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Homework #3 will be posted soon.
Several “out-of-class” activities will also be
posted shortly.
Email announcements will be sent
Around 1500 B.C., Egyptians developed a sundial, onto which
they divided the daylight hours into 10 equal parts.
Around 1500 B.C., Egyptians developed a sundial, onto which
they divided the daylight hours into 10 equal parts.
They designated two additional parts (“hours”) to signify twilight
time (morning & evening)
They divided the night time into twelve portions based upon
crossing of the meridian by evenly spaced “clock stars”
Ever since then, we have divided the day into twentyfour portions (hours)
Why do clocks run in the
“clockwise” direction?
Apparent Solar Time
Mean Solar Time
Apparent Solar Time: Based on the location of
the sun in the sky relative to the local meridian
Because of the Earth’s variable orbital speed
(due to its noncircular orbit) and to the inclination
of Ecliptic to Equatorial plane, the rate at which
the sun appears to move is not uniform.
This leads to variable length days!
Not very useful to have hours and
days that are not uniform in length!
Solution: Create fictitious sun
which moves at a uniform rate
equal to the mean motion of the
Mean Solar Time
Location of “average” sun
relative to the local meridian.
This average sun moves at a
constant speed relative to the
celestial equator – equal length
Mean solar time
can run from 17
minutes earlier
than apparent
solar time to 15
minutes later.
between two is
given by the
Both apparent and mean solar time are defined locally.
Need more uniform time keeping scheme.
Standard Time: Time zones within which the time is
approximately the same as the mean solar time at the
center of the zone.
The Year
Important in societies that need to keep
track of “annual” events, such as seasons
Based upon orbit of Earth about the Sun
Complicated by uneven number of days in
Sidereal Year: Length of time required
for Earth, Sun, and stars to return to
same configuration.
Tropical Year: Length of time between
successive Vernal Equinoxes.
These differ in length because of the
precession of the Earth’s axis.
The Earth’s axis “wobbles”, similar to a top, causing the
direction of rotation to change with time.
The orientation returns to its original direction every 26,000 years.
Precession causes movement of:
Celestial poles
Celestial Equator
Position of Vernal Equinox
These, in turn, mean that the celestial
coordinates (RA and declination) of an
object change with time because the
coordinate system moves
Must specify “epoch” of coordinates
(e.g., 2000.0)
“Sun Signs” shift in sky – popular
“signs” really relate to positions 2000
years ago
From historical
and practical
perspectives, it is
desirable to have
a calendar in
which seasons fall
at the same time
each year.
Egyptian Calendar: (~4200 BCE) – calendar
of 365 days
but tropical year ~ 365 ¼ days in length, so
seasons got out of sync.
Julian Calendar: (46 BCE). Introduced
concept of leap year. One day added to
calendar every four years. Spring set to
March 24.
However, tropical year actually ~ 11
minutes short of 365 ¼ days. By late
1500s, the beginning of spring (Vernal
Equinox) was falling on March 11.
Gregorian Calendar: (1582). Designed to
maintain March 21 date of Vernal Equinox.
Added leap centuries to calendar. Leap year for
“hundred's year” only if century divisible by 400.
Oct. 1, 2, 3, 4, 15, 16 ... 1582
The Gregorian Calendar was adopted at
different times around world; 1752 in England
and American colonies, 1912 in China, 1919 in
Other calendars are in active use. Some of
these are lunar/solar hybrids, e.g., the Jewish
calendar periodically adds months to the year
in order to keep pace with the seasons.
The year is divided into 12 months, based upon lunar cycles
Months are divided into Weeks:
* The week is traditionally divided into 7 days named for
Sun, Moon, Mercury, Venus, Mars, Jupiter, and Saturn
Roman Anglo-Saxon English
You will be shown the relative positions of the planets for
today’s date, as seen from high above (to the north) of the solar
Keep in mind that the planets
• all orbit in approximately the same plane,
• all orbit in the same direction (prograde),
• inner planets have shorter orbital periods than planets on
larger orbits,
Given this, think about and answer the following questions.
1. Which planets are visible at 9 pm? At 3 am?
2. Mercury and Venus appear in the sky only shortly after sunset, at
which time they are called “evening stars”, OR shortly before
sunrise (“morning stars”). What are these two planets currently?
3. The orbit planes of all of the planets are near a plane for which
we have already discussed. What is the name of this plane?
What defines it?
4. Do we expect to ever see either the inferior planets (Venus &
Mercury) or the inferior planets (all the rest) close to the North
Celestial Pole? At southern celestial latitudes? Explain.
Inner solar system
Outer solar system
Planets in the Sky
The five “naked eye”
planets are very easy to
see –> bright <-
Planets Known in Ancient Times
difficult to see; always close to Sun in sky
very bright when visible — morning or evening
noticeably red
very bright
moderately bright
Close grouping of
these five planets in
April 2002.
Note that these planets
plus the recently set
Sun all lie essentially
along a line . . . .
Where in the sky should we look
to see the planets?
The planets are always seen
near the ecliptic.
This is a consequence of the
planets orbiting in planes that
are near each other.
(1) Planets, including
the Earth, orbit the
(2) Planets closer to
the Sun have shorter
orbital periods than
planets farther from
the Sun
As we “pass” a planet, it
appears to move backwards
(as seen from Earth)
Parallax Angle
Apparent shift of a star’s position due
to the Earth’s orbiting of the Sun
The ancient Greeks rejected the notion
that the Earth orbits the sun. Why?
It ran contrary to their senses.
If the Earth revolved about the Sun,
then there should be a “great wind” as
we moved through the air.
Greeks knew that we should see stellar
parallax if we orbited the Sun – but
they could not detect it.
Parallax Angle
Apparent shift of a star’s position due
to the Earth’s orbiting of the Sun
The nearest stars are
much farther away than
the Greeks thought.
So the parallax angles
of the star are so
small, that you need a
telescope to observe
Possible reasons why stellar
parallax was undetectable:
Stars are so far away that stellar parallax is too small for
naked eye to notice
Earth does not orbit Sun; it is the center of the universe
Unfortunately, with notable exceptions like Aristarchus, the
Greeks did not think the stars could be that far away, and
therefore rejected the correct explanation (1)…
Thus setting the stage for the long, historical showdown between
Earth-centered and Sun-centered systems.
We have now set the stage for discussing
the historical development of astronomy
Close grouping of five
planets in April 2002.
Note that these planets
plus the recently set
Sun all lie essentially
along a line . . . .
This is a pattern that
was well known to the
Locations of planets in the sky
Mercury: always close to Sun in sky
Venus: always close to Sun in sky
 Mars: no restrictions on distance from Sun in sky
 Jupiter: no restrictions on distance from Sun in sky
 Saturn: no restrictions on distance from Sun in sky
What causes these differences?
Motions of the planets
 On short term (diurnal motion), planets appear to move
with the stars, east to west, making a full circuit around the
sky (meridian to meridian) in approximately one day
 Most of the time, planets move slowly eastward each day
relative to the stars: different planets at different rates
What causes these motions?
 Planets are always
close to the “ecliptic”,
the apparent annual
path of the sun through
the sky.
Close grouping of five
planets in April 2002.
This is a pattern that
was well known to the
Why are the planets
restricted to these
Some planets occasionally reverse their motion
relative to the stars, moving slowly westward
relative to the stars, for a few days
apparent retrograde motion
What causes this?
What causes this?
What causes the observed motions of the
stars, sun, moon, and planets in the sky?
The Greeks developed a model for the
Universe that lasted for nearly 15
It did a reasonably good job explaining
these motions.
Claudius Ptolemy (100-170 CE)
Developed a model of the
universe designed to fit
the observational data.