Transcript January 14 - Astronomy

The Night Sky
Discussion
Describe how the stars move in the sky as
viewed from the Earth’s north pole.
Where in the sky is the north celestial pole,
the south celestial pole and the celestial
equator?
Discussion
Describe how the stars move in the sky as
viewed from the Earth’s equator.
Where in the sky is the north celestial pole,
the south celestial pole and the celestial
equator?
Discussion
Describe how the stars move in the sky as
viewed from a latitude of 35 degrees north.
Where in the sky is the north celestial pole,
the south celestial pole and the celestial
equator?
North celestial
pole
South celestial
pole
What is the
latitude of this
picture?
Discussion
Which star (A, B, C, or D) will spend the least
amount of time above the horizon during
the course of a day?
How long do stars on the celestial equator
spend above the horizon?
If taken in the northern hemisphere, are
the stars rising or setting in this picture?
Discussion
What is the difference between the solar day
and the sidereal day?
Solar and sidereal days
Solar day – 24 hours is the average time
between two solar meridian crossings
Sidereal day – 23 hours 56 minutes is the
actual rotation period of the Earth
Discussion
What is the ecliptic?
The ecliptic
The ecliptic is the annual path through the
sky that the Sun appears to take.
In other words, the ecliptic is the plane of
Earth’s orbit projected onto the stars.
The constellations
Traditionally, a constellation is a grouping of
stars in the same part of the sky.
Orion
The constellations
In modern astronomy, the constellations are
88 irregular areas that completely cover the
sky. Thus, every celestial object lies within the
boundaries of a constellation.
Ursa Major
Discussion
Why do you think ancient astronomers
invented the constellations and made up
stories to go with them?
Canis Major
Constellations of the Zodiac
The ecliptic passes through 12 constellations
(actually 13) during the year. This are know as
the zodiacal constellations.
Discussion
About what time is it in this picture?
Discussion
In what constellation will the Sun be in at 6 pm?
Discussion
In what constellation will the Sun be in one
month from the time of this picture?
The Seasons
Due to the Earth’s rotation axis being tilted
by 23.5 degrees from perpendicular to the
plane of its orbit.
Near edge-on view
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
Why is it hard for
people to learn to
ride a bike, but
once they learn it is
considered easy?
Discussion
Where is the Sun today on the previous picture?
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
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 the Earth’s rotation axis were exactly
perpendicular to the ecliptic, would we still
experience seasons?
Which planet has most the
extreme seasons?
Discussion
Another student tells you that the seasons
are caused the Earth being closer to the Sun
in the summer and farther from the Sun in
the winter. What evidence could you use to
refute this claim?
The arctic circle
The arctic circle is the northern latitude at
which on the summer solstice the Sun
never sets and on the winter solstice the
Sun never rises above the horizon.
Discussion
The tropics mark the farthest points north
and south where the Sun can appear at the
zenith. What is the latitude of the tropics?
Discussion
What is the latitude of the arctic circle?
Discussion
From Cleveland on the vernal equinox,
describe how the sun moves through the sky
during the course of a day. (Where does it
rise, where does it cross the meridian, and
where does it set.)
Discussion
From Cleveland on the first day of summer,
describe how the sun moves through the sky
during the course of a day.
Discussion
From Cleveland on the first day of winter,
describe how the sun moves through the sky
during the course of a day.
Precession: a complication
The Earth’s rotation axis is not fixed in
space over long periods of time
The Earth’s rotation rate is nearly
constant, its speed is not
The speed at which points on the Earth’s
equator are moving is larger than points on
the Earth at higher latitudes. At the equator
you would be moving at 1,650 km/hr, while at
the north pole you would not be moving at all,
just rotating around a point.
Discussion
Why is Kennedy Space Center in Florida and
not in Maine?
Florida is closer to the equator and a space
shuttle just sitting on the launch pad is
moving about 1,550 km/hr. If we moved
the launch pad to Maine, the space shuttle
sitting on the launch pad is only moving
1,275 km/hr or about 275 km/hr less than
in Florida. To launch the space shuttle in
Maine would require more fuel to
accelerate the shuttle the extra 275 km/hr.
Equatorial Bulge
The extra velocity at the equator pushes the
matter out at the equator.
The same thing happens on a merry-go-round.
The closer you are to the edge, the faster you
will be moving, and the greater the force
pushing you off.
The Earth is not a perfect sphere
The Earth bulges out at its equator, that is, its
diameter measured along its equator is 43
km larger than its diameter from the north to
the south pole.
Oblateness
Precession
The Sun and Moon apply a
torque to the Earth
Because the Earth is not a perfect sphere
and its rotation axis is tilted 23.5 degrees
from its orbital plane, the Sun and the Moon
pull on the extra mass in Earth’s equatorial
bulge and try to straighten out the tilt.
Precession
This off axis force, or torque on the Earth
causes Earth’s rotation axis to vary slightly,
or precess, over long periods of time.
Thus, the celestial poles trace out a circle
against the stars over a period 26,000 years.
Discussion
The Great Pyramid at Giza has a tunnel which
points toward the north celestial pole. At the
time the pyramid was built, around 2600 BCE,
toward which star did it point?