the autumn of 1609. They represent the first
realistic depiction of the Moon in history.
Kepler
was born on the 27th of December, 1571, at Weil, in the Duchy of Wurtemberg.
Johannes Kepler was a German mathematician and astronomer who discovered that the Earth
and planets travel about the sun in elliptical orbits. He gave three fundamental laws of planetary
motion. He also did important work in optics and geometry.
Kepler's Laws of Planetary Motion
Kepler's three laws describe how planetary bodies orbit the Sun. They describe how
(1) planets move in elliptical orbits with the Sun as a focus,
(2) a planet covers the same area of space in the same amount of time no matter where it is in its
orbit, and
(3) a planet’s orbital period is proportional to the size of its orbit (its semi-major axis.
ISAAC NEWTON
Isaac Newton was born on the 25th of December (old style), 1642, at Woolsthorpe, in Lincolnshire,
about a half-mile from Colsterworth, and eight miles south of Grantham.
The earliest of Newton's great achievements in natural philosophy was his detection of the
composite character of light. That a beam of ordinary sunlight is, in fact, a mixture of a very great
number of different-coloured lights, is a doctrine now familiar to everyone who has the slightest
education in physical science. We must, however, remember that this discovery was really a
tremendous advance in knowledge at the time when Newton announced it.
Newton proposed that all objects in the Universe pulled on each other through gravity. It was the
reason why planets move in orbits and why objects fall to the Earth.
We here give the little diagram originally drawn by Newton, to
explain the experiment by which he first learned the composition
of light. A sunbeam is admitted into a darkened room through an
opening, H, in a shutter. This beam when not interfered with will
travel in a straight line to the screen, and there reproduce a bright
spot of the same shape as the hole in the shutter. If, however, a
prism of glass, A B C, be introduced so that the beam traverse it,
then it will be seen at once that the light is deflected from its original
track. There is, however, a further and most important change
which takes place. The spot of light is not alone removed to
another part of the screen, but it becomes spread out into a long
band beautifully coloured, and exhibiting the hues of the rainbow.
At the top are the violet rays, and then in descending order we
have the indigo, blue, green, yellow, orange, and red.
Not only did Newton decompose a white beam into its constituent
colours, but conversely by interposing a second prism with its
angle turned upwards, he reunited the different colours, and thus
reproduced the original beam of white light. In several other ways
also he illustrated his famous proposition, which then seemed so
startling, that white light was the result of a mixture of all hues of
the rainbow. By combining painters' colours in the right proportion
he did not indeed succeed in producing a mixture which would
ordinarily be called white, but he obtained a grey pigment. Some
of this he put on the floor of his room for comparison with a piece
of white paper. He allowed a beam of bright sunlight to fall upon
the paper and the mixed colours side by side, and a friend he
called in for his opinion pronounced that under these
circumstances the mixed colours looked the whiter of the two.
Isaac Newton built his reflecting telescope as a proof for his
theory that white light is composed of a spectrum of
colours.[5] He had concluded that the lens of any refracting
telescope would suffer from the dispersion of light into
colours (chromatic aberration).
The Newtonian telescope, also called the Newtonian
reflector or just the Newtonian, is a type of reflecting
telescope invented by the English scientist Sir Isaac Newton
(1642–1727), using a concave primary mirror and a flat
diagonal secondary mirror.