Celestial Bodies and Names

Planets, stars, the Sun, and the Moon were the subject
of mystical powers well before the development of the
foundations of science appeared in the ancient
Mediterranean cultures

Ancient Egyptian, Chinese, and Indian mythology
identified the powers of their Gods with the imaginary
forces of the heavenly bodies

Our western culture inherited the Egyptian,
Mesopotamian, and Greek mythological identities and
names of the planets and many of the bright stars

The names we use for the planets and many of the
moons are the Roman names adopted from the Greek
names
Celestial Bodies and Names
Planet
Greek Name
Roman Name
Mercury
Hermes - The Messenger God
Mercury - The Messenger
God
Venus
Aphrodite - Goddess of Love
Venus - Goddess of Love
Earth
Gaea or Terra - Represented the
Earth. Created the Universe and gave
birth to the first race of Gods (the
Titans), and to the first humans
Terra - Goddess of the
Earth
Mars
Aires - God of War
Mars - God of War
Jupiter
Zeus - King of the Gods and son of
Gaea
Jupiter - King of the Gods
Saturn
Cronus - God of Agriculture
Saturn - God of Agriculture
Uranus
Uranus - God of the Sky
Uranus - God of the Sky
Neptune
Poseidon - God of the Sea; brother of
Neptune - God of the Sea
Jupiter
Pluto
Hades - God of the Underworld
Pluto - God of the
Underworld
Celestial Bodies and Names

Later discoveries of planets using telescopes followed the
convention of Roman/Greek mythological gods

Moons are generally derived from the Greek and Roman
mythological characters
 The moons of Uranus are named after Shakespearean
literary figures (e.g., Cordelia, Ophelia, and Bianca)

Asteroids (minor planets) were first named after mythological
and literary characters
 With the discovery of tens of thousands of asteroids came
a more practical naming scheme that includes the year and
month of discovery, and at times, the name of the
discoverer

Comets are named after the discoverer(s) and the sequence of
discovery
Celestial Bodies and Names
Stars

Stars and star names originated in the ancient
cultures, with the western world introduced to the
names and positions of stars coming from the
Mesopotamians and Greeks

Groups of stars called constellations came from the
Babylonians and/or Sumerians and were copied by the
early Greeks

These early constellations is recorded in Ptolemy's
Almagest, along with more than a thousand stars, and
a summary of all of Greek astronomical knowledge
Celestial Bodies and Names
Celestial Bodies and Names
Stars
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Stars were first classified by their parent
constellation and their relative brightness within
that constellation using the Greek alphabet
Alpha Ursa Majoris for example is the brightest star
in the constellation Ursa Major
With the advent of powerful telescopes and the
ability to resolve billions of stars and galaxies, the
convention for identifying stars is their right
ascension and declination (latitude and longitude)
in the sky
Celestial Bodies and Names
Stars

Twelve constellations that surround the equatorial belt
of the celestial sphere are called the constellations of
the zodiac
 Zodiacal constellations circle the sky in the Sun’s
apparent path through the stars
 Known also as the 12 signs of the Zodiac
 Zodiac is Greek for “circle of animals”
 Origin of the longitude on the celestial sphere is the
vernal equinox
Celestial Bodies and Names
Stars

Although no more or less important than the other
constellations, these have become commonplace
because of their position in relation to the Sun's path
through the equatorial belt
 Commonly associated with astrology, a pseudoscience that was separated from astronomy some
500 years ago
Celestial Bodies and Names
Calendars and
Time
Time and Calendars
Calendars are based on the seasonal differences in the
year and the lunar cycle

Egyptian calendar was based on twelve months per
year with 30 days in each month, totaling 360 days in a
year which was more than five days short of the actual
solar year

To correct for the difference five days were inserted into
the calendar as a festival during the harvest season
 This festival called heb set was a part of the
Egyptian's 365 day calendar

Egyptian calendar was 12 months/year x 30 days/month
+5 days harvest festival (heb set) = 365 days/year
Time and Calendars
Egyptian calendar was off by about ¼ day per
year but used by the Greeks and the Romans

A calendar correction was made by inserting
one-quarter day per year, but by using a full
day every four years called leap year


Called the Julian Calendar, after the Roman ruler
Julius Caesar who commissioned the change
Julian Calendar – similar to Egyptian calendar but
added 1 day every four years (leap year) = 365.25
days/year
Time and Calendars
The ¼ day/year correction was also inaccurate, although
less so than without the leap year added
The actual value of 365.24219... days per year could be
approximated by adjusting the number of leap years in
a longer period
Over a century, the difference between 365.25 and
365.24219.. was less than a day and the accumulated
difference of one day was vary close to four centuries
As a solution, the Roman Catholic Church adopted a
method that would resolve the problem of drift in the
calendar and reconcile the vernal equinox and Easter
Time and Calendars

That calendar correction omitted leap years every
century except those divisible by 400

This was called the Gregorian calendar that was
named for Pope Gregory and the calendar in general
use today

Gregorian Calendar – similar to Julian calendar but
leap year is removed on century dates except those
divisible by 400
Time and Calendars
Gregorian calendar

Ten days were removed in October,
1582, to reconcile the accumulated
difference in time from the earlier
Julian calendar. For this reason,
astronomers use dates based on the
original Julian day calendar - called
the Julian date - for continuous time
and date reference

The last day of the Julian calendar
was Thursday 4 October, 1582, which
was followed by the first day of the
Gregorian calendar Friday, 15
October, 1582
Time and Calendars
A second type of calendar is based on the Moon's orbital
period around the Earth

The lunar calendar is not fixed with reference to the solar
calendar base which results in a progressive shift of the
Islamic calendar's cardinal dates on the Gregorian calendar

A third type of calendar called the lunisolar calendar is
based on the lunar cycle, but reconciled (intercalated) with
the tropical year every few years
 This creates a discontinuity in both of the lunisolar
calendars that use this dual time base - the Chinese and
the Hebrew calendars
 Hence, the Chinese New Year is not on the same day on
the Gregorian calendar from year to year
Time and Calendars
Time is ultimately based on the Earth's orbit around
the Sun


This orbital period defines dynamical time which
must account for the irregular motion of the Earth
Other definitions of the Earth's orbit period around
the Sun include the period measured from one
vernal equinox to the next
 Average is 365 days, 5 hours, 48 minutes, 45.51
seconds
 Called the vernal year
Time and Calendars
The average over all seasonal
positions is called the mean
tropical year


Time with respect to the
background stars called the
sidereal year is slightly different
than solar and vernal year
because of the Earth's changing
rotational axis orientation
called precession
 Period of approximately
26,000 years
This difference due to this
precession is approximately
20.40 minutes per year
Time and Calendars
The two most common time references which define the
year are called solar time and sidereal time

Solar time incorporates the Sun as the primary
reference

Sidereal time uses the background (fixed) stars as the
primary reference
 Background stars are also useful as an inertial
(fixed) reference for navigation and astronomy

Solar time - Sun used as primary reference

Sidereal time - Background stars are used as
primary reference
Time and Calendars

The Earth’s day is defined roughly as one rotation of the Earth on
its axis every 24 hours

Since the Earth also orbits the Sun, there is a difference between
a complete rotation with reference to a fixed background versus
the moving solar background

As the Earth rotates and orbits the Sun, it most rotate slightly
farther to make up for the 0.99o orbital motion per day (360o in
365.242 days)

This produces a 3.94 minute per day difference between solar
(orbit) time and sidereal (star) time

Because the Earth's orbit also produces one rotation, there is a
difference of one extra day per year
Time and Calendars

The difference between solar and sidereal time is calculated using:
1/365.242 days/year x 24 hr/day x 60 min/hr = 3.943 min/day

This calculation is also the time difference representing the apparent
motion of stars in the night sky of 3.943 minutes per day, or, 24
hours per year

Using 30 day months, the star motion is approximately 2 hours per
month (30 days x 4 minutes/day = 120 minutes/month) or 1,440
minutes per year = 24 hr

Sidereal time is shorter than solar time by:

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4 min per day
2 hours per month
24 hours per year
Time and Calendars
Prague
Astronomical
Clock
Prague
Astronomical
Clock
Time and Calendars
Development
of Science
Development of Science
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3,000 BC Constellations named by the Minoan,
Sumerian and Babylonian cultures
2,000 - 3,000 BC The Egyptian culture made
extensive celestial observations that were used for
their calendars, for characterizing their
mythological deities, and for agricultural and
ceremonial purposes
300 BC The first scientists in the Classical Greece
period used logic to develop the foundation of the
sciences by using knowledge, measurements, and
reasoning to help understand their surroundings
Development of Science


The cornerstones of science were later dismantled
by a number of the well-known Pythagorean
philosophers who proposed that data was
unnecessary in modeling the physical world, and
that a science like astronomy was a futile attempt
in trying to discover the secrets of the universe
True science was replaced with a divine practice
using logic that pursued the same questions but
with ratios
“So if we mean to study astronomy in a way which makes
proper use of the soul's inborn intellect, we shall proceed
as we do in geometry -- working at mathematical
problems -- and not waste time observing the heavens
(Plato's' Republic)”
Development of Science
Planetary motion was also characterized in Aristotle’s
teachings as follows:
Solid crystalline spheres in the sky carried the five visible
planets, the Sun, and the Moon
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


The outermost sphere held the stars
The Earth was at the center of the universe and did not
move
Sublunar world was imperfect, consisting of everything
under the Moon, with a natural motion towards Earth
Superlunar world was perfect, consisting of everything
beyond the Moon (stars and planets) with circular orbital
motion around the Earth
Aristotle’s Universe
Development of Science


250 BC Aristarchus of Samos developed
the first sun-centered (heliocentric) concept
of the "universe" which consisted of the
solar system and visible stars
140 A.D. Ptolemy refined the Greek view of
the geocentric (Earth-centered) universe
with eccentric, epicycle, and equant
geometries to describe the motions of the
Sun, Earth, planets, and the Moon
Development of Science

These complicated refinements remained as
the accepted explanation of the unusual
motion of the planets (from planetes, or
"wanderers" in Greek) until the Copernican
Revolution in the 1500s
 The incorrect theory of the geocentric solar
system is often referred to as the Ptolemaic
universe because of its increased
refinement by the astronomer and
mathematician Claudius Ptolemaeus (90-168
AD)
Development of Science


1543 Nicolaus Copernicus published On
the Revolutions of the Celestial Spheres
which established the Sun as the center of
the solar system
His theory was far more convincing than
Ptolemy's geocentric model, but the new
concept was inconvenient to adopt for
scientists and the church

There were also inaccuracies in the Copernican
theory since his circular orbit model for the
planets still did not account for the irregular
planetary motion, including the retrograde
motion of Mars.
Development of Science
1570-1601 Tycho Brahe made careful measurements
of the stars and planets which would be used for
the more refined heliocentric theory of Kepler


Although Tycho was meticulous in his observations,
he believed in the Ptolemaic geocentric solar
system
Because the model could not be reconciled with his
own observations, Tycho constructed his own
theory using a modified Ptolemaic geocentric solar
system that had all planets except for the Earth
and the Moon orbiting the Sun, with that collection
orbiting the Earth
Tycho Brahae’s Model
Development of Science

1600 Johannes Kepler begins work for Tycho
Brahe and applies mathematics to the observations
to arrive at his three laws, and an accurate physical
model for the solar system
Kepler's laws
1. The planets orbit the Sun in elliptical orbits, with
the Sun at one focus of the ellipse
2. Equal areas are swept out in equal times by the
orbiting planets
3. The period of orbit for a planet squared is equal
to its semi-major axis cubed
Development of Science
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1600-1640s Galileo Galilei established a
much more accurate definition than
Aristotle for force and motion which would
later become the fundamentals of
mechanics
His demonstration of the uniformity of
falling bodies due to the force of gravity
was only part of the relationship
established between forces, velocity and
velocity changes, and inertia
Galileo also demonstrated the first basic
understanding of gravity
Development of Science

Galileo applied the optical
telescope for the first time to
make accurate astronomical
observations and found:
 The inner planets have phases
similar to the Moon (proving
heliocentric theory)

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There are four large moons of
Jupiter (Galilean moons)
Numerous stars make up the
diffuse Milky Way galaxy
Lunar mountains, valleys and
seas looked similar to those on
Earth
Development of Science

1660-1720s Isaac Newton developed precise
mathematical and physical laws of gravity and the
laws of physical motion

Newton also constructed the basic theories of light,
and created one of the most important mathematical
tools - calculus – at the same time as and independent
of Gottfreid Leibniz

Newton would also develop analytical tools and
approximation methods useful for two-body and nbody orbit calculations
Development of Science
Newton's three laws of motion
1. Newton's first law (also known as the law of inertia)
An object at rest will stay at rest and an object in
motion will stay in motion unless acted upon by an
external force.
2. Newton's second law
An applied force equals the rate of change of (the
derivative of) momentum. This is a statement that
force equals mass times acceleration, or F=ma.
3. Newton's third law
For every action there is an equal and opposite
reaction. This is the basic principle of rocket
propulsion.
Development of Science

Joseph Lagrange (1736-1813) - In addition to
developing a number of mathematical tools for
physics and astronomy applications,
Lagrange was intrigued with planetary motion
and orbital motion in general

Lagrange also developed a powerful
technique for approximating the motion of
small objects throughout the solar system
using the simplified three-body problem

His work led to the stability and motion
characterization of the solar system's
members, from the largest planets to the
smallest particles
Development of Science

1900 - Max Planck theorized that
electromagnetic energy (light) is quantized in
units called photons

This gave rise to the beginning of the theories
that constitute quantum mechanics, a
powerful tool in understanding the physical
laws of the atomic world
Development of Science

1905 - Albert Einstein postulated the
photoelectric effect which corroborates the
quantum mechanical view of the atomic world


Although Einstein was uncomfortable with the later
developments of quantum mechanics and its
implications, he received the Nobel prize in physics
in 1912 for his early work on the quantum
photoelectric effect
1910-1920s - Albert Einstein developed the
special and general theories of relativity which
greatly advanced our understanding of gravity
(mass), motion, time and space

These theories are still used for understanding
observations of the distant universe, galaxies, and
even nearby stars
Development of Science

1920s Edwin Hubble and
Harlow Shapely establish
the concept and scale of our
galaxy, the Milky Way (also
known as The Galaxy)

His pioneering work
established the expanding
universe and later the Big
Bang theory

Hubble's groundbreaking
work that measured the
receding galaxies continues
today with much greater
accuracy
Development of Science

As more distant and more precise
measurements are made of distant
galaxies, the Big Bang theory has
become much more complex

Our understanding of the structures in
the universe and the foundation of the
Big Bang theory have evolved from
these early observations
Questions?