earth, the moon, and the solar system

6.1.1 earth, moon, and planetary motion

axis and rotation

the axis is an imaginary line around which the earth rotates.

• runs from antarctica to the arctic ocean
• earth completes one rotation in approximately 24 hours
• day and night are caused by rotation about the axis

equator and hemispheres

the equator is an imaginary line that divides the earth into two halves.

• above the equator is the northern hemisphere
• below the equator is the southern hemisphere

seasons

• the earth is tilted at an angle of 23.5°
• sunlight does not strike both hemispheres at the same angle
• the hemisphere tilted towards the sun experiences summer
• summer has longer days and more direct sunlight

orbit of the moon

• the moon orbits the earth approximately once every month
• only the sunlit side of the moon reflects light
• different amounts of the lit side are visible from earth
• this causes the phases of the moon

orbital speed of planets

orbital speed is calculated using distance travelled divided by time.

speed = distance / time

for a circular orbit:

distance = 2πr

speed = 2πr / orbital period

6.1.2 the solar system

components of the solar system

• the sun is a star
• there are eight planets: mercury, venus, earth, mars, jupiter, saturn, uranus, neptune
• moons orbit planets
• minor planets include pluto and similar bodies
• asteroids are mainly found between mars and jupiter
• comets are made of dust and gas

planetary orbits

• planets have elliptical orbits
• the sun is not at the centre of the orbit
• planets orbit a point between themselves and the sun

formation of planets

• the first four planets are small and rocky
• the outer four planets are large and gaseous

formation process:

• hydrogen and other elements were spread throughout space
• gravity pulled particles together to form the sun
• remaining gas and dust formed a rotating accretion disc
• gravity pulled dust together to form rocks and planets
• heat from the sun pushed lighter gases outward
• this formed the gaseous outer planets

gravitational field strength

• gravitational field strength depends on mass
• it decreases with increasing distance from the object
• the sun has the strongest gravitational field in the solar system
• orbital speeds of planets decrease with distance from the sun

changing speed of a comet

• comets have highly elliptical orbits
• comets move faster when closer to the sun
• gravitational potential energy decreases as distance decreases
• kinetic energy increases as speed increases
• total energy remains constant

total energy = gravitational potential energy + kinetic energy