Apparent Motions of Celestial Objects

An apparent motion is a motion that a celestial object appears to make across the sky. The “actual

motion” may be different.

Apparent Motions of

Stars

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.

Polaris

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.

East

South North

Westhorizon

zenithSolar Noon – Position of the Sun when it is at its highest altitude in the sky.

Apparent path of the Sun

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.