1. Seasons of the Year

2. 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.

3. Seasons

4. We have four seasons: why?
The Earth’s axis of rotation is tilted at 23.5° from the perpendicular

5. Most important variables
Earth’s orbit is 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

6. 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!!!

7. Seasons What two things cause the difference in the seasons?
The angle of the sun’s rays
The length of the days

8. There are two kinds of rays from the Sun:
Direct- occurs when Earth is tilted toward the Sun
Indirect-occurs when Earth is tilted away from the Sun

9. 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)

11. Seasons in the Northern Hemisphere
The primary cause of the seasons is the 23.5 degree of the Earth's rotation axis with respect to the plane of the ecliptic
As the Earth goes around its orbit the Northern hemisphere is at various times oriented more toward and more away from the Sun

12. The Seasons in the Northern Hemisphere

13. The Seasons in the Southern Hemisphere

14. Earth-Sun Relations

15. 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.

16. 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.

18. On all days of the year the earth is tilted by 23
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.

20. 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?

21. 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.

22. 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.

23. Night and Day

24. 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.

25. One important question…
What direction does the Earth rotate?
To simply answer the question the Earth rotates to the east.
Read more:

26. What causes night and day?
The rotation of the Earth

27. Earth-Sun Relations

28. Rotation and Revolution
What causes night and day?
What causes the seasons to change?

29. Rotation What 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

30. 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.

31. Summary What 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.

32. 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.

33. 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

34. 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.

35. There are two kinds of rays from the Sun:
Direct- occurs when Earth is tilted toward the Sun
Indirect-occurs when Earth is tilted away from the Sun

36. What type of movement causes night and day?
The Earth’s rotation
What type of movement causes the change of seasons?
The Earth’s revolution- the angle of the sun, and the length of the day.

37. How do we know that the Earth rotates?
Star trails- trails made with time exposure, circular paths that occur because the planet is moving and not stationary.
Winds curve because of the Earth’s rotation (Coriolis Effects)
Ocean currents curve clockwise in the northern hemisphere, and counterclockwise in the southern hemisphere.

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

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

40. 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.

41. Eclipses

42. Solar and Lunar Eclipses
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

43. 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.
Use the interactive figure to show the conditions for the 3 types of solar eclipse.

44. Solar Eclipses
The 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.

45. 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

46. 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.
Use the interactive figure to show the conditions for the 3 types of lunar eclipse.