The Reason for the Seasons

The Sun in the Sky

Ecliptic is the Sun’s apparent path through the celestial sphere.

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.

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 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

What causes the seasons?

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.

How do we mark the progression of the seasons? • We define four special points:

summer solsticewinter solsticespring (vernal) equinoxfall (autumnal) equinox

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.

Phases of the Moon

• Why do we see phases of the Moon?

• What causes eclipses?

Our goals for learning:

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 Although the Moon is Moon is

always ½ lit always ½ lit by the Sun, by the Sun,

we see we see different different

amounts of amounts of the lit portion the lit portion

from Earth from Earth depending on depending on

where the where the Moon is Moon is

located in its located in its orbit.orbit.

• Moon is illuminated (always ½) by Sun

• We see a changing combination of the bright and dark faces as Moon orbits the Earth

Animations at links from our website

The full moon rises at approximately:

A.MidnightB. SunsetC. SunriseD.9 or 10 p.m.E. 3 or 4 a.m.

If you were on the Moon, would the Earth,

A.Show no phasesB. Show phases the same as the moon (when it is full Moon it

is full Earth, etc.)C. Show phases opposite to the Moon (when it is full Moon it

is new Earth, etc.)

Make a sketch to decide!

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