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Motions in the Sky
Motion of the Sun
Diurnal motion – rises in the east sets in
the west with a 24 hour period
Annual motion – with respect to the stars
the Sun moves eastward with a yearly
period
Constellations of the Zodiac
The ecliptic passes through 12 (actually 13)
constellations during the year. This are know as
the zodiacal constellations.
Discussion
Which constellation is the Sun in today?
Which constellation is highest in the southern
sky at midnight?
Discussion
In this drawing, about what time is it?
Discussion
In what constellation will the Sun be in at 6 pm?
The seasons and ellipticity
The Earth’s orbit is nearly circular – distance
from the Sun varies by only 3%
Earth is closest to the Sun in January and
furthest from the Sun in July
Summer in northern hemisphere is winter in
southern hemisphere
The Seasons and tilt
The seasons are caused by the 23.5 degree tilt
of the Earth’s rotation axis relative to the
normal (perpendicular) of Earth’s orbital
plane.
Conservation of angular
momentum
Anything that spins on an axis or revolves
around another object has angular
momentum.
Conservation of angular momentum requires
that the rate of spinning remains constant
with time. Also, the axis of rotation of any
spinning object remains in a fixed direction in
space.
Discussion
Think back to when you were just learning
how to ride a bike. What was it that made it
so difficult to learn?
Near edge-on view
Discussion
Where is the Sun today on the previous picture?
Why is summer warmer?
1. The Sun, being above the celestial
equator, remains in the sky longer during
the summer, the longest daylight time
occurring on the summer solstice.
2. The Sun rays hit the earth more directly
during the summer months, i.e. the
summer hemisphere receives more
energy per square meter.
Discussion
If someone tells you that the seasons are
caused by the varying distance from the Sun,
what evidence could you cite to disprove this?
Motion of the Moon
Diurnal motion – east to west in sky
Monthly motion – along ecliptic, west to
east against the stars
The Moon
As we pass through the month, the appearance
of the Moon changes each night. We say the
Moon goes through a series of phases.
Discussion
Many people believe incorrectly that the
phases of the Moon are caused by the
shadow of the Earth on the Moon. What
simple observation of the Moon could you
make to prove that they are wrong?
First quarter Moon
The motions of the Planets
Diurnal motion – daily motion from east to west
staying close to ecliptic
Direct motion – as with the Sun and Moon, move
west to east as compared to the stars, each at a
different rate
Retrograde motion – each spends some time
moving east to west as compared to stars
Path of Mars
Two types of planets
Superior planets – appear in sky at
opposition or opposite the Sun
Inferior planets – always stay close to the
Sun and are never seen at opposition