Apparent Motions of Celestial
Objects
Apparent
Motions of Stars
Polaris
An apparent motion is a motion
that a celestial object appears to
make across the sky. The “actual
motion” may be different.
Stars “appear” to rise
from the east and set
in the west along a
pathway known as an
arc.
Each star’s
“apparent” motion in
the sky is at a
constant rate -- 15°
per hour.
Some stars near
Polaris (North Star)
move in a complete
circle.
Apparent Motions
of the Planets and
the Moon
The “apparent” path of the
planets and Moon in the sky
is the same as that of the
stars – from an east-to-west
direction.
The position of planets in
the sky does change
throughout the year due to
their revolutions around the
sun.
The position of the Moon in
the sky changes each
evening – rising from the
east 50 minutes later each
evening and shifting more
eastward (retrograde
motion) when compared to
background stars.
Apparent Motions
of the Sun – The
Celestial Sphere
The “apparent motion” of the Sun is the same as that
of the stars, planets and Moon. The Sun “appears” to
rise from the east and set in the west.
A celestial sphere is a model to help illustrate the
“apparent motions” of objects in the sky – such as the
Sun.
Apparent path of the Sun
Solar Noon
– Position
of the Sun
when it is
at its
highest
altitude in
the sky.
zenith
horizon
West
South
North
East
Apparent Motions
of the Sun – The
Celestial Sphere
Throughout the year, the “apparent
motion” of the Sun in the celestial
sphere changes in altitude, origin and
length with the seasons.
In the Northern Hemisphere:
The Sun rises due east and sets due west in
the beginning of spring and fall.
In the Northern Hemisphere:
The Sun rises north of east and sets
north of west in the summer.
The sun’s altitude at noon is highest
during the year.
The sun’s “apparent path” across the
sky is at its longest (greater than 12
hours).
In the Northern Hemisphere:
The Sun rises south of east and sets
south of west in the winter.
The sun’s altitude is low in the sky.
The sun’s “apparent path” across the
sky is short (less than 12 hours).
Geocentric
Model
Throughout human history, ancient
civilizations and modern science have
sought to explain the “apparent motions
of celestial objects in the sky.
The geocentric model places the Earth
as a stationary body at the center of the
universe – with most celestial objects
revolving around it.
This model was widely
accepted by European
cultures until the 16th century.
The theory fails to explain the
“apparent motions” of planets and
some motions of the Earth.
Heliocentric
Model
The heliocentric model places the Sun
as a stationary body at the center of the
Solar System – with celestial objects
revolving around it.
This model is widely accepted
by most modern cultures.
In this model, the Earth rotates
upon an imaginary axis and
revolves around the Sun.
The heliocentric model helps to
explain the “apparent motions” of
the stars, planets, Sun and Moon
along the celestial sphere.