Download DTU 8e Lecture PPT Chap 1 Discovering the Night Sky v2

Neil F. Comins • William J. Kaufmann III
Discovering the Universe
Eighth Edition
CHAPTER 1
Discovering the Night Sky
Cyclic motions of the Sun and stars in our sky
are due to motions of the Earth
1. ROTATION = the spin of the Earth on its axis. It takes
one day (24hrs) for the Earth to complete one rotation. If
observing Earth from the north pole, the earth rotates
counter-clockwise.
2. REVOLUTION = the movement of the Earth in orbit
around the sun. It takes one year (365.25 days) for the
Earth to complete one revolution.
A sidereal month is the time it takes for the Moon to orbit the Earth with
respect to the stars and is 27.3 days long.
A synodic month is the time it takes for the Moon to orbit the Earth with
respect to the Sun and is 29½ days long.
The two times
are different
because the
Earth moves in
its orbit around
the Sun as the
Moon moves in
its orbit around
the Earth.
Another familiar cycle is the lunar cycle. When the Moon orbits the Earth,
the amount of the side facing the Earth that is lit changes, creating the
Moon’s phases. This phase cycle is called the synodic period and is 29½
days long.
Synchronous Rotation of the Moon
The motion of the Moon around Earth as seen from above Earth’s
north polar region (ignoring Earth’s orbit around Earth-Moon
barycenter). For the Moon to keep the same side facing Earth as it
orbits our planet, the Moon must rotate on its axis at precisely the
same rate that it revolves around Earth.
One common
misconception is that
the Moon is only
visible at night.
However, the time of
day in which the
Moon is in our sky
varies depending on
its phase. This
picture clearly
displays the Moon,
visible during the day.
During a new or full moon phase, when the Moon, Sun, and Earth are
aligned, the Moon may enter the shadow of the Earth, or the shadow of the
Moon may reach the Earth, creating eclipses. However, these eclipses do
not occur during every full or new moon because the Moon’s orbit is tilted
by 5 with respect to the Earth-Sun (ecliptic) plane.
Types of Lunar Eclipses:
PENUMBRAL = the Moon appears
dimmed.
PARTIAL = part of the Moon enters the
umbra of the Earth’s shadow and is
darkened.
TOTAL = all of the Moon enters the Earth’s
shadow and becomes a reddish color, only
lit from light bending around the Earth’s
atmosphere.
During a total lunar eclipse, the Moon moves in and out of the
umbra of the Earth’s shadow.
Solar Eclipses
Solar Eclipses:
If you are located where
the umbra of the Moon’s
shadow reaches, you will
see a total solar eclipse,
during which the entire
disk of the Sun is covered
by the Moon, revealing
the faint solar corona
surrounding the Sun.
Unlike lunar eclipses,
solar eclipses occur
at specific places on the
Earth, indicated by the
arrow.
Partial Solar Eclipse:
occurs in places
covered by the
penumbra of the Sun’s
shadow
Annular eclipses:
Sometimes eclipses occur
when the Moon is too far away
from the Earth to completely
cover the Sun in our sky. When
this occurs, the Moon appears
in the center and a thin ring, or
“annulus,” of light surrounds it.
These are called annular
eclipses.
http://www.youtube.com/
watch?v=CTbIufApsSk
Eclipse Paths for Total and Annular Eclipses 2001–2020
http://eclipse.gsfc.nasa.gov/eclipse.html
This map shows the eclipse paths for the 14 total solar and 13 annular
eclipses that occur between 2001 and 2020. In each eclipse, the
Moon’s shadow travels along the eclipse path in a generally eastward
direction across the Earth’s surface.
Summary of Key Ideas
Patterns of Stars
The surface of the celestial sphere is
divided into 88 unequal areas called
constellations.
 The boundaries of the constellations run
along lines of constant right ascension or
declination.

Earthly Cycles


The celestial sphere appears to revolve around
the Earth once in each day-night cycle. In fact, it
is the Earth’s rotation that causes this apparent
motion.
The poles and equator of the celestial sphere
are determined by extending the axis of rotation
and the equatorial plane of the Earth out onto
the celestial sphere.
Earthly Cycles


Earth’s axis of rotation is tilted at an angle of
231⁄2° from the perpendicular to the plane of the
Earth’s orbit (the plane of the ecliptic). This tilt
causes the seasons.
Equinoxes and solstices are significant points
along the Earth’s orbit that are determined by
the relationship between the Sun’s path on the
celestial sphere (the ecliptic) and the celestial
equator.
Earthly Cycles


The Earth’s axis of rotation slowly changes
direction relative to the stars over thousands of
years, a phenomenon called precession.
Precession is caused by the gravitational pull of
the Sun and Moon on the Earth’s equatorial
bulge.
The length of the day is based upon the Earth’s
rotation rate and the average motion of the Earth
around the Sun. These effects combine to
produce the 24-hour day upon which our clocks
are based.
Earthly Cycles

The phases of the Moon are caused by the
relative positions of the Earth, Moon, and Sun.
The Moon completes one cycle of phases in a
synodic month, which averages 29½ days.

The Moon completes one orbit around the Earth
with respect to the stars in a sidereal month,
which averages 27.3 days.
Eclipses


The shadow of an object has two parts: the
umbra, where direct light from the source is
completely blocked; and the penumbra, where
the light source is only partially obscured.
A lunar eclipse occurs when the Moon moves
through the Earth’s shadow. During a lunar
eclipse, the Sun, Earth, and Moon are in
alignment with the Earth between the Sun and
the Moon, and the Moon is in the plane of the
ecliptic.
Eclipses


A solar eclipse occurs when a strip of the Earth
passes through the Moon’s shadow. During a
solar eclipse, the Sun, Earth, and Moon are in
alignment with the Moon between the Earth and
the Sun, and the Moon is in the plane of the
ecliptic.
Depending on the relative positions of the Sun,
Moon, and Earth, lunar eclipses may be
penumbral, partial, or total, and solar eclipses
may be annular, partial, or total.
Key Terms
angle
angular diameter
(angular size)
annular eclipse
arc angle
celestial equator
celestial pole
celestial sphere
circumpolar star
constellation
declination
degree
diurnal motion
eclipse path
ecliptic
equinox
gravitation
line of nodes
lunar eclipse
lunar phase
north celestial pole
partial eclipse
penumbra
penumbral eclipse
precession
precession of the
equinoxes
revolution
right ascension
rotation
scientific notation
sidereal month
sidereal period
solar corona
solar day
solar eclipse
south celestial pole
summer solstice
synodic month
terminator
time zone
total eclipse
umbra
vernal equinox
winter solstice
zenith
zodiac
WHAT DID YOU THINK?
Is the North Star—Polaris—the brightest
star in the night sky?
 No. Polaris is a star of medium brightness
compared with other stars visible to the
naked eye.

WHAT DID YOU THINK?


Do astronomers regard constellations as the
familiar patterns of stars in the sky?
Astronomers sometimes use the common
definition of a constellation as a pattern of stars.
Formally, however, a constellation is an entire
area of the celestial sphere and all the stars and
other objects in it. Viewed from Earth, the entire
sky is covered by 88 different-sized
constellations. If there is any room for confusion,
astronomers refer to the patterns as asterisms.
WHAT DID YOU THINK?
What causes the seasons?
 The tilt of the Earth’s rotation axis with
respect to the ecliptic causes the seasons.
They are not caused by the changing
distance from the Earth to the Sun that
results from the shape of Earth’s orbit.

WHAT DID YOU THINK?
When is the Earth closest to the Sun?
 On or around January 3 of each year.

WHAT DID YOU THINK?
How many zodiac constellations are
there?
 There are 13 zodiac constellations, the
lesser-known one being Ophiuchus.

WHAT DID YOU THINK?
Does the Moon have a dark side that we
never see from Earth?
 Half of the Moon is always dark.
Whenever we see less than a full Moon,
we are seeing part of the Moon’s dark
side. So, the dark side of the Moon is not
the same as the far side of the Moon,
which we never see from Earth.
