movements of earth & the moon the night sky: 1.3

Movements of Earth & the Moon

The Night Sky: 1.3

Earth and Motion• Rotation vs. Revolution

– Because of Earth’s rotation, everything appears to rise in the East and set in the West

– Because of Earth’s revolution, different constellations are visible at different times of year

Rotation

Earth’s Orbit• Orbits result from 2 types of motion:

– The straight line motion of the planets past the sun

– The motion of the planet towards the sun (due to gravitational pull)

• As a result, the planet falls into a stable orbit around the sun

Earth’s Orbit (not even close to scale)

Earth’s Orbit• The path around the

Sun is not a perfect circle – it’s an ellipse!– A stretched out circle

• An ellipse has 2 focal points-defines the shape

• The Sun is at one of the focal points

Earth’s Orbit

• The Earth’s orbit brings it closest to the Sun in January– Perihelion

• It is furthest from the Sun in July– Aphelion

Earth’s Orbit

Reason for the Seasons

•But wait!• Why do we experience winter when

Earth is actually closest to the sun??• Earth does not rotate upright

– Axis is tilted at 23.5°

Reason for the Seasons• In the summer, the northern

hemisphere is tilted towards the Sun• In the winter, the northern hemisphere

is tilted away from the Sun

• The opposite occurs in the southern hemisphere– Aussies have Christmas

in 30°C heat!

Reason for the Seasons• The distance between Earth and the

Sun has a smaller effect than the tilt of the axis towards the sun

• Even though Earth is further from the sun during the summer, the northern hemisphere is tilted towards the sun

Reason for the Seasons

• Winter solstice ~ Dec 22 N. pole tilted farthest from the Sun– Shortest day of the year

• Summer solstice ~ June 22 N. pole tilted towards sun– More direct (intense) sunlight

for longer periods

Reason for the Seasons

Moon’s Motion• Theory of origin: A mars-sized body hit

a young Earth, blasting material into orbit around it– Material fused together to form the moon

and was retained by Earth’s gravitational pull

• During the night, the moon seems to move across the sky. Why?– Due to Earth’s rotation

Moon’s Motion• As the moon completes 1 orbit around

Earth, it rotates once on its axis (27.3 days)– We only observe the same side of the moon

• We cannot see the other side of the moon – nicknamed “the dark side of the moon”– Misnomer: it’s really just the far side of the

moon

Phases of the Moon• The revolution of the moon around Earth

makes the moon appear as if it is changing shape in the sky

• This is caused by the different angles from which we see the bright part of the moon’s surface

• The moon doesn’t generate any light itself, it reflects the light of the sun

Phases of the Moon

• Monthly progression of changes in appearance of the moon are called phases– Resulting from different portions of the

moon’s sunlight side being visible from Earth

• There are 8 distinct, traditionally recognized stages– Beginning with New Moon…

New Moon?

Phases of the Moon

1. New Moon

2. Waxing Crescent Moon

3. Quarter Moon

4. Waxing Gibbous Moon

5. Full Moon

6. Waning Gibbous Moon

7. Last Quarter Moon

8. Waning crescent Moon

Eclipses

• Eclipse: Phenomenon in which one celestial object moves directly in front of another celestial object– Lunar eclipse– Solar eclipse– Bella eclipse?

• https://www.youtube.com/watch?v=nXseTWTZlks

Lunar Eclipses• Earth’s shadow is divided into 2 parts

– Umbra: inner shadow– Penumbra: outer shadow

• During a lunar eclipse, the full moon passes through the umbra

Solar Eclipses• Phenomenon where the shadow of the moon

falls on Earth’s surface– Moon is between the sun and Earth

• ~ 2/year, but you need to be exactly in moon’s umbra very rare!

Tides• What causes tides?

– Forces of gravity exerted by the moon and Earth

• Gravitational force depends on the masses of 2 or more objects and the distance between them– The larger the masses, the greater the force– The lesser the distance, the greater the force

• The moon is held in orbit due to gravitational forces– The moon’s gravity pulls on the pulls on Earth– The earth’s gravity pulls on the moon

Tides

• It is not the direct pull of the moon’s gravity that causes tides– It’s the difference between 2 forces:

• The force of gravity on the side of Earth closest to the moon

• The force of gravity on the side of earth furthest from the moon

– This results in a stretching force, called tidal force

Tidal Force

• https://www.youtube.com/watch?v=WGKgKayuC2M