Seasons of the Year

• If the Earth's axis were perpendicular to the ecliptic, as in the drawings here, the Sun's position in the sky would be halfway between the celestial poles, and its daily path, seen from any point on Earth, would stay exactly the same, day after day.

Seasons

We have four seasons: why?

The Earth’s axis of rotation is tilted at

23.5° from the perpendicular

Most important variables

• Earth’s orbit is 365.25 days long

• Earth’s axis of rotation tilts 23.5° from the perpendicular

– These two variables account for the seasonal changes in weather patterns that we observe in a given year

The Four Seasons

• Due to the tilt of the Earth's axis relative to the plane of its orbit.

• NOT due to changes in the distance of the Earth from the Sun!!!

SeasonsWhat two things cause the

difference in the seasons?

1. The angle of the sun’s rays

2. The length of the days

There are two kinds of rays from the Sun:

1. Direct- occurs when Earth is tilted toward the Sun

2. Indirect-occurs when Earth is tilted away from the Sun

In the Summer, we get the direct rays from the sun and the days are longer. (But we are farther away from the Sun)

In the Winter, we get indirect rays from the Sun and the days are shorter. (But we are closer to the Sun)

The Seasons in the Northern Hemisphere

The Seasons in the Southern Hemisphere

Earth-Sun Relations

Equinox and Solstice

• The angle between the Earth's axis and the Earth-Sun line changes throughout the year.

• Twice a year, at the spring and fall equinox (around March 21 and September 22--the exact date may vary a bit) the two directions are perpendicular.

• Twice a year, the angle is as big as it can get, at the summer and winter solstices, when it reaches 23.5 degrees.

• In the summer solstice (around June 21) the north pole is inclined towards the Sun, in the winter solstice (around December 21) it faces away from it.

• On all days of the year the earth is tilted by 23.5 degrees from the vertical.

• This tilt causes the seasons. • On the equinox, the earth's tilt is perfectly

perpendicular to the sun's radiation, thus from the sun's perspective the earth is tilted neither toward or away from the sun.

• At the equator, the sun will be directly overhead at solar noon on the equinox. Shadows will be cast straight down at the equator at solar noon.

Fun fact

• If June 21 is the day when we receive the most sunshine, why is it regarded as the beginning of summer and not its peak? And similarly, why is December 21, the day of least sunshine, the beginning of winter and not mid-winter day?

Answer

• Blame the oceans, which heat up and cool down only slowly. By June 21 they are still cool from the winter time, and that delays the peak heat by about a month and a half. Similarly, in December the water still holds warmth from the summer, and the coldest days are still (on the average--not always! ) a month and a half ahead.

Seasons

• During the year, the seasons change depending on the amount of sunlight reaching the Earth as it revolves around the Sun.

• The seasons are caused as the Earth, tilted on its axis, revolves (travels in a loop) around the Sun each year.

• Summer happens in the hemisphere tilted towards the Sun, and winter happens in the hemisphere tilted away from the Sun.

• As the Earth travels around the Sun, the hemisphere that is tilted towards or away from the Sun changes.

Night and Day

DAY

• Among the many things we seek to understand about the Earth is how it moves. This is important since it orbit and rotation affect many important aspects of life on planet Earth. Earth’s solar orbit determines the length of our year and the rotation of the earth gives us the day and night that helps to regulate the climate of our planet.

One important question…

• What direction does the Earth rotate?

• To simply answer the question the Earth rotates to the east.

• Read more: • http

://www.universetoday.com/74460/what-direction-does-the-earth-rotate/#ixzz2PVvOqFEU

The rotation of the Earth

What causes night and day?

Earth-Sun Relations

Rotation and RevolutionWhat causes night and day?What causes the seasons to

change?

RotationWhat does Rotation mean?Rotation occurs when something is

spinning around an axis.

What are some examples of things that rotate?

Tops, Ballerinas, Figure skaters, cyclones, Tornadoes, a merry-go-round, Earth

• Earth's rotational axis points in the same direction relative to the stars, so that the North Pole points towards the star Polaris.

• Think of the Earth as a spinning top, tipped over to one side.

SummaryWhat is the difference between

rotation and revolution?Rotation occurs when an object

spins around an axis.Revolution occurs when an object

spins around another object.

Give me an example of something that revolves.

Day/Night

• The Earth rotates around once in 24 hours - that's a rate of 1000 miles per hour!

• The time it takes for the Earth to rotate completely around once is what we call a day.

• It's Earth's rotation that gives us night and day.

Earth’s rotation

• Only 50% of the Earth’s surface can be “lit” by the sun

• Because the Earth’s axis is tilted the area covered by this 50% varies throughout the year

The tilt of the Earth's axis affects

• The Earth is tilted on it’s axis at a 23.5 degree angle affecting...

• The amount of direct sunlight (Insolation)

• The length of the day • This tilt is what causes our days to get longer in the summer and shorter in the winter.

There are two kinds of rays from the Sun:

1. Direct- occurs when Earth is tilted toward the Sun

2. Indirect-occurs when Earth is tilted away from the Sun

How do we know that the Earth rotates?

1. Star trails- trails made with time exposure, circular paths that occur because the planet is moving and not stationary.

2. Winds curve because of the Earth’s rotation (Coriolis Effects)

3. Ocean currents curve clockwise in the northern hemisphere, and counterclockwise in the southern hemisphere.

• The side of the Earth that is facing the sun has daylight, the side of the Earth away from the sun has night.

• The time it takes for the Earth to rotate completely around on its axis is what we call a day. It’s Earth’s rotation that gives us night and day. One complete rotation actually takes 23 hours, 56 minutes and 4.1 seconds.

• It takes 24 hours for the Earth to complete one rotation.

•Read more: http://www.universetoday.com/14700/how-long-is-a-day-on-earth/#ixzz2PVxVtDMd

• A year on earth is 365 days 5 hours 48 minutes and 46 seconds.

• A year is 365.24 days. Or 8,765 hours, or 526,000 minutes, or 31.6 million seconds.

• We use a 365 day year, with a leap year every 4 years.

• This tilt is what causes our days to get longer in the summer and shorter in the winter.

• Because of the Earth’s rotation the sun appears to rise in the east and set in the west.

Eclipses

Solar and Lunar EclipsesEclipse:

The total or partial obscuring of one celestial body by another…

The obscuration can be either• One celestial body blocking

the view to the other:– Solar eclipse---Moon

blocking Earth’s view to the Sun…

• One celestial body is in the shadow of another:– Lunar eclipse---Moon

is in the shadow of the Earth…

Lunar eclipse image from http://www.mreclipse.com

Solar eclipses

• Solar eclipses occur when the shadow of the Moon falls on the surface of Earth– Only people in the

shadow can see the eclipse.

• Solar eclipses can be partial, annular, or total.

• Solar eclipses can occur only at new moon.

Solar EclipsesThe solar corona is revealed during a total solar eclipse• The corona is about

one millionth times fainter than the disk of the Sun.

• Similar to observing stars next to the Sun, the light from the disk must be blocked (by the moon, or by special occulter in the telescope) before we can see the solar corona.

What Causes Eclipse? The Earth and Moon cast shadows. When either passes through the other’s shadow, we have an eclipse. Because the Sun is an extended bright object, there are two different

regions of the shadow: Penumbra is partially illuminated Umbra is completely dark

Click on the image to start animation

Lunar eclipses

• Lunar eclipses happens when the Moon passes through the shadow of the Earth– Everybody on the night side

of Earth can see the lunar eclipse.

• Lunar eclipses can be partial, penumbral, or total.

• Lunar eclipses can occur only at full moon.