The Seasons

Anticipation guide # 2 1) The seasons are caused by Earth’s tilt and revolution around the Sun. 2) The length of the day is the most important factor in heating of the Earth. 3) Eclipses can only occur during full or new moon. 4) The planets in the solar system have circular orbits. 5) A light year is a measure of time.

Objective 1 Describe the characteristics of the Earth’s Moon (gravity, revolution, rotation, phases, distance from the earth, temperature) and surface features. 2 Diagram and explain the difference between a lunar and solar eclipse 3 Relate the rise and fall of tides to the gravitational pull of the Sun and Moon.

The Seasons As we can see the weather changes throughout the year. The length of the day varies from very short in December to quite long in July. As we have mentioned before these annual changes are the result of the Earth’s orbital motion around the sun and the fact that Earth does not “stand up” straight. We are tilted on our axis.

Earth’s Tilt Earth’s axis is tilted approx 23.5 degrees relative to the ecliptic plane. This orientation remains the same as earth revolves around the sun, this causes our four seasons

A Different Apparent Motion of the Sun Depending upon where you are the sun will either cross higher in the sky each day (winter and spring) or lower each day (summer and autumn) This seasonal apparent motion affects the amount of sunlight that falls on the hemisphere

The Seasons The seasons are reversed depending upon which hemisphere is being observed. Note that the months themselves DO NOT change.

The Seasons Australians are perfectly happy to celebrate Christmas with a trip to the beach. They live in the Southern Hemisphere and December 25th comes in the beginning of their summer not winter.

The Seasons Summer in the Northern Hemisphere occurs when that half of the Earth is tilted towards the sun

The Seasons Winter in the Northern Hemisphere occurs when that half is tilted away from the sun

The Seasons Spring and Autumn are the days where the tilt is neither towards nor away from the sun

The Seasons

Solstices For each revolution Earth experiences two occasions where the apparent motion of the sun reverses and heads the other way.

Solstices The Winter Solstice occurs around December 21st in the Northern Hemisphere and the Sun begins to move higher across the sky each day. Since the Sun moves higher the amount of daylight increases each day as well

Solstices The Summer Solstice occurs around June 21st in the Northern Hemisphere and marks the day where the apparent motion of the Sun moves lower in the sky each day. The amount of daylight decreases each day until the next Winter Solstice.

Equinoxes Obviously as the apparent motion of the Sun increases or decreases between each Solstice there will be two times per year when the amount of daylight and darkness match up and are equal. In the Northern Hemisphere this occurs on or around the 22nd of March (Vernal) and 22nd of September (Autumnal). The term equinox means “equal nights”

Equinoxes The Vernal equinox marks the beginning of Spring

Equinoxes The Autumnal Equinox marks the beginning of Fall

The Tropics Look at a globe and you will see two distinct lines of latitude labeled Tropic of Cancer in the Northern Hemisphere and Tropic of Capricorn in the Southern Hemisphere.

The Tropics These lines occur at 23.5 degrees of latitude and coincide with the 23.5 degree tilt of the earth on her axis. They mark the maximum latitudes that will receive direct sunlight during the year

The Tropics The Sun is directly over the Tropic of Cancer at the summer solstice in the Northern Hemisphere. (June)

The Tropics The sun is directly over the Tropic of Capricorn at the summer solstice in the Southern Hemisphere (December)

The Tropics The sun is directly over the equator on the two equinoxes

The Tropics The Arctic and Antarctic circles define areas that go with all daylight or all night on the solstices