Transcript Zodiacal Constellations.

Quiz #1 Recap
PSC 1010
20110915
Scientific Method
• Hypothesis: initial idea/observation on physical
phenomenon (or, an answer to the question how does
something work)
– Must be testable
• Theory: widely accepted explanation for a physical
phenomenon
• Hypothesis evolves into a theory after
– Rigorous testing in controlled experiments
– Communicating results to peers (publishing)
– More testing by independent researchers/teams
– An accepted theory will be able to make accurate
predictions about physical phenomena
Motions of Sun, Moon and Planets
•The apparent motion of the Sun, Moon, and planets relative to the background
stars (through the Zodiacal constellations) as measured from night-to-night is a
consequence of the facts that
•The Moon orbits the Earth and
•The Earth and planets orbit the Sun and
•These orbits lie in approximately the same plane in space (within the band
of the Zodiac).
•Our line of sight to any of the planets, Sun, and Moon changes relative to
the background stars and constellations as the Earth, Moon, and planets
orbit the Sun.
•Since all orbits are in approximately the same plane, the Sun, Moon and
planets all cross in front of the same group of constellations, all on the
ecliptic plane: the Zodiac.
The Zodiacal Constellations
•The path in the sky that the Sun,
Moon, and planets follow throughout
the year, and year after year is called
the ecliptic.
•The constellations that the ecliptic
passes through are called the Zodiacal
Constellations.
•As with constellations anywhere in the
sky, only those Zodiacal constellations
in a direction away from the sun are
visible at any given time of year in the
night sky.
The Zodiacal Constellations
• The Zodiac
constellations are
those constellations
that the Sun, Moon,
and planets pass
through during the
course of a year, and
repeating year after
year (yellow dots on
figure indicate the
ecliptic path).
If the Earth’s axis was not
tilted…
• Every day
would be an
equinox
• For any single
spot on the
Earth, sunlight
angle of
incidence
would not
change from
day-to-day
• There would
be no
seasons
•Figure shows sunlight angle of incidence on an
equinox for an observer at latitude 38
(Washington, D.C.)
North/South Hemisphere
Seasons
• Seasons in the Southern Hemisphere are opposite those
in the Northern Hemisphere. Therefore, on the day of
Quiz #1, 15 Sep. 2011, the season in South Africa was
winter (soon to be spring)
North 
Spring
Summer
Fall
Winter
South 
Fall
Winter
Spring
Summer
Length of Continuous Sunlight
(Day) or Darkness (Night) at the
Poles
• 6-months
• http://astro.unl.edu/naap/motion3/animatio
ns/sunmotions.html
Seasons by Hemisphere
December Solstice Example
• Northern Hemisphere
– 1st day of winter
– Compare the
amount of light
vs. dark at the
observer’s
latitude on the
globe at right:
Longest night
time of the year
– Sun lowest in the
sky throughout
the day: winter
•Southern Hemisphere
•1st day of summer
•Longest day time of the year
•Sun highest in the sky throughout the day
Equinoxes
• The
Equinoxes
days with
equal
daylight and
darkness
occur in
March and
September
The Seasons 1
• We experience seasons on the Earth
– because the Earth’s rotation axis (the NorthSouth line through the Earth) is tilted (by
23.5°) with respect to the direction of its
motion as it orbits the Sun, or in other words
• because the Earth’s rotation axis is tilted with
respect to the plane of the Earth’s orbit by 23.5°.
The Seasons 2
and the consequences of this tilt are:
• day time and night time are not of equal length in
time for every day throughout the year
• when days are longer (and the Sun is high in the
sky throughout the day, and the sunlight is more
concentrated on the ground), we have summer
• when nights are longer (and the Sun is low in the
sky throughout the day, and the sunlight is less
concentrated on the ground ), we have winter
The Seasons 3
additional consequences of this tilt are:
• the sun rises and sets at different positions on the horizon from day
to day throughout the year
– when the Sun rises at the northern most point along the eastern
horizon, that day is the June solstice
– when the Sun rises at the southern most point along the eastern
horizon, that day is the December solstice
The Seasons 4
additional consequences of this tilt are:
• The seasons are not the same on the Northern
and Southern hemispheres of the Earth
– Northern Hemisphere: spring, summer, fall, winter
– Southern Hemisphere: fall, winter, spring, summer
• Therefore, when summer ends in the U.S. (and fall
begins in the U.S.), spring will start in South Africa.
Lunar Cycle – First Week
Waxing
One week after the
moon is new, the moon
phase is First Quarter.
Lunar Cycle – Second Week
Waxing
After one more week, the
moon is full. Therefore,
two weeks elapse between
the new moon and the
full moon.
Solar and Lunar Eclipses
Solar Eclipse: Lunar phase
is new
(compare with Lunar Cycle:
First Week slide).
Lunar Eclipse: Lunar phase
is full
(compare with Lunar Cycle:
2nd Week slide).
Full Moon Rises at Sunset,
Full Moon Sets at Sunrise
The Moon’s Orbital Plane is
do not occur every
Tipped • Eclipses
month because the Moon’s
Earth
The Earth-Moon system to scale
Moon
orbital plane around the
Earth is not aligned with the
Earth’s orbital plane around
the
• Since the Moon is far and
small it’s easy for its
shadow to miss the Earth if
the two aren’t closely
aligned with the Sun.
• An alignment happens
where the orbital planes
cross each other
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Full Moon is on the Night Side of
the Earth
Can see any lunar phase at some
time during the day except full
However, the different
phases are visible only
certain times during the
lunar cycle. Those times
are the same from one
lunar cycle to the next.