The Reason for the Seasons

The Sun in the Sky Ecliptic is the Sun’s apparent path through the celestial sphere. If you do not have a model of the celestial sphere to bring to class, you might wish to use this slide; you will probably want to skip it if you have a model that you can discuss instead…

We can recognize solstices and equinoxes by Sun’s path across sky: Summer (June) solstice: highest path; rise and set at most extreme north of due east Winter (December) solstice: lowest path; rise and set at most extreme south of due east Equinoxes: Sun rises precisely due east and sets precisely due west. Of course, the notes here are true for a N. hemisphere sky. You might ask students which part written above changes for S. hemisphere. (Answer: highest and lowest reverse above, but all the rest is still the same for the S. hemisphere; and remind students that we use names for the N. hemisphere, so that S. hemisphere summer actually begins on the winter solstice…)

But…CLOSER means MORE right?

CLOSER means MORE right? Heat The closer you are the hotter it is

CLOSER means MORE right? Heat The closer you are the hotter it is Sound The closer you get, the louder it is

CLOSER means MORE right? Heat The closer you are the hotter it is Sound The closer you get, the louder it is Light The closer you get, the brighter it is

Complete this statement! When the Sun is ______ it is summer, and when the Sun is _______ it is winter.

Complete this statement! When the Sun is CLOSER it is summer, and when the Sun is FARTHER it is winter.

When the Sun is high in the sky, the amount of direct sunlight received is greater. This results in SUMMER

When the Sun is low in the sky, the amount of direct sunlight received is less. This results in WINTER

When the Sun is high in the sky, the amount of direct sunlight received is greater. This results in SUMMER When the Sun is low in the sky, the amount of direct sunlight received is less. This results in WINTER

What causes the seasons? Misconceptions about the cause of the seasons are so common that you may wish to go over the idea in more than one way. We therefore include several slides on this topic. This slide uses the interactive version of the figure that appears in the book; the following slides use frames from the Seasons tutorial on the Astronomy Place web site.

Summary: The Real Reason for Seasons Earth’s axis points in the same direction (to Polaris) all year round, so its orientation relative to the Sun changes as Earth orbits the Sun. Summer occurs in your hemisphere when sunlight hits it more directly and is in the sky longer; winter occurs when the sunlight is less direct and is not in the sky as long. AXIS TILT is the key to the seasons; without it, we would not have seasons on Earth.

Why doesn’t distance matter? Variation of Earth-Sun distance is small — about 3%; this small variation is overwhelmed by the effects of axis tilt. The two notes should be considered optional. If you cover the first note, you might point out that since Earth is closer to the Sun in S. hemisphere summer and farther in S. hemisphere winter, we might expect that the S. hemisphere would have the more extreme seasons, but it does not because the distance effect is overwhelmed by the geographical effect due to the distribution of oceans.

How do we mark the progression of the seasons? We define four special points: summer solstice winter solstice spring (vernal) equinox fall (autumnal) equinox Here we focus in on just part of Figure 2.13 to see the four special points in Earth’s orbit, which also correspond to moments in time when Earth is at these points.

We can recognize solstices and equinoxes by Sun’s path across sky: Summer solstice: Highest path, rise and set at most extreme north of due east. Winter solstice: Lowest path, rise and set at most extreme south of due east. Equinoxes: Sun rises precisely due east and sets precisely due west. Of course, the notes here are true for a N. hemisphere sky. You might ask students which part written above changes for S. hemisphere. (Answer: highest and lowest reverse above, but all the rest is still the same for the S. hemisphere; and remind students that we use names for the N. hemisphere, so that S. hemisphere summer actually begins on the winter solstice…)

Phases of the Moon Our goals for learning: Why do we see phases of the Moon? What causes eclipses?

The changing phases of the Moon inspired the concept of the month

Phases of the Moon: 29.5-day cycle Waxing Moon visible in afternoon/evening Gets “fuller” and rises later each day Waning Moon visible in late night/morning Gets “less full” and sets later each day

Although the Moon is always ½ lit by the Sun, we see different amounts of the lit portion from Earth depending on where the Moon is located in its orbit.

Animations at links from our website Moon is illuminated (always ½) by Sun We see a changing combination of the bright and dark faces as Moon orbits the Earth You may want to do an in-class demonstration of phases by darkening the room, using a lamp to represent the Sun, and giving each student a Styrofoam ball to represent the Moon. If you lamp is bright enough, the students can remain in their seats and watch the phases as they move the ball around their heads. Animations at links from our website

The full moon rises at approximately: Midnight Sunset Sunrise 9 or 10 p.m. 3 or 4 a.m. Answer: B

If you were on the Moon, would the Earth, Show no phases Show phases the same as the moon (when it is full Moon it is full Earth, etc.) Show phases opposite to the Moon (when it is full Moon it is new Earth, etc.) Make a sketch to decide! Answer: C

We see only one side of Moon Synchronous rotation: the Moon rotates exactly once with each orbit That is why only one side is visible from Earth Use this tool from the Phases of the Moon tutorial to explain rise and set times for the Moon at various phases. As usual, please encourage your students to try the tutorial for themselves.