Introduction to the seasons

Earth orbits the Sun once every year. In other words, the Earth’s path through space is a (roughly) circular path. It takes about 365.25 days for the Earth to complete one orbit around the Sun.

Earth spins on its axis once every 24 hours. This is what causes day and night because as your location spins from sunrise to sunset and back to sunrise again you go from Earth’s shadow to the sunlit side of the Earth, back into the shadow and around again into the light. This axis of rotation is tilted relative to Earth’ orbit around the Sun. The tilt of the Earth’s axis is 23.5° from perfectly vertical. That is, the axis tilts 23.5° down from the direction that is at 90° to the orbit of the Earth around the Sun.

Earth’s axis currently points at a point in the heavens very near to the star Polaris. That is why Polaris is called the North Star or Pole Star. No matter where the Earth is in its orbit around the Sun its axis always points toward Polaris. The relationship between the direction that the axis points and the direction of the Sun determines the seasons. It is Summer in the Northern Hemisphere (North of the equator) when the North Pole is pointing right at the Sun. It is Winter when the North Pole is pointing directly away from the Sun. Spring and Autumn begin when the axis of the Earth is at right angles to the direction of the Sun.

The seasons in the Southern Hemisphere of the Earth (South of the equator) are the opposite of the seasons in the Northern Hemisphere. For example, when it is Summer here in Maine it is Winter in Australia. When it is Autumn in England it is Spring it Argentina.

There are four special dates during the year that are considered the first day of each season in the Northern Hemisphere:

March 21^st
Vernal Equinox
1^st Day of Spring
Equal Day/Night

June 21^st
Summer Solstice
1^st Day of Summer
Longest Day

September 21^st
Autumnal Equinox
1^st Day of Autumn
Equal Day/Night

December 21^st
Winter Solstice
1^st Day of Winter
Shortest Day

Answer the following questions based on your class notes and the information above. Use complete sentences in your answers: you will need to use more than one sentence in some cases.

1. Using the dates for each of the Equinoxes and Solstices (given above) figure out what fraction of the year passes between the beginning of each of the seasons. Draw a picture of the orbit of the Earth around the Sun. You may draw it as if from above and show the orbit as a circle with the Sun in the middle. Draw the Earth on its orbit four times, once for each of the special dates.

2. Use the pictures below to draw in the Earth’s Axis and shadow for each of the special dates. Call the location on the bottom of each picture the Vernal Equinox. Label the other positions appropriately. The location of Polaris is above the paper and to your left as you look at the paper flat on the desk. Make sure the axis always points at Polaris.

3. There are four pictures below. 1 Label each of the four in order starting with the Vernal Equinox. 2 On each picture draw a line showing the orbit of the Earth. 3 Draw the Earth’s axis relative to the Sun. The North Pole should be toward the top of the page. In these pictures the axis will not always point in the same direction: sometimes the axis is parallel to the Earth’s orbit sometime perpendicular and sometimes in between. 4 Finally, shade the side of the Earth that is in shadow due to the illumination of the Sun.

4. Pick a point on the surface of the Earth in the Northern Hemisphere and mark it on each of the pictures above. To answer the following questions imagine what Day and Night would be like for a person standing in the spot you picked. Remember that as the Earth turns on its axis the spot you mark will move perpendicular to the axis that you drew. Check with your teacher to be sure that you have the axis drawn correctly for each picture. Or look it up online.
   1. A day is 24 hours long. Approximately what fraction of the 24 hours is the spot you picked in sunlight on the Vernal Equinox? So how long is the night by comparison?
   2. Is the day longer than the night or vice versa on the Summer Solstice for someone in the Northern Hemisphere? Why?
   3. What can you say about the relative length of day and night on the Autumnal Equinox?
   4. Is the day longer than the night or vice versa on the Winter Solstice for someone in the Northern Hemisphere? Why?

5. The word Equinox comes from Latin and means ‘equal night’. Explain why this is appropriate for the Vernal and Autumnal Equinoxes.
6. What is the longest day in the Northern Hemisphere? What is the longest day in the Southern Hemisphere? Why is it different?
7. Why are the seasons in the Southern Hemisphere the opposite of the seasons in the Northern Hemisphere?
8. Why do you think it is hot in the Summer and cold in the Winter?
9. Earth is 3% closer to the Sun in January than in July. Does Earth’s distance from the Sun have anything to do with what causes the seasons?
10. What hemisphere did the people live in who decided on the names of the four special dates (Vernal Equinox, etc.)? How do you know?