The Reason for Seasons

"Beautiful is your shining forth at heaven's edge, o living Aten, beginning of life!
When you arise on the eastern horizon,you fill every land with your beauty.
You are lovely, great and glittering, and go high above every land." - Amenhotep IV of Egypt

Can you answer these questions?

1. The longest day of the year for us (in the northern hemisphere) occurs on:

a) July 4

b) June 21

c) September 22

d) December 21

e) April 14 (before taxes are due!)

2. The hottest month(s) of the year for us in the northern hemisphere are usually:

a) June-July

b) July-August

c) August-September

d) December-January

e) Whenever your car's air conditioning breaks!

3. Planet Earth is closest to the Sun during the month of:

a) June

b) January

c) August

d) July

e) Hah! You can't fool me! We are always the same distance!

4. Seasons in the southern hemisphere are:

a) Much more severe (hotter in summer, colder in winter)

b) Much milder

c) Exactly the same times and duration as ours

d) Opposite of those in the northern hemisphere

5. Summer for us in the northern hemisphere occurs because

a) The Earth is closest to the Sun during this season

b) The Earth is tilted so that the Northern Hemisphere is closer to the Sun than the Southern Hemisphere

c) The Earth is tilted so that the Sun stays up higher in the sky, for a longer time.

d) There are fewer clouds and more sunny days.

Instructions & Answers to Reasons for the Seasons!

Often we create explanations for phenomena based on our prior experiences, only to find out that those explanations are not entirely correct. But these incorrect misconceptions are very hard to dispel, to forget, to replace with the truth. Our prior ideas return again and again; we have to work hard to eliminate them altogether.

Explaining seasons is one area where we most of us have an internal "guess" or "feeling" for an explanation, created from prior schooling, information from parents or friends, and what we have heard or read. For many people who haven't studied astronomy, that personal explanation is often incorrect!

Instructions

1. Ask three friends or relatives who are NOT taking astronomy to answer the questions above, and to explain why there are seasons. Record their educational background - their level of school completed. Write down their explanations verbatim. Try to resist telling them whether they are right or wrong - instead just state that you need their opinions for your research. Did any of your respondents mention distance from the Earth to the Sun as the reason we have seasons - that we are closer to the sun in summer, and farther in winter?

2. From your reading of the textbook (and in-class discussion) you already know that distance from the sun does NOT account correctly for the seasons. But many folks aren't convinced unless they see "numbers". Examine the following data about the distance from the Earth to the Sun during the year:

Date	Earth-Sun Distance	Date	Earth-Sun Distance
January 12	147,122,000 km (91,417,000 miles)	July 12:	152,076,000 km (94,496,000 miles)
February 11:	147,623,000 km (91,729,000 miles)	August 17	151,470,000 km (94,119,000 miles)
March 26:	149,206,000 km (92,712,000 miles)	September 14:	150,499,000 km (93,516,000 miles)
April 10:	149,891,000 km (93,198,000 miles)	October 15:	149,194,000 km (92,705,000 miles)
May 23	151,452,000 km (94,108,000 miles)	November 16	147,534,000 km (91,673,000 miles)
June 15	151,947,000 km (94,415,000 miles)	December 15:	147,249,000 km (91496,000 miles)
	Perihelion		Aphelion
Jan 4	147,097,641 km (91,402,237 miles)	July 4:	152,087,478 km (94,502,778 miles)

Sources:

NASA (2006) Astronomical Unit. http://neo.jpl.nasa.gov/glossary/au.html

Espanak, F. (1999) NASA Reference Publication 1349. Twelve Year Planetary Ephemeris: 1995-2006. Goddard Space Flight Center. NASA. Available online at: http://sunearth.gsfc.nasa.gov/eclipse/TYPE/sun2.html#su2006

Questions to Ponder:

Try these on your friends and family as you explain the real reasons for seasons:

a) If distance to the sun was the most important factor, when "should" summer be (based on the table)?

b) If distance to the sun was the most important factor, and knowing the Earth is about 8,000 miles (Would the seasons for the southern hemisphere occur at the same time, or at a different time of year, compared with the northern hemisphere? Why?

3. For some folks, no amount of numbers or words will convince them. So build a model! (We do this in science all the time.)

Grab an apple, pear, or similar-sized object that is handy. Be sure it has an identifiable "top" - like the stem, or put a toothpick into the object so that you can create an "axis of rotation". Pick out a light elsewhere in the room - a lamp with a lightbulb is best, in the middle of the room, but anything will do. This light will represent the sun. Since the earth is inclined to the plane of our orbit around the sun (the ecliptic), in the summer months we (in the northern hemisphere) are tilted towards the Sun, and in winter we are tilted away. Suppose it is summer.

a) First, spin the object in your hand around that stem or toothpick. What does this motion represent?
(Answer: one day!)

b) Second, tilt the object in your hand so that the top - the stem part - is inclined a bit towards the lightbulb. What does this orientation represent?
(Answer: Summer, for those on the "top" half of the object - i.e., observers in the northern hemisphere.)

c) Third, move the object in a big circle AROUND the lightbulb. What does this represent?
(Answer: The Earth's Orbit!)

You'll have to move to do this correctly! You can't do this sitting down. But don't keep the top tilted towards the light - that would mean that the Northern Hemisphere is ALWAYS in summer! Instead, keep the object tilted the same direction (relative to your house or apartment or room) the entire time. Half of the time around the light, the stem-end will be tilted towards the light; the other half of the time, the other end (the "bottom") will be - if you are doing this step correctly.

4. When the northern hemisphere is tilted toward the sun, the north pole will be nearer to the sun, and the southern hemisphere will be farther away. (Based on the angle our planet is tilted, the northern hemisphere on June 21st is about 2,500 km closer to the sun than is the center of the planet.) Could this fact - that the northern hemisphere is closer to sun than the southern hemisphere, account for our summer and their winter? Before you answer, consider how far the northern hemisphere is from the Sun in July vs. January, regardless of whether it is tilted toward the sun.

Answers to the Reasons for Seasons Quiz:

1. (b) The longest day in the northern hemisphere is June 21st, the summer solstice. The word solstice comes from sol (the Sun) and stice (meaning stop). On this day, the Sun "stops", reaching its highest point in the noon sky, and on the succeeding days, is lower in the sky.

2. (b) The hottest months are usually July and August, after the oceans have warmed up. Note that the days are longer in June and July, but the Northern Hemisphere and its oceans are still "chilled" from winter and spring. Also note that local temperature variations may make some areas warmer in August and September ("Indian Summer").

3. (b) The Earth is actually closest to the Sun in JANUARY! Check the chart of distances if you don't believe me!

4. (d) The Southern Hemisphere has summer in December - February, and winter in June - August, exactly opposite to us in the Northern Hemisphere. And when it is spring in the northern hemisphere, it is autumn in the southern. And when it is autumn in the northern hemisphere, it is spring in the southern.

5. (c) Seasons result from the Earth's tilt of 23.5°, which means that each hemisphere will alternately be tilted towards the Sun, exposing the ground to more direct sunlight for longer periods. Just because the planet is tilted with the northern hemisphere tilted toward the Sun does NOT mean that the northern hemisphere is appreciably closer to the Sun. Recall that in July, the entire planet is much farther from the Sun than it is in January. So if distance was the key, the entire planet would still have winter, regardless of which hemisphere was tilted towards or away. The size of the earth is inconsequential compared with the difference in distance (millions of miles) between January and July.