Transcript NATS 1311-From the Cosmos to Earth

NATS 1311 - From the Cosmos to Earth
A model of the celestial
sphere shows the
patterns of the stars, the
borders of the 88 official
constellations, the
ecliptic, and the celestial
equator and poles.
NATS 1311 - From the Cosmos to Earth
Latitude and Longitude
Latitude - lines of latitude parallel
to Earth’s equator - labeled north
or south relative to equator - from
90º N to 90º S
Longitude - lines of longitude
extend from North Pole to South
Pole - by international treaty,
longitude 0 (the prime meridian)
runs through Greenwich,
England
Dallas:
latitude = 32.78º N
longitude = 96.78º W
NATS 1311 - From the Cosmos to Earth
A circumpolar constellation never rises or sets - they are always visible.
Your latitude determines the portion of the celestial sphere visible in your
sky.
(a) A Northern Hemisphere sky.
(b) A Southern Hemisphere sky.
At what latitude would you see the entire sky?
NATS 1311 - From the Cosmos to Earth
Star Trails
The Southern Hemisphere
The Northern Hemisphere
The Earth's rotation causes stars to trace daily circles around the sky. The north
celestial pole lies at the center of the circles. Over the course of a full day,
circumpolar stars trace complete circles, and stars that rise in the east and set
in the west trace partial circles. Here, the time exposure lasted about 6 hours we see only about one-quarter of each portion of the full daily path.
NATS 1311 - From the Cosmos to Earth
Finding the Celestial Poles
You can always find north using the North Star. Polaris can be found
using the big dipper. Draw a line through the two “pointer” stars at the
end of the big dipper and follow it upwards from the dipper about four
outstretched hand’s width. The big dipper is circumpolar in the US so is
always above the horizon. The south celestial pole can be found using
the Southern Cross. There is no “South Star”
NATS 1311 - From the Cosmos to Earth
The Big and Little Dippers
NATS 1311 - From the Cosmos to Earth
Motion of the Night Sky Animation
NATS 1311 - From the Cosmos to Earth
The height in degrees of the north star above the horizon is the same as
your latitude.
NATS 1311 - From the Cosmos to Earth

The angle  between the horizon and Polaris is the latitude of the observer.
If Dallas is at 33º latitude, where is Polaris in the sky? Where is it at the
Equator?
NATS 1311 - From the Cosmos to Earth
Angular Size
Distances in the sky
measured by angular
distance:
Minute of arc = 1/60th of
a degree
Second of arc = 1/3600th
of a degree
Angular diameter angular distance from
one side of an object to
the other
NATS 1311 - From the Cosmos to Earth
Revolution
Earth travels around the sun (orbits) once per year in the same direction it
rotates. It’s orbit is not quite a perfect circle - it is elliptical. The location in
the orbit of the minimum and maximum distances from the Sun are called
perihelion and aphelion. The plane of the orbit is called the ecliptic.
NATS 1311 - From the Cosmos to Earth
Earth’s Axial Tilt
Ecliptic
Plane
The Earth’s axis is currently tilted 23.5º to the ecliptic. It varies over time
between 22º and 25º due the the gravitational forces from Jupiter and the
other planets.
NATS 1311 - From the Cosmos to Earth
The axis remains at the same tilt angle - pointed at Polaris - throughout the
orbit because of conservation of angular momentum. The ecliptic plane is the
plane of the Earth’s orbit. Looking from the Earth, it is the apparent path of
the Sun (and planets) in the sky.
NATS 1311 - From the Cosmos to Earth
The Relationship of the Celestial Equator and the
Ecliptic Plane
NATS 1311 - From the Cosmos to Earth
The Zodiac
The Sun appears to move steadily eastward along the ecliptic, through the
constellations of the zodiac. As Earth orbits the Sun, we see the Sun
against the background of different zodiac constellations at different times
of year. For example, on August 21 the Sun appears to be in the
constellation Leo. Defines astral calendar.
NATS 1311 - From the Cosmos to Earth
Sun’s Path Through the Zodiac
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Celestial
Sphere
The apparent
Sphere of the
sky
Celestial
Poles
The points
about which
the celestial
sphere
appears to
rotate
Celestial
Equator
Projection of
the Earth’s
equator on the
celestial
sphere
Ecliptic
Apparent
annual path of
the sun on the
celestial
sphere
NATS 1311 - From the Cosmos to Earth
Seasons occur because even though the Earth's axis remains pointed toward
Polaris throughout the year, the orientation of the axis relative to the Sun
changes as the Earth orbits the Sun. Around the time of the summer solstice,
the Northern Hemisphere has summer because it is tipped toward the Sun, and
the Southern Hemisphere has winter because it is tipped away from the Sun.
The situation is reversed around the time of the winter solstice when the
Northern Hemisphere has winter and the Southern Hemisphere has summer. At
the equinoxes, both hemispheres receive equal amounts of light.
NATS 1311 - From the Cosmos to Earth
Why Does Flux Sunlight Vary Animation
NATS 1311 - From the Cosmos to Earth
Antarctica
June 21
December 21
NATS 1311 - From the Cosmos to Earth
In the summer hemisphere, the sun follows a longer and higher path.
The sunlight is more intense - more direct and more concentrated. In the
winter hemisphere, the sun follows a shorter and lower path. The sunlight
is less direct and less intense.
Why are the warmest days one to two months after summer solstice?
NATS 1311 - From the Cosmos to Earth
The Relationship of the Celestial Equator and the
Ecliptic Plane
The solstices are when the celestial equator and the ecliptic have the greatest
separation - when the Sun is highest in the sky.
The equinoxes are when ecliptic and the celestial equator cross - when the
Sun crosses from north/south of the celestial equator to south/north of the
equator
Northern spring equinox is when the Sun passes from south of the celestial
equator to north.
NATS 1311 - From the Cosmos to Earth
Why are the seasons more extreme in the Northern hemisphere?
NATS 1311 - From the Cosmos to Earth
1. Most of Earth’s land mass in in the Northern Hemisphere. Water takes
longer to heat or cool than soil or rock. The water temperature remains
relatively constant, thereby moderating the climate.
2. Earth is slightly farther from the sun during northern summer solsitce moves slower in its orbit so summer/winter is 2 - 3 days longer/shorter. This
effect is more important then the slightly more intense sunlight due to Earth
being closer/farther away.
NATS 1311 - From the Cosmos to Earth
Five Major Circles of Latitude
1. The Arctic Circle (66.5 degrees N)
2. Tropic of Cancer (23.5 degrees N)
3. The Equator
4. The Tropic Capricorn (23.5 degrees S)
5. The Antarctic Circle (66.5 degrees S)
What is special about these latitude circles?
NATS 1311 - From the Cosmos to Earth
Five Major Circles of Latitude
The Arctic and Antarctic Circles - One day a year the sun shines all day and
one day a year it doesn’t shine at all.
Tropic of Cancer (Capricorn) - The sun is never directly overhead at higher
latitudes.
NATS 1311 - From the Cosmos to Earth
Daily Paths of the Sun at Equinoxes and Solstices
At the
north
pole
At 40º
latitude
At the
equator
At the
tropic of
Cancer
NATS 1311 - From the Cosmos to Earth
Celestial Coordinate System
Equatorial System
–Declination -  - Angle of a
star North or South of the
Celestial Equator Measured in Degrees
–Right Ascension -  Measure of Angular
Distance of a Star East of
the Spring Equinox Measured in Hours,
Minutes, Seconds
In this coordinate system the position of a star is specified by the right
ascension and the declination. This system is used more than any other by
astronomers.
NATS 1311 - From the Cosmos to Earth
Coordinate Systems
Geographic
Latitude - lines of latitude parallel to
Earth’s equator - labeled north or south
relative to equator - from 90º N to 90º S
Celestial
Declination - lines of declination parallel to
celestial equator - labeled positive or
negative relative to celestial equator - from 90º to +90º
Longitude - lines of longitude extend from
North Pole to South Pole - by international Right ascension - lines of right ascension
run from north celestial pole to south
treaty, longitude 0 (the prime meridian)
celestial pole - by convention 0 runs through
runs through Greenwich, England
spring equinox - measured in hours, minutes
and seconds east of spring equinox - one
hour is 15º
NATS 1311 - From the Cosmos to Earth