Transcript on the Celestial Sphere

Motions in the sky:
The celestial sphere
Slide 1
What is the celestial sphere?
• An imaginary sphere surrounding Earth
that contains all the objects we see when
we look at the sky.
• Imagine that there is a giant, spherical
projector screen around the earth and all
the light from objects in space is shining
onto it to make the patterns we see (like
constellations).
Slide 2
Star trails
Why do stars appear
to move across the
sky?
Slide 3
Diurnal motion
The entire sky appears to rotate once in 24 hours. This is termed the daily or
diurnal motion of the celestial sphere, and results from the daily rotation of the
earth on its axis.
The diurnal motion affects all objects in the sky and does not change their positions
relative to each other.
Some objects also appear to move with respect to the other objects on the celestial
sphere. These are the "wanderers" of the ancient astronomers: the planets, the
Sun, and the Moon. This is due to the Earth’s orbit around the sun.
Slide 4
The Celestial Sphere
We can use the “celestial sphere” idea to
help locate objects in the sky.
The stars rotate around the North and South Celestial Poles. These are
the points in the sky directly above the Earth’s north and south poles.
Fortunately, for those in the northern hemisphere, there is a fairly bright star
very close to the North Celestial Pole called Polaris (or the North star).
Another important reference marker is the celestial equator: an imaginary
circle around the sky directly above the Earth's equator. All the stars
rotate in a path that is parallel to the celestial equator.
Slide 5
Stars that are close to the celestial poles
never appear to rise or set. These are
called circumpolar stars.
Slide 6
Slide 7
Slide 8
p. 16
Slide 9
The "Road of the Sun" on the Celestial Sphere
1. The sun has a diurnal (daily) motion from east to west
due to the earth’s spinning around its axis, ~ 24 h
2. The sun also changes its position in the sky ~ 1
degree per day, ~ 365.25 days
Slide 10
What is the ecliptic?
• The Earth and
sun orbit each
other in the same
plane, called the
ecliptic.
• That means the
path of the sun
across the sky will
be on the ecliptic.
Slide 11
Constellations of the Zodiac
•Your zodiac “sign” is the constellation the sun
was in on the day you were born.
•Since the Earth orbits the sun, when standing
on Earth and looking up at the sun, there will be
a different constellation in the background
depending on the time of year.
Slide 12
Slide 13
Ecliptic and Zodiac
Sun travels 360o/365.25 days ~ 1o/day
Slide 14
axis
Celestial
equator
As a result, planes of the ecliptic and celestial equator make an angle 23.5o
Slide 15
The Solstices
• The solstices are
the two spots
farthest away from
the celestial equator.
• Summer solstice:
longest day of the
year
• Winter solstice:
shortest day of the
year
Slide 16
Slide 17
Longer day
p. 23
The Equinoxes
• The ecliptic and the
celestial equator
cross in two places.
• In these two spots,
there is an equal
amount of day and
night (12 hours
each). These are
the equinoxes.
Slide 18
Slide 19
Shorter day
p. 23
What causes the seasons?
The tilt of the Earth’s axis, NOT distance from
the sun!!!
Slide 20
Tilt of the axis
• When the northern hemisphere is tilted
towards the sun, we receive more direct
sunlight.
• When we are tilted away from the sun, its
rays are spread out over a larger area so
they don’t give us as much warmth.
Slide 21
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p. 22
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p. 23
Seasons - summary
1.
Seasons are NOT caused by varying distances from the Earth to the Sun
2. The primary cause of seasons is the 23.5 degree tilt of the
Earth's rotation axis with respect to the plane of the ecliptic.
The Seasons in the Northern Hemisphere
Note: the Earth is actually closest to the Sun in January 4!
Slide 24
Thus, we experience Summer in the Northern Hemisphere when the
Earth is on that part of its orbit where the N. Hemisphere is
oriented more toward the Sun and therefore:
1. the Sun rises higher in the sky and is above the horizon longer,
2. The rays of the Sun strike the ground more directly.
Likewise, in the N. Hemisphere Winter the hemisphere is oriented
away from the Sun, the Sun only rises low in the sky, is above the
horizon for a shorter period, and the rays of the Sun strike the
ground more obliquely.
Slide 25