Transcript NATS1311_090908_bw

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
NATS 1311 - From the Cosmos to Earth
New Moon  First Quarter  Full Moon
NATS 1311 - From the Cosmos to Earth
Full Moon  Third Quarter  New Moon
NATS 1311 - From the Cosmos to Earth
Earthshine
The dark portion of the lunar face is not totally dark - you can see the
outline of the full face of the Moon even when the Moon is not full - in
particular the crescent phase.
Because the crescent phase is nearly a new moon as seen from Earth, the
Earth is nearly full as viewed from the moon.
The light of Earth illuminates the night moonscape - just as the full moon
illuminates the Earth landscape.
Because Earth is much larger than the Moon, the full earth is much bigger
and brighter in the lunar sky than the full moon is in Earth's sky. This faint
light illuminating the “dark” portion of the Moon's face is often called the
ashen light or earthshine.
NATS 1311 - From the Cosmos to Earth
The “Dark” Side of the Moon
Near Side
Far Side
The Moon is tidal locked with the Earth - one side faces the Earth at all
times - term dark side would better be called the far side - the hemisphere
that never can be seen from Earth. Was not seen until first spacecraft
orbited the moon and sent back pictures of the far side.
NATS 1311 - From the Cosmos to Earth
Names of the Full Moons
January
February
March
April
May
June
July
August
September
October
November
December
Wolf Moon
Snow Moon
Worm Moon
Pink Moon
Flower Moon
Strawberry Moon
Buck Moon
Sturgeon Moon
Harvest Moon
Hunter's Moon
Beaver Moon
Cold Moon
NATS 1311 - From the Cosmos to Earth
Blue Moon
Modern folklore - a Blue Moon is the second full Moon in a calendar month can occur in any month but February, which is always shorter than the time
between successive full Moons (29 1/2 days).
Ecclesiastical version - occurs when a season has four full Moons, rather
than the usual three - the third is the Blue Moon - found only in February,
May, August, and November, one month before the next equinox or solstice.
The result of following rules laid down as part of the Gregorian calendar
reform in 1582. The ecclesiastical vernal (spring) equinox always falls on
March 21st, regardless of the position of the Sun. Lent begins on Ash
Wednesday, 46 days before Easter, and must contain the Lenten Moon,
considered to be the last full Moon of winter. The first full Moon of spring is
called the Egg Moon (or Easter Moon, or Paschal Moon) and must fall within
the week before Easter. Only by naming the third moon the Blue Moon will
the names of the other full Moons, such as the Moon Before Yule and the
Moon After Yule, fall at the proper times relative to the solstices and
equinoxes.
NATS 1311 - From the Cosmos to Earth
Astronomical Time Periods
NATS 1311 - From the Cosmos to Earth
Definitions of a Day
•
Sidereal Day
– Time from one transit of a star across the meridian to the next.
– Related to the Stars
•
Apparent Solar Day – Time from one transit of sun across the meridian to the next.
– From one high noon to the next
– Related to the sun
•
Mean Solar Day
– Time between successive transits of mean sun.
– Average of apparent solar days over one year.
– Defined to be 24 Hours
NATS 1311 - From the Cosmos to Earth
Sidereal Day
Sidereal - “related to the stars” - the time it takes for any star to make a
circuit of the sky - about 23 hours 56 minutes. Measure of the Earth’s
rotation - varies about 1 second in 45,000 years. Today defined relative to
an ensemble of extra-galactic radio sources.
NATS 1311 - From the Cosmos to Earth
Solar Day
The time it takes for the Sun to make one circuit around the local sky length varies over course of year (up to 25 seconds longer or shorter)
but averages 24 hours.
NATS 1311 - From the Cosmos to Earth
Why is a Sidereal Day Shorter than a Solar Day?
One full rotation represents a sidereal day - but while orbiting the
Sun, Earth travels in its orbit (about 1 degree per day). So the Earth
must rotate slightly farther to point back at the Sun - solar day.