Unit 1
Movement of the Earth, Seasons,
Coordinates and Observing the night sky
Celestial Sphere
Stars seem to be on
the inner surface of
a sphere
surrounding the
Earth.
Celestial Sphere
Stars that appear close in the sky may not actually be close
in space.
Celestial Sphere
Celestial Poles are
points on the
celestial sphere
north and south of
the Earth’s north and
south poles.
Celestial Equator
Imaginary
circle on the
celestial
sphere
directly above
the Earth’s
equator.
Celestial Coordinates
• Horizon – Edge of our
local sky
– We can only see half of the
celestial sphere at one
time.
• Zenith – The point straight
overhead on the celestial
sphere.
– What you see if you lie on
your back and look straight
up
Celestial Coordinates
• Altitude – How many degrees above the horizon
• Azimuth – How many degrees along the horizon from the north
Celestial Coordinates
• Declination (Dec)
• Degrees north or
south of celestial
equator
• Right ascension (Ra)
• Measured in hours,
minutes, and seconds
eastward from
position of the Sun at
vernal equinox
Celestial Coordinates
Compare and contrast
Ra, Dec, Latitude and
Longitude.
Ecliptic
• The apparent path
of the sun on the
celestial sphere
over the course of a
year.
• Tilted at an angle of
23.5° with respect
to the celestial
equator
Ecliptic
The 12 constellations the Sun moves through
during the year are called the zodiac
Location of all
planets when
they are visible
in the sky.
Diurnal Motion
Apparent rising and
setting of stars
Earth’s Orbital Motion
• Revolution – One body moves around another
• Rotation– Movement of an object around its axis
Earth’s Orbital Motion
• Solar Day – Our observed daily cycle (Noon to Noon)
• Sidereal Day – Time it takes for a star to be in the exact
same position in the sky. (23.93 hours)
Earth’s Orbital Motion
Summer Solstice is June 21 – Sun at it’s northernmost
point (above celestial equator)
Winter Solstice is December 21 – Sun at it’s southernmost
point
This is caused by the tilt
of the earth on it’s axis.
Responsible for seasons
(not distance from Sun)
Points where path of
the sun crosses
celestial equator are
vernal (March 21) and
autumnal (Sept 21)
equinoxes.
Tropical Year = 365 solar days
Interval of time from one equinox to the next
Earth’s Orbital Motion
Precession: Rotation of Earth’s axis itself
makes one complete circle in about 26,000 years
Earth’s Orbital Motion
Earth’s Orbital Motion
A sidereal year is the time it takes for Earth to orbit
once around the sun relative to fixed stars.
Tropical year follows seasons (365 days)
Sidereal year follows constellations (365.256363004 days)
In 13,000 years July and August will still be summer, but Orion will be a
summer constellation.
Why we have leap year!
Moon Phases
Moon Phases
Phases are due to different amounts of sunlit
portion being visible from Earth.
The Motion of the Moon
Synodic Month – Time it
takes the moon to go
through a whole cycle of
phases (29.5 days).
Sidereal Month - Time to make
full 360°revolution around
Earth (about 2 days shorter
than synodic month).
The Tides
Tides are due to
gravitational force
on Earth from the
Moon
The force on near
side of Earth is
greater than the
force on far side.
Water can flow
freely in
response.
The Tides
The Sun has less
effect, but it does
modify the lunar
tides.
Spring tidesmore tidal
differentiation
Neap tides – less
tidal
differentiation
The Tides
Tides tend to exert a “drag” force on Earth,
slowing its rotation.
This will continue until Earth rotates
synchronously with the Moon, so that the same
side of Earth always points toward the Moon.
The Tides
This has already
happened with
the Moon,
whose near side
is always toward
Earth.
Lunar Eclipse
Occurs when Earth
is between the
Moon and Sun
• Partial when only
part of the Moon is
in shadow
(Penumbra)
• Total when all is
in shadow (Umbra)
Lunar Eclipse
Solar Eclipse
Occurs when the Moon
is between Earth and
Sun
Solar Eclipse
Annular Solar Eclipse
Moon is too far
from Earth for
total.
Eclipses don’t occur every month
because Earth’s and the Moon’s orbits are
not in the same plane.
Solar Eclipse Tracks, 2010 - 2030
Triangulation
Measure baseline
and angles, and
you can calculate
distance.
Tan = opp / adj
Tan B = Distance to Object / baseline
Distance to Object = (Tan B)(Baseline)
Parallax
Similar to
triangulation, but
looking at
apparent motion
of object against
distant
background from
two vantage
points
Parallax
The greater the
parallax, the closer
the star
Angular Measure
The basic unit of angular
measure is the degree
(°).
Astronomers use angular
measure to describe the
apparent size of
a celestial object
The angular diameter is proportional to the actual diameter
divided by its distance. If any two of these quantities are
known, the third can be determined.
Angular Measure
Review
 Stars can be imagined to be on inside of celestial
sphere; useful for describing location.
 Plane of Earth’s orbit around Sun is ecliptic; at
23.5° to celestial equator.
 Angle of Earth’s axis causes seasons.
 Moon shines by reflected light, has phases.
 Solar day ≠ sidereal day, due to Earth’s rotation
around Sun.
 Synodic month ≠ sidereal month, also due to
Earth’s rotation around Sun
Review
 Tropical year ≠ sidereal year, due to
of Earth’s axis
precession
 Distances can be measured through triangulation
and parallax.
 Eclipses of Sun and Moon occur due to
alignment; only occur occasionally as orbits are
not in same plane.
 Apparent size can be determined using angular
measurement