Download January 19

Review Question
Describe how the stars move through the
sky if you were at the earth’s north pole.
Star trails…
Review Question
Describe how the stars move through the
sky if you were at the earth’s equator if you
are facing north, south, east and west.
Star trails…
Review Question
What are the two reasons that is it warmer
during the summer than during the
winter?
Review Question
Describe the path the Sun will take across
our sky on the day of the autumnal equinox.
Review Question
Describe the diurnal motion of the Sun in
our sky on the summer solstice.
Review Question
Describe the path the Sun will take across
our sky on the day of the winter solstice.
Review Question
What is precession?
Review Question
What causes the Earth rotation axis to
precess?
Precession
Precession of the Equinoxes
If the celestial poles change with time, so to
must the celestial equator, which is 90 degrees
away from the poles. If the celestial equator
changes with time, than the intersection of
ecliptic and the celestial equator will also
change with time.
Age of Aquarius?
The “age” is defined by the constellation the
Sun is in on the vernal equinox. Currently, it
is the age of Pisces.
Discussion
Why is the changing of the equinox position
important?
Tropical year
The length of our calendar year is defined by
the time it takes between two vernal
equinoxes. This is called the tropical year
and is 365 days 5 hours 48 minutes and 46
seconds long.
Sidereal year
However, because the equinox positions
slowly changes, this is not the Earth’s actual
orbital period around the Sun. The actual
orbital period is called the sidereal year and
is 365 days, 6 hours, 9 minutes and 10
seconds, or 20 minutes 24 seconds longer.
Leap Years
Notice that our tropical year is not an even
number of days. The extra 5 hours, 48
minutes and 46 seconds is almost, but not
quite ¼ of a day. To make up for this, every 4
years we have to add an extra day to our
calendar.
What is the difference?
If we used the sidereal year for our calendar
year the seasons would slowly migrate through
the different months over time. Spring would
come one day earlier every 70 years.
Astronomical coordinates
Because of precession and the changing
position of the celestial poles and the vernal
equinox, celestial coordinates of astronomical
objects change from year to year.
Astrology
Precession breaks the cause and effect
relationship between the position of the
stars and seasonal changes.
TRADITIONAL
ASTROLOGICAL DATES
Constellation Date Span
ACTUAL ASTRONOMICAL DATES
TODAY (c. 2000 C.E.)
Days
Constellation
Date Span
Days
Aries
21 Mar - 19 Apr 30
Aries
19 Apr - 13 May
25
Taurus
20 Apr - 20 May 31
Taurus
14 May - 19 Jun
37
Gemini
21 May - 20 Jun 31
Gemini
20 Jun - 20 Jul
31
Cancer
21 Jun - 22 Jul
32
Cancer
21 Jul - 9 Aug
20
Leo
23 Jul - 22 Aug
31
Leo
10 Aug - 15 Sep
37
Virgo
23 Aug - 22 Sep 31
Virgo
16 Sep - 30 Oct
45
Libra
23 Sep - 22 Oct
Libra
31 Oct - 22 Nov
23
Scorpio
23 Oct - 21 Nov 30
Scorpius
23 Nov - 29 Nov
7
Ophiuchus
30 Nov - 17 Dec
18
30
Sagittarius
22 Nov - 21 Dec 30
Sagittarius
18 Dec - 18 Jan
32
Capricorn
22 Dec - 19 Jan
29
Capricornus
19 Jan - 15 Feb
28
Aquarius
20 Jan - 18 Feb
30
Aquarius
16 Feb - 3/11
24-25
Pisces
19 Feb - 20 Mar 30-31
Pisces
12 Mar - 18 Apr
38
Motion of the Moon
The Moon’s motion
Diurnal motion – rises in the east crosses the
meridian and sets in the west every day.
Monthly motion – moves eastward with
respect to stars moving its diameter in about
an hour.
The Moon
As we pass through the month, the appearance
of the Moon changes each night. We say the
Moon goes through a series of phases.
Discussion
Many people believe incorrectly that the
phases of the Moon are caused by the
shadow of the Earth on the Moon. What
simple observation of the Moon could you
make to prove that they are wrong?
1st Quarter Moon
The Earth is round
The “shadow” on a first or last quarter Moon
is a straight line dividing the face of the
Moon in half. The Earth is round and will
always cause a curved shadow on the Moon.
When the shadow of the Earth does fall on
the Moon we call this a lunar eclipse.
The real reason
The Moon’s phases are actually cause by
the changing position of the Moon with
respect to the Sun. The side of the Moon
facing the Sun is always fully illuminated
(except during a lunar eclipse). But because
the angle we view the Moon differs from
that of the Sun, we see varying degrees of
the Moon’s surface facing us illuminated.
New Moon
Occurs when the Moon is in the same part of
the sky as the Sun. Thus the Moon is up
during the day, and the illuminated side of
the Moon faces away from the Earth. The
Moon is not visible when it is new.
Waxing crescent
When less than ½ of the Moon’s face is
illuminated. It is the western side of the
Moon’s face that is illuminated and the Moon
is visible in the sky before Sunset. Waxing
means increasing.
First Quarter
When exactly ½ of the Moon’s face is
illuminated. It is the western side of the
Moon’s face that is illuminated and the Moon
is up before sunset. Although ½ of the Moon’s
face is illuminated it is called a quarter
because the Moon is ¼ of the way through it’s
cycle and occurs about one week after the
new phase. The Moon is 90 degrees away
from the Sun in the sky.
Waxing Gibbous
When more than ½ of the Moon’s western
face is illuminated.
Full Moon
when the Moon’s face is fully illuminated. At
this time the Moon is directly opposite the Sun.
Occurs about 2 weeks after the new phase.
Waning Gibbous
when more than ½ of the Moon’s face is
illuminated. But now it is the eastern face that
is illuminated and waning means decreasing.
Last Quarter
when ½ of the Moon’s face is illuminated. Again
it is the eastern half that is illuminated and the
Moon is visible after sunrise. Occurs about 3
weeks after the new phase.
Waning Crescent
When less than ½ of the eastern half of the
Moon’s face is illuminated.
What time is it in this drawing?
What time is it in this drawing?
What time is it in this drawing?
What time is it in this drawing?
What time is it in this drawing?
What time is it in this drawing?
Lunar eclipse
Discussion
What phase must the Moon be in for there
to be a lunar eclipse?
Discussion
Why don’t we have a lunar eclipse every
time there is a full Moon?
Total Lunar eclipse of July 16, 2000
Discussion
Why do you think the eclipsed Moon
appears red?
Discussion
The Moon goes through its phases in 29.53
days. This is called the synodic month, i.e.
the month relative to the Sun. The actual
orbital period of the Moon is 27.32 days, the
sidereal month. Why is there a difference in
the synodic and sidereal months?
In the 27.32 days that the Moon has taken to
orbit the Earth the Earth has moved in its
orbit relative to the Sun. The Earth moves
about 1 degree per day so that after 27 days
the Moon needs to move through an extra 27
degrees in the sky to get back to the same
position relative to the Sun.
Synodic and Sidereal months
Discussion
If the Earth moves faster in its orbit does the
length of the sidereal month change?
Discussion
If the Earth moves faster in its orbit, does the
synodic month get longer or shorter?
Discussion
The Moon always keeps the same side facing
the Earth. Does this mean the Moon does not
spin on its axis? Explain?
Synchronous Rotation
If the Moon did not spin on its axis we would
see the Moon’s entire surface over the month.
The Moon therefore does rotate but the
rotation period is exactly equal to its orbital
period and rotates such that we always see
the same face. Thus, the Moon’s rotation
period is synchronous with its orbital period.
Discussion
What is the length of the sidereal and mean
solar days on the Moon?
Mean solar day on the Moon
29.5 Earth days, the Sun rises in the east sets in
the west 14.75 days later, followed by 14.75
days of darkness.
The Moon’s sidereal day is its actual rotation
(and revolution) period of 27.32 days.
The motions of the Planets
Each planet follows the same diurnal
motion as the Sun, Moon and stars, rising
in the east and setting in the west each
day.
Like the Sun and Moon, each planet moves
eastward with respect to the stars. This is
called direct motion. In addition, all the
planets stay close to the ecliptic.
Path of Mars
Ptolemy’s explanation for
retrograde motion
• Each planet moves on a small circle called
and epicycle.
• The center of each epicycle moves along a
larger circle centered near the Earth called
a deferent.
Ptolemaic system
• Very successful at predicting positions of
the planets but was not perfect
• Offered no explanation of why the planets
moved on deferents and epicycles
• There was no relationship between period
of revolution and epicycle size
Alternatives to Ptolemy’s model
Aristarchus proposed a heliocentric model of
the Solar System in the 3rd century B.C.E.
Reintroduced in the 16th century by
Copernicus
Heliocentric model
• Sun at the center
• Diurnal motion explained by rotation of the
Earth
• All the planets including Earth revolve
about the Sun in circular orbits with
different speeds
Advantages of heliocentric
model
• Provides natural explanation of retrograde
motion.
• Provides natural explanation for the motion of
mercury and Venus as inferior planets, i.e. their
orbits are interior to that of the Earth.
• Provides a relationship between distance from
Sun and orbital period. Planets farther from the
Sun took longer to complete an orbit.
Retrograde motion