Earth’s Movements and Eclipses

Earth’s Orbit of Sun

• There are 365 days in a year. Every day the sun rises and sets. This proves that the Earth is in a state of motion.

• DEFINITIONS:– Revolution: movement of one object travelling

around another object

– Rotation: spinning of an object around its axis

Rotation• Earth spins like a top would.

• It takes Earth 24 hours to complete one rotation.

• This is what defines a day.

• Earth rotates from east to west. This causes the sun to rise in the east and set in the west.

Revolution• Earth moves through space around

the sun.

• The Earth completes one revolution around the Sun (940 000 000 km) in 365.24 days. This defines a year.

• The shape of Earth’s orbit is called an ellipse.

Revolution• The sun is not exactly in the centre of

the ellipse. So the Earth is not the same distance from the sun at all times.

• The point in the orbit where the Earth is closest to the sun is called the perihelion. Occurs in January (i.e. Earth is closest to Sun in winter)

• The point in the orbit where the Earth is farthest from the sun is called the aphelion. Occurs in July (i.e. Earth is farthest from Sun in summer)

Seasons

• Seasons are caused by the tilt of Earth’s axis.

Seasons• Summer Solstice (First Day of Summer)

– When Earth is tilted towards Sun– June 21 in North America– Longest day of the year (most daylight)

• Winter Solstice (First Day of Winter)– When Earth is tilted away from Sun– December 21 in North America– Shortest day of the year (least daylight)

• Equinoxes (First Day of Fall/Spring)– When Earth’s axis is not tilting toward/away from Sun– September 20 and March 22 in North America– Day and night are same length (12 hours of daylight)

The Moon• The moon is made up of elements

like:– uranium, thorium, potassium,

oxygen, silicon, magnesium

• Origin of the Moon:

– The “impact theory” states that the Moon formed around 4.5 billion years ago when Earth collided with a large object.

– The ejected materials were caught in a stable orbit around Earth.

Moon – Lunar Phases• As the moon orbits the Earth, the sun

shines on different parts of the moon on the side we can see.

• The moon goes from new moon (dark) to full moon (bright).

• Different phases:– Waxing: growing (becoming more full)– Waning: getting smaller (becoming more

dark)

Lunar Eclipses• A lunar eclipse occurs when the

moon passes through the Earth’s shadow. An eclipse can only occur at full moon.

• This doesn’t happen all of the time because the moon’s orbit is tipped slightly, so most of the time it passes above or below the Earth’s shadow.

• There are two parts to the Earth’s shadow: the umbra (blocks all sun) and penumbra (blocks part of the sun)

• Three types of lunar eclipses:

– Penumbral: moon passes through penumbral shadow. Very difficult to detect.

– Partial: part of the moon passes through the umbral shadow. Easy to detect.

– Total: entire moon passes through Earth’s umbral shadow.

Solar Eclipses• A solar eclipse occurs when the Moon

blocks light from the Sun and causes a shadow to be cast on a small area of the Earth. This can only occur at a new moon when the moon is between the Earth and the Sun.

• This doesn’t happen all of the time because the moon’s orbit is tipped slightly, so most of the time it passes above or below the Earth’s shadow.

• The moon has two parts to its shadow:– Penumbra: faint, outer shadow– Umbra: inner, dark shadow

• Solar eclipses can be dangerous to your eyes. Never look at one directly.

• Three types of solar eclipses:

– Partial: The moon partially overlaps the sun and blocks only part of the sun from our view

– Total: The sun and the moon appear to be the same size and the moon exactly covers the sun.

– Annular: The moon appears smaller than the sun, looks like a bull’s eye