2. Historical Perspective
Astronomical timeline can be dated back to 2000 B.C. when
ancient people became aware of astronomical events such as
the phases of the moon and eclipses. About 140 B.C. the
Greek astronomer Hipparchus used triangulation to estimate
the sun, as seen from two locations during the solar eclipses.
3. In 1609, Italian scientist, as well as,
Galileo Galilei or best known as
Galileo, discovered the craters on the
earth’s moon, as well as dark spots
of the Sun also known as the
sunspots. He discovered that the
sunspots changed their position from
day to day.
4. He also looked at Jupiter and saw four
smaller objects orbiting it, the Galilean
moons. With this such discovery, he also
looked at Saturn but appeared to have a
round disk that remained stationary on
either side of the planet - which he later
on found out as Saturn’s rings.
5. In the year Galileo died, Isaac
Newton made astounding
contributions to Mathematics and
science. His work conceived from
the studies about motion, optics
and gravity. The law of universal
gravitation is one of the most
important rules of nature that he
6. In 1705, Sir Edmund Halley
proposed that some comets move
around the sun like planet, whereas
others return at regular intervals.
The comet that reappears about
every 76 years was named after
him. He used methods developed
by Newton and found comets that
seen in 1531, 1607, and 1682 had
similar orbits and predicted that
these were the same object that
appear in 1759.
7. The Nature of Planetary Orbits
A few years before Tycho Brahe
died he hired a young assistant
named Johhanes Kepler. Kepler was
a clever and hardworking man with
strong geometrical background with
plenty of unusual ideas. His
knowledge of geometry with his
stunning ideas, led him to visualize
the geometric figures between the
8. Furthermore, Kepler’s measurements of Mar’s
orbit revealed that the sun is located not at the
center of the ellipse but off-center, one focus of
the ellipse which is called the eccentricity.
12. Light Year
Astronomical distances were adopted from the speed of
light which is 299,793km/s = 2.998 x 10⁵ km/sec, or usually
expressed as 3 x 10⁸ m/sec in a vacuum. The speed of the
light defines how far light travel in a certain perspective.
The sun is 8.3 light minutes away, which means that the
light from Earth which is more distant than light minutes or
light hours, mostly are expressed in light-years.
13. Astronomical Unit
Astronomers also use the term astronomical
unit (AU) to define distance between the
earth and sun. It is equivalent to 149 million
kilometers or 93 million miles.
14. If the distance is bigger, and if it seems that the unit
light year will also run out to define the distance
of a certain astronomical object, astronomers
also use the term parsec (pc). One parsec is
3.26 light years.
15. The Coordinate System
Cartesian Coordinates or Cartesian plane is a straight
line that specifies each point in a plane, by a pair of
numerical coordinates which are the signed
distances a point from two perpendicular lines. The
Cartesian coordinates have X and Y axis Y axis as
the horizontal line; the left part is in negative, while the
right part is positive. The Y axis the vertical line; the
upper part of the Y axis is in positive while the bottom
part of the Y axis is in negative
(Horizontal Line or Parallel Line) is an angular distance;
it is represented in degrees, minutes and seconds, from
the north or south equator.
(Vertical Line) is also an angular distance; it is
represented in degrees, minutes and seconds. They
point from east or west of the prime (Greenwich)
meridian. Longitude is also called the meridian.
18. Celestial Sphere It is an imaginary sphere in the
sky, showing the rising and
setting points of celestial
bodies (sun, moon, planets
and stars). It determines their
positions in the sky.
19. Celestial Sphere There is another coordinate system
in celestial sphere, which is the
altitude-azimuth system. The
altitude (vertical) of a star is the
number of degrees above your
horizon to the star. The azimuth
(horizontal) is measured eastward
along with the horizon from the north
to the intersection of the horizon line
from the zenith through the star to
• The north pole of the celestial sphere on directly in the Earth’s North Pole. Look for the
North Star Polaris as it will tell where the north is.
• The equator of the celestial sphere is directly on the Earth’s equator.
• The angle of the star in the celestial equator corresponds to the degrees of the latitude on
which star is above the Earth.
• The Earth is considered at rest and the celestial bodies travel their diurnal motion from east
• The path of the Sun is called the ecliptic. It makes an angle of 23.35 degrees with the
celestial equator. Each month, The sun moves slowly eastward relative to the stars, it is
located in one different constellation called the zodiacal constellation. The moon and the
Planets have paths that are very close to the ecliptic. Thus they will always be in one
different zodiacal constellation.
• The ecliptic crosses the equator twice a year. The points of crossing are called points of
equinox. One of these points is the vernal quinox where the point is 0 degree right
ascension. The other point is called the autumnal equinox which is 180 degrees. Also there
are some called solstice it is where the sun reaches its highest and lowest excursion
related to the celestial equator. The solstice is divided into two: summer solstice and the
- The summer solstice is where we experience longer day’s shorter nights. While winter
solstice is where we experienced shorter days and longer nights. Equinox and solstice are
related in season.
- The angular distance of a star in north or south on the celestial equator is called the
declination (dec. or horizontal) if the star is in the celestial equator it is 0 degree, if the star
in in the north it is 90 degrees and if the star is in the south declination is negative. Right
ascension (RA or vertical) is measured eastward from the vernal equinox along with the
celestial equator, it represents the unit of time. The lines of the right ascension run from the
north celestial pole (NCP) to the south celestial pole (SCP).
22. Tools used in the observation
There are many and different tools used in observing
the night sky. Astronomers study the heavens and
collect data based from their observations. The most
familiar tool used in astronomy is the Telescope.
28. Cosmology is the study of the structure and evolution of the universe as
a whole. Cosmologists seek to answer: is the universe infinite? Why
does it exist? Where did it come from? What will be it’s fate? It has
become an apple of the eye of society. It’s hard to tell whether
statements are true or not. Many of the attempted answers have
become part of humanity’s religious heritage. But we have made many
wonderful and surprising discoveries. Current evidence indicates that the
universe was born about 13.7billion years ago out of a hot, dense,
violent state of matter and energy called Big Bang. It has been the most
acceptable theory of our beginning because of it’s remarkable evidences
that is given by the universe itself. It has been expanding ever since, and
it is filled with radiation from the early stages of inflation.
29. The Milky way
Our Sun is one of billions of stars in galaxy called Milky Way. It’s name
“milky” is derived from it’s appearance as a dim glowing band arching
across the night sky in which naked eye cannot distinguish individual
stars. Our galaxy is barred spiral galaxy that contains many types of
stars: giants, dwarfs, how and cold young and old, stable and
exploding. All of these star types combine to define the stellar
population of stars in our galaxy.
30. The Milky way
According to the two-stage collapse model, our galaxy began as a
vast, slowly rotating gas cloud, a few million light-years in a diameter,
containing several hundred billion solar masses of gas. The cloud was
composed of hydrogen and helium. As gravity began shrinking this
immense cloud, clumps of gas within it grew forming stars. Until the
next generation of stars are formed. It is the reason why stars have