Download What is the “Meridian”?

Groups of dark brown streaks
have been photographed by
the Mars Reconnaissance
Orbiter on melting pinkish
sand dunes covered with light
frost. Dark sand on the
interior of Martian sand dunes
near the north pole of Mars
became more and more
visible as the spring Sun
melts the lighter carbon
dioxide ice. When occurring
near the top of a dune, dark
sand may cascade down the
dune leaving dark surface
streaks. Objects about 25
centimeters across are
resolved on this image
spanning about one kilometer.
Homework #1 has been posted.
Due: Thursday, January 28, 4:00 pm.
Be sure to have your answers determined
before accessing the homework on Oncourse.
Celestial Sphere:
How do you find things on
a sphere?
On the Celestial sphere
any location can be
given by its:
Declination (Celestial
Latitude, measured north
or south of the Celestial
equator)
 Right Ascension
(Celestial Longitude,
measured (angular
distance from the
“Vernal Equinox”)
The celestial sphere is
“fixed” in space.
We view it in the
context of the local sky
We have a sphere (the Earth) within a sphere
(Celestial sphere). If both were fixed and
unchanging, we would always look to the same point
in the sky to see the same object. BUT…
 We can be located anywhere on the Earth
 The Earth is rotating
 The Earth is orbiting the Sun
 The Moon is orbiting the Earth
 The planets are also orbiting the sun
We need to take all of this into account to understand
changes in the sky, time, seasons, etc.
Half of the Celestial
Sphere is visible in
the local sky, the
other half is below
the horizon.
Rotation of the Earth
causes the portion of
the celestial sphere
visible in the local
sky to change with
time.
The Earth rotates
west to east .
This causes the
(fixed) celestial
sphere to appear to
rotate east to west
The apparent motion
of the stars is referred
to as “diurnal motion”.
Diurnal circle
Diurnal motion leads
stars to “move”
around the celestial
sphere at a constant
angular distance
from the celestial
pole.
As you move around on the Earth, the
portion of the celestial sphere visible in
your local sky changes.
Half of the celestial sphere is always
visible above the horizon.
Moving north or south (changing your
latitude), leads to viewing a different
portion of the celestial sphere.
How does the altitude of
the North Celestial Pole
change with the latitude of
an observer?
(hint: imagine observer’s
at the North Pole, at the
Equator, and at a midlatitude location, such as
Bloomington).
Location: Fairbanks, AL
(Figures from Nick Strobel’s
“AstronomyNotes” website www.astronomynotes.com)
Location: Seattle, WA
Location: Los Angeles
Location: Equator
IMPORTANT POINTS!!
1. The altitude of the celestial pole is
equal to the latitude of the observer
2. The Celestial Equator ALWAYS
intersects the East and West points on
the horizon.
Depending upon your location on
Earth, some stars will either never
set (i.e., they are always above
the horizon) and some stars will
never rise (they are always below
the horizon). These stars are
called “Circumpolar Stars”
Circumpolar Stars:
never set
The star named “Polaris” is very near the
North Celestial Pole (within one degree).
It is also referred to as the “North Star”.
What is the “Meridian”?
The longitude passing through Greenwich England
The point directly over an observer’s head
A great circle half way between the observer’s zenith
and nadir
An imaginary line in the sky that passes from the north
point on horizon, through the zenith, to the south point
on horizon
What is the “Meridian”?
The longitude passing through Greenwich England
The point directly over an observer’s head
A great circle half way between the observer’s zenith
and nadir
An imaginary line in the sky that passes from the north
point on horizon, through the zenith, to the south point
on horizon
Stars which are not circumpolar:
 rise in the eastern half of the sky (i.e.,
east of the meridian),
 increase their altitude until they cross
the meridian ,
 set in the western half of the sky (i.e.,
west of the meridian).
ConceptTest
You are in Bloomington and observe a star
rising directly to the east. When this star
reaches its highest point above the
horizon, where will it be?
(a) high in the northern sky
(b) high in the eastern sky
(c) high in the southern sky
(d) high in the western sky
(e) at the zenith
ConceptTest
You are in Bloomington and observe a star
rising directly to the east. When this star
reaches its highest point above the
horizon, where will it be?
(a) high in the northern sky
(b) high in the eastern sky
(c) high in the southern sky
(d) high in the western sky
(e) at the zenith
ConceptTest
Where are circumpolar stars at their
lowest point in the sky?
In the western sky
In the eastern sky
In the southern sky
On the meridian
ConceptTest
Where are circumpolar stars at their
lowest point in the sky?
In the western sky
In the eastern sky
In the southern sky
On the meridian
In Bloomington, where is the North Celestial Pole?
 On the Celestial Equator
 On the meridian at an altitude of approximately
40 degrees.
 Near the zenith
 At the north point on the horizon
In Bloomington, where is the North Celestial Pole?
On the Celestial Equator
 On the meridian at an altitude of approximately
40 degrees.
Near the zenith
At the north point on the horizon
You are in Bloomington and observe a star
rising directly to the east. Where does this
star lie on the Celestial Sphere?
ConceptTest
As you move from the Earth's equator
toward the North Pole, the number of
circumpolar stars:
increases
decreases
stays the same
ConceptTest
As you move from the Earth's equator
toward the North Pole, the number of
circumpolar stars:
increases
b. decreases
c. stays the same
We have been examining the
daily motion of the stars through
the local sky.
What about the Sun’s motion?
Got here
 The Sun moves east to west, full circuit around the sky,
each “day” (result of Earth’s rotation)
 Relative to the stars, the Sun moves slowly eastward
each day (~ 1 degree/day): full circuit around Celestial
Sphere in one “year” (result of Earth’s orbit)
 The Sun moves east to west, full circuit around the sky,
each “day” (result of Earth’s rotation)
 Relative to the stars, the Sun moves slowly eastward
each day (~ 1 degree/day): full circuit around Celestial
Sphere in one “year” (result of Earth’s orbit)
 Path of Sun on Celestial
Sphere due to Earth’s orbital
is a great circle called
“Ecliptic”
motion
the
 The Sun moves east to west, full circuit around the sky,
each “day” (result of Earth’s rotation)
 Relative to the stars, the Sun moves slowly eastward
each day (~ 1 degree/day): full circuit around Celestial
Sphere in one “year” (result of Earth’s orbit)
 Path of Sun on Celestial Sphere due to Earth’s orbital
motion is a great circle called the “Ecliptic”
 Constellations the Ecliptic passes through are referred to
as the “zodiac”
Two measures of the Earth’s rotational period
Solar day: Rotational period relative to the sun
(successive sun crossings of the meridian). This is
the common measure of the day.
Sidereal Day: Rotational period relative to the stars
(successive star crossings of the meridian, 23h 56m)
“angular distances”
Full circle = 360 degrees
Right angle = 90 degrees
Fist at arm’s length ~ 10 degrees
Moon & sun ~ ½ degree wide
1 Hour = 15 degrees
1 minute = 1/60 hour = 1/4 degree
Definitions
ecliptic plane = The plane
of the Earth’s orbit
ecliptic = The annual path
of the Sun on the sky
equatorial plane = The
plane of the Earth’s
equator
celestial equator =
Projection of the equatorial
plane onto the sky
The Earth orbit is prograde, i.e., it is in the
same direction as its rotation.
Thus, the Earth’s orbital motion causes the sun
to appear to move eastward (increasing R.A.)
along the ecliptic around the celestial sphere.
Today we divide the
celestial sphere into
88 regions, referred
to as constellations
(in a manner similar
to dividing the
Earth’s surface into
countries).
The constellations along the ecliptic are
called the zodiac (see applet)
The Ecliptic & Celestial
Equator are Great
Circles that:
 are inclined by 23.5 degrees
to each other
 intersect with at two points
 One of these points of
intersection is where the sun is
located on the celestial sphere
at the moment Spring begins.
This point is referred to as the
“Vernal Equinox”
The Vernal Equinox
serves as the origin for
measuring Right
Ascension (R.A. =
celestial longitude)
Right Ascension - R.A.
(Celestial Longitude)
Angular distance
east of the Vernal
Equinox
Measured in “time”
o
units (1 hour = 15 )
Winter Solstice – sun at
farthest point south
Vernal (Spring) Equinox
– Sun on Celestial
equator moving north
Summer Solstice – sun
at farthest point north
Autumnal (Fall) Equinox
– Sun on Celestial
equator moving south