1. Apparent Motions of Celestial Objects

2. Apparent Motions of Stars
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.
Polaris

3. 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.

4. 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
zenith
Solar Noon – Position of the Sun when it is at its highest altitude in the sky.
horizon
West
South
North
East

5. 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).

6. 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.

7. This model is widely accepted by most modern cultures.
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.