Transcript Powers of ten notation

Motion in the Sky
Discussion
How do we estimate how far away things
are from us in everyday life?
Parallax
Discussion
Does this work with astronomical objects, like
the stars, the Sun, the Moon and the planets?
Can you tell by looking at them which is further
from you than the others?
The celestial poles and equator
north celestial pole – point in the sky directly
above the Earth’s north pole
south celestial pole – point in sky directly
above the Earth’s south pole.
celestial equator – midway between the
celestial poles (90 degrees away), lies directly
above the Earth’s equator.
Diurnal (daily) motion of the Sun
The Sun rises in the east and sets in the west.
Thus, each day it crosses the meridian.
AM – ante meridian
PM – post meridian
When the Sun crosses the meridian it is also
the farthest from the horizon.
Discussion
Can you tell by observing the Moon that it is in
fact closer to you than the stars? If so, how?
Occultation of Regulus
Occultation of Saturn
Discussion
Why does the Moon look much bigger near
the horizon than when it is high in the sky?
Does it get further away as it rises?
Circumpolar stars
Not all stars rise and set below the horizon.
Some stars, called circumpolar, always appear
above the horizon.
Southern Circumpolar stars
Fake star
trail picture
All the stars appear to circle a point in the
sky called the celestial pole.
In the north this point lies near the star
Polaris, the north star.
In the south this point lies near the
Southern Cross.
Star trails on the celestial
equator
Discussion
On Earth’s surface, also a sphere, we specify
positions using latitude and longitude. How
does this system work?
Discussion
If we were at the North pole, how far above
the horizon (in degrees) would the North
celestial pole be?
Discussion
How high above the north horizon in
degrees is the north celestial pole from
Earth’s equator?
Discussion
How high above the northern horizon
would the north celestial pole appear in
Cleveland which has a latitude of about
42 degrees?
Measuring latitude
No matter where on Earth you are, you
can easily determine your latitude by
measuring the angle of the celestial pole
from the horizon.
Positions in the sky
If we imagine the sky as the inside surface of a
sphere, we can specify any point on this
celestial sphere using two coordinates.
Celestial coordinates
The angle from the celestial equator is called
the declination and the angle from the celestial
“prime meridian” is called the right ascension.
The celestial equator has a declination of 0
degrees, while the north celestial pole has a
declination of 90 degrees. The RA is measured
in hours where 360 degrees = 24 hours.
Earth’s rotation
The diurnal motion is due to Earth’s rotation
The stars take 23 hours 56 minutes from
one meridian crossing to the next, this is
Earth’s actual rotation period called the
sidereal day
The annual motion of the Sun
The Sun on the other hand, takes an average
of 24 hours between successive meridian
crossings. The difference is due to Earth’s
revolution about the Sun.
The Sun moves on average 4 minutes
eastward each day relative to the stars,
staying in the sky longer each day than a star
at the same declination.
Solar and sidereal days
Mean solar day – 24 hours
Sidereal day – 23 hours 56 minutes is the
actual rotation period of the Earth
Earth’s orbit speed is not
constant
The Earth moves faster when it is closer to
the Sun (in January) and slower when it is
further away (in July). This means that the
length of the day is not a constant 24 hours
during the year. Instead of varying the length
of the day throughout the year we use the
mean or average position of the Sun to define
our 24-hour day.
Discussion
If we stopped the Earth in its orbit, how
would this change the length of the
sidereal day?
How would this change the length of the
solar day?
Discussion
If the Earth moved faster in its orbit would
the length of the solar day be longer or
shorter? Why?
Annual motion of the stars
The same stars are not visible all year long.
Any given non-circumpolar star will set 4
minutes early each day relative to the Sun,
until it becomes lost in the glare of the setting
Sun.
The ecliptic
The ecliptic is the annual path through the
sky that the Sun appears to take.
In actuality, the ecliptic is the plane of Earth’s
orbit projected onto the stars.
The constellations
In modern astronomy, the constellations are
88 irregular areas that completely cover the
sky. Thus, every celestial object lies within the
boundaries of a constellation.
Constellations of the Zodiac
The ecliptic passes through 12 constellations
(actually 13) during the year. This are know as
the zodiacal constellations.
Summary
Diurnal motion due to rotation of the Earth
Sidereal rate is actual rotation period
Position of celestial poles determined by latitude
Sun, Moon and planets all move eastward relative
to the stars