Transcript NATS 1311-From the Cosmos to Earth

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
NATS 1311 - From the Cosmos to Earth
Annual Motion of the Sun
The R.A. of the Sun…
increases about 2 hours
per month
The Declination of the Sun…
varies between –23º and
+23º
NATS 1311 - From the Cosmos to Earth
Local Skies
Lines of constant declination cross the sky at different altitudes,
depending on your location on Earth.
declination line = your latitude - goes through your zenith
the altitude of the N or S celestial pole = your latitude
NATS 1311 - From the Cosmos to Earth
Local Skies
NATS 1311 - From the Cosmos to Earth
Sun’s Altitude vs Latitude and Season Animation
NATS 1311 - From the Cosmos to Earth
Celestial Navigation
Determining latitude
Find celestial pole - latitude equal to angular altitude - in northern
hemisphere Polaris is within 1º of celestial pole
For more precision - use star
with known declination determine angular altitude as it
crosses your meridian imaginary half circle drawn from
your horizon due south, through
zenith (point directly overhead)
to horizon due north - or when
star is at its highest altitude in
the sky. Ancients used crossstaff or Jacob’s ladder to
determine angular altitude.
Modern device called a sextant.
Sextant
NATS 1311 - From the Cosmos to Earth
Vega crosses your meridian in the southern sky at 78º 44’. You know it
crosses you meridian at 38º 44’ north of the celestial equator. So the celestial
equator must cross you meridian at an altitude of 40º so your latitude is 50º.
The formula for latitude is
  Elevation  declination
north 
90 
south 

north/south of zenith. Sun can also be used if you know the date and the
Sun’s declination on that date.

NATS 1311 - From the Cosmos to Earth
Celestial Navigation
Determining longitude
Need to compare current positions of objects in your sky with positions at
known longitude - Greenwich (0º Longitude). For instance - use sundial to
determine local solar time is 3:00 PM. If time at Greenwich is 1:00 PM, you
are two hours east of Greenwich and your longitude is 15º X 2 = 30º East
Longitude.
Accurate determination of longitude required invention of clock that could
remain accurate on a rocking ship. By early 1700s, considered so
important, British government offered large monetary prize for the solution claimed by John Harrison in 1761. Clock lost only 5 seconds during a 9week voyage.
NATS 1311 - From the Cosmos to Earth
Precession
(a) A spinning top slowly wobbles, or precesses, more slowly than it spins. (b)
The Earth's axis also precesses. Each precession cycle takes about 26,000
years. Note that the axis tilt remains about the same throughout the cycle, but
changing orientation of the axis means that Polaris is only a temporary North
Star.
NATS 1311 - From the Cosmos to Earth
Precession Movie
NATS 1311 - From the Cosmos to Earth
Gravitational
Attraction
The Sun’s gravity (and the Moon’s to a lesser degree) tugs on the Earth trying
to straighten out its rotational axis. However, like any rotating object, the
Earth tends to keep spinning around the same axis. The result is that gravity
succeeds only in making the axis precess.
NATS 1311 - From the Cosmos to Earth
Sun’s Path Through the Zodiac Animation
When astrology began - few thousand years ago - astrological sign
supposed to represent the constellation Sun appeared in on your
birth date.
However, because of precession - no longer case for most people signs are about a month off.
Sign actually corresponds to constellation the Sun would have
appeared in on your birthday 2000 years ago
NATS 1311 - From the Cosmos to Earth
26,000 yrs
Climate Changes
41,000 yrs
100,000 yrs
Changes in Earth’s orbit and orientation cause cyclic changes in
climate - ice ages. Mildest period about 5,000 years ago - headed for
another ice age.
NATS 1311 - From the Cosmos to Earth
Milankovitch Theory
Variations in Earth's orbit, the resulting changes in solar energy flux at high
latitude, and the observed glacial cycles.
Milankovitch Theory - precession of equinoxes, variations in tilt of Earth's
axis (obliquity) and changes in eccentricity of the Earth's orbit responsible for
observed 100 kyr cycle in ice ages by varying amount of sunlight received by
the Earth particularly noticeable in high northern latitude summer.
NATS 1311 - From the Cosmos to Earth
Lunar Phases
Moon’s appearance and rise and set times change with the
cycle of the lunar phases
The sun illuminates one side (half) of the moon - appearance
varies with side of moon facing the Earth - the angle between
the sun-Earth line and the Earth-moon line.
NATS 1311 - From the Cosmos to Earth
NATS 1311 - From the Cosmos to Earth
Lunar Phases
NATS 1311 - From the Cosmos to Earth
Lunar Phases
NATS 1311 - From the Cosmos to Earth
Lunar Phase Terminology
Phases of the Moon’s 29.5 day cycle
new
crescent
first quarter
gibbous
waxing
full
gibbous
last quarter
crescent
waning