Download time astro 2014 - Fort Thomas Independent Schools

Earth’s Motion
Days and Years
What is Time?
 The measurable period during which an
action, process or condition exists or
continues.
 the indefinite continued progress of existence
and events in the past, present, and future
regarded as a whole.
 a point of time as measured in hours and
minutes past midnight or noon.
 "the time is 9:30”
What is Time?
Basic units of time…
 Seconds, Minutes, Hours, Days, Years
Forward Motion
Relativistic time : decreases with increasing
velocity with respect to outside observer.
How do we measure time?
 Presently, time is measured in seconds
as corresponding to the frequency
[9,192,631,770 hertz (Hz =
cycles/second)] of radiation emitted
from the 133 Cs atom as excited outer
electrons change or “jump” energy
states.
How do we measure time?
 Sunrise to sunset?
 Has the earth’s rotational speed been
constant throughout geologic time?
 Solar day is 24 hours (4.5 by ago the Earth
day was 6 hours and 620 my ago it was 21.9
hours)
 Note: the Earth is slowing down by roughly
1.7 milliseconds per century.
Cesium 133 Wrist Watch
$ 6000.00
Development of Time Zones
 What if everyone kept the same time…then 12
midnight for some would be dark and for others it
would be light.
 What led to the development of time zones?
 Synchronization of human activities over long
distances led to the standardization of local times.
 How were the time zones determined?
 It takes the Earth approximately one hour to rotate
15 degrees.
 Because of this, time zones are separated by
generally 1 hour or 15 degrees longitude.
Different parts of the world experience different
times of day as the Earth rotates.
TIME ZONES which can be used to calculate the
time of day in any given part of the world.
Sidereal Period
 Sidereal Period: The orbital period
of one object about another
measured with respect to the
background stars.
The Sidereal Day
 It takes 23 hours, 56 minutes and 4 seconds for the
Earth to complete one rotation with respect to the
background stars.
 Notice that a sidereal day falls short of 24 hours.
 As the Earth rotates, it also revolves around the
Sun at a rate of nearly 1 degree per day.
 To compensate for this motion, the Earth must
rotate an additional 4 minutes and 4 seconds to
complete a full solar day (from the perspective of
the Sun).
Mean Solar Day vs. Sidereal Day
 Solar Day: One complete rotation with respect to
the Sun. The average of this defines our 24-hour
day. That is why it is referred to as a mean solar
day.
 Sidereal Period: The orbital period of one object
about another measured with respect to the
stars.
 Sidereal Day: the length of one rotation of the
Earth with respect to the background stars. Our
sidereal day is 23 hours 56 minutes 4.1 seconds.
Days
 Why is a mean solar day longer than
a sidereal day?
 http://bcs.whfreeman.com/universe
7e/content/ch02/0203003.html
The Sidereal Year
 The sidereal year is the time taken
for the Earth to orbit the Sun and to
return to the same position with
respect to the background stars.
A sidereal year is equal to
365.25636042 mean solar days.
The significance between sidereal
and mean solar day
 The stars rise approximately 4 minutes earlier
each night. This is due to the difference in the
length of the mean solar day (24 hours) and
sidereal day (23 hours 56 minutes 4.1 seconds).
 With each night that passes, the Earth falls four
minutes short of returning to its starting point
with reference to the Sun. So each night, the
Earth falls behind by four minutes.
The significance between sidereal
and mean solar day
 So, from your viewing location at the same time each
night, the Earth points towards a different part of the
universe, giving us a slightly different view of the
stars.
 1st night: you see a constellation at a specific
coordinate at a specific time
 2nd night: you see the constellation at the same
coordinate, but in order to see this, you must view
the night sky four minutes earlier.
 3rd night: you see the constellation, but eight minutes
earlier…and so on…
Year
 Tropical year
 One revolution of Earth around the Sun relative to the
position of the equinoxes.
 365.242219 mean solar days
 Sidereal year
 One revolution of Earth around Sun relative to fixed stars
 365.25636 mean solar days
 Difference between the tropical and sidereal year is
the caused by the precession of the equinoxes (Earth’s
gradual precession causes the orientation of the
equatorial plane to shift slightly relative to the ecliptic
plane.
Leap Year
 To compensate for the extra .24 days
of the tropical year (365.242219 mean
solar days)—spring to spring or
vernal equinox to vernal equinox.
 Add a year if the year is divisible by 4.
 Skip all centuries unless it is divisible
by 400.
What is the analemma?
 The figure-eight pattern signifying the path of the Sun
over the course of a year.
 What causes this?
 The analemma is a product of the variation in the
speed of the Earth’s orbit. (the Earth’s orbit is slightly
elliptical)
 Demo: Chapter 9 (Analemma)
 Click on analemma-A
 Note the path of the sun over the course of a year—
 If the Earth has a circular orbit, how would this
change the pattern of the Sun during the course of a
year?
Starry Night Demo
 From Chapter 4 Lab (Earth’s Orbital Motion)
 Orbit A: the view as time passes shows no change in
the location of the background stars. This represents
successive sidereal days, not mean solar days. If you
click one step ahead to the next day, you will see that
the time is four minutes and four seconds earlier than
the day before.
Starry Night Lab Demo
 Orbit A: if the sidereal day is switched to day, this
view shows the background stars moving. Notice
that each step forward changes only the day, but not
the time. This shows a solar day (24 hours). The
additional 4 minutes and 4 seconds of rotation causes
the night sky and the Sun to shift approximately 1
degree to the east.
Starry Night Demo
 Chapter 5 (Seasons and the Length of Day)
 View over S Pole
 What is the difference between the north ecliptic
pole and the north celestial pole?
 What is the angular separation of these two
poles?
 View from Sun Centre
 Why is the Earth’s axis pointing in different
directions during the course of the year?
 How does this affect the angle of sunlight striking
the planet?
Starry Night Lab Demo
 In Ch. 3 (diurnal motion), the sky appears to rotate
from east to west.
 Diurnal A: this view reinforces that the Sun, stars, and
planets appear to move east-west (actually, in arcs).
 Diurnal motion is caused by the rotation of the Earth.
Starry Night Demo
 Ch. 3 Lab (Diurnal Motion)
 Seattle view: stars and planets rise at the same angle
as the Sun rose. The apparent motion of the sky is an
illusion caused by the…?
 rotation of the Earth.
Questions
 Describe what is observed in the sky during the Earth’s
diurnal motion.
 Explain the difference between the sidereal day and the
mean solar day.
 Why does the night sky appear to be different each night
when observing at the same time?
 Why does the night sky look the same when you observe it
after one full sidereal day?
 If you want to find a particular star in the exact location
you observed it on the previous night, when should you
look for it?
 30.
What is the insolation angle and how
does it influence the surface temperatures and
seasons on Earth?