Transcript jun8 - Astronomy

Motion in the Sky
Discussion
What is a year?
Discussion
What is a day?
Discussion
Why are there 7 days in a week?
Time
Day – Time it takes the Sun to return to the
same position above the horizon.
Month – Time it takes the Moon to go
through its phases.
Year – Time it takes the Sun to return it the
same position relative to the stars.
Note: Ancient definitions differ from the
modern usage!
Discussion
Do you know how the hour was originally
defined?
An hour is the time it takes the Moon to move
its diameter against the stars.
Fixed stars and Planets
Although the stars change their positions over
the course of a night and from night to night,
their relative positions (their positions relative
to each other) remain constant year after year.
Planet means “wanderer.” They change their
positions relative to that of the fixed stars
The Planets of antiquity
Sun
Moon
Mercury
Venus
Mars
Jupiter
Saturn
The days of the Week
Sunday – the Sun’s day
Monday – The Moon’s day
Tuesday – Tiw’s day – Mars
Wednesday – Woden’s day – Mercury
Thursday – Thor’s day – Jupiter
Friday – Freia’s day – Venus
Saturday – Saturn’s day
Ordering of the day names
Planets ordered by apparent length of
period of revolution:
Saturn
Jupiter
Mars
Sun
Venus
Mercury
Moon
Discussion
Assign each planet to an hour of the day:
Saturn 1, Jupiter 2, Mars 3, Sun 4, Venus 5,
Mercury 6, Moon 7, Saturn 8, Jupiter 9, …
What is the first hour of each day of the week?
Discussion
How do we estimate how far away things
are from us?
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 farther
from you than the others?
Discussion
At any given time we can only see at most
half of the celestial sphere. How come?
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 do you think the Moon looks much
bigger near the horizon than when it is high
in the sky?
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.
Circumpolar stars
Not all stars rise and set. Some stars, called
circumpolar, always appear above the horizon.
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
Where on Earth would all visible stars be
circumpolar?
Discussion
If we were at the North pole, how far above
the horizon would the North celestial pole
be?
Discussion
Where on Earth would none of the visible
stars be circumpolar?
Discussion
How high above the north horizon 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.
Discussion
On Earth’s surface, also a sphere, we specify
positions using latitude and longitude. How
does this system work?
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 the Earth moved faster in its orbit would
the length of the sidereal day be longer or
shorter? How about the solar day?
Explain.
Hint: How would the length of the days
change if we were to stop Earth in its orbit?
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 traditionally through 12
constellations during the year. (In modern
astronomy it is actually 13 constellations.)
These 12 constellations are know as the zodiacal
constellations.
Discussion
Based on the previous slide, in which
constellation is the Sun in today? Which
constellation is high in the sky at midnight?
Earth’s axial tilt & the seasons
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