Download Chapter 2 Knowing the Heavens

Guiding Questions
1. What role did astronomy play in ancient civilizations?
2. Are the stars that make up a constellation actually
close to one other?
3. Are the same stars visible every night of the year?
What is so special about the North Star?
4. Are the same stars visible from any location on Earth?
5. What causes the seasons? Why are they opposite in the
northern and southern hemispheres?
6. Has the same star always been the North Star?
7. Can we use the rising and setting of the Sun as the
basis of our system of keeping time?
8. Why are there leap years?
Naked-eye astronomy had an important
place in ancient civilizations.
Chichén Itzá in Yucatán
Stonehenge in British Isles
Medicine Wheel in Wyoming
Casa Grande in Arizona
Eighty-eight
constellations
cover the entire
sky.
Eighty-eight
constellations
cover the entire
sky.
Eighty-eight constellations cover the
entire sky.
• 6000 stars visible to
unaided eye (only half
are above the horizon).
• 88 semi-rectangular
groups of stars called
constellations.
• Some stars in the
constellations are quite
close while others are
very far away.
Eighty-eight
constellations
cover the
entire sky.
Constellation
names are derived
from the myths and
legends of
antiquity.
Clip
The appearance of the sky changes
during the course of the night and from
one night to the next.
Diurnal Motion of the Night Sky
• Each night, most stars appear to rise in the east,
move across the sky, and set in the west because
of Earth’s rotation.
Looking toward the North
• The North Star
(Polaris) does not
appear to move.
• Stars in the northern
sky seem to move in a
counter-clockwise
sense.
• Northern stars that
never set are called
circumpolar stars.
As Earth orbits our
Sun, different
constellations are
visible at different
times of the year.
The circumpolar
constellations are
always the same
because they are visible
no matter where Earth
is in its orbit.
It is convenient
to imagine that
the stars are
located on a
celestial sphere.
Celestial equator:
splits the sky into a
northern half and a
southern half.
North celestial pole:
the point directly
above Earth’s
rotation axis.
Celestial
Coordinates
pinpoint positions
on the celestial
sphere.
Right Ascension:
how far objects are to
the east of the vernal
equinox (Sun’s position on
March 21).
Declination: how far
objects are above or
below the celestial
equator.
The point directly
overhead is called
the ZENITH.
The line that
splits the sky into
eastern and
western halves is
called the
MERIDIAN.
The celestial sphere
seems to spin
around the Earth.
The horizon is the
line that separates
what can be seen in
the sky and what
cannot.
For observers on
Earth at a latitude of
35º, the NCP is
located at an
altitude of 35º
above the horizon.
Stars near the SCP
are never visible.
Over the course of a year, the Sun’s
position in the sky changes.
The Sun’s daily path across the sky
Dec. 21
Winter Solstice
March 21
Vernal Equinox
June 21
Summer Solstice
Sept. 21
Autumnal equinox
The ecliptic is
the Sun’s
apparent path
around the
celestial sphere
over the course
of a year.
The ecliptic is
the Sun’s
apparent path
around the
celestial sphere
over the course
of a year.
The seasons are caused by the tilt of
Earth’s axis of rotation.
The seasons are caused by the tilt of
Earth’s axis of rotation.
The Moon helps to cause precession, a
slow, conical motion of Earth’s axis of
rotation.
12,000 years
from now, the
bright star
Vega will be
the “new
North Star”
because of
precession.
Positional astronomy plays an
important role in keeping track
of time.
Key Question: When is the Sun on the
meridian (directly in the south)?
• Apparent solar day: the interval between two successive
meridian transits of the Sun (varies around 24 hrs as
Earth orbits the Sun at varying speeds).
• Mean solar day: the interval between two successive
meridian transits of the Sun IF it moved at a constant rate
(exactly 24 hrs).
• Sidereal time: the interval of time between two
successive meridian transits of a star (23 hrs 56 min).
When is the Sun
overhead at my
location?
Astronomical observations led to the
development of the modern calendar
• Our Sun takes 365.24220 days to move around the celestial
sphere once (one year).
• 0.24220 fractional days is 5 hours, 48 minutes, and 46
seconds – a fraction that has caused endless headaches for
calendar makers who would rather the year was exactly 365
days long!
• In 45BC Julius Caesar decreed that years are 365 days with
one extra day added in February, every four years (good to
one day in 128 years).
• In 1582, Pope Gregory XIII introduced the currently used
Gregorian calendar that does not allow leap years in
Centuries unless the year is evenly divisible by 400 (good to
one day in 3300 years).
Guiding Questions
1. What role did astronomy play in ancient civilizations?
2. Are the stars that make up a constellation actually
close to one other?
3. Are the same stars visible every night of the year?
What is so special about the North Star?
4. Are the same stars visible from any location on Earth?
5. What causes the seasons? Why are they opposite in the
northern and southern hemispheres?
6. Has the same star always been the North Star?
7. Can we use the rising and setting of the Sun as the
basis of our system of keeping time?
8. Why are there leap years?