WinterSolstice

Dec. 21, 22

SpringVernal Equinox

March 20,21

SummerSolsticeJune 21

Fall Autumnal Equinox

Sept. 22,23

Seasons

• Patterns of temperature changes and other weather trends over the course of a year are known as seasons.

153,000,000 km

148,000,000 km

April

May

July

August October

November

January

February

Which hemispherewould have

theseparticular seasons?

Not to scale

• What causes the seasons on Earth?

The tilt of the earth on its axis at a 23.5 degree angle.

• Not because we are closer to the sun!!!

•What is it about the answer to the previous question that actually makes it warmer or hotter in the summer and cooler or colder in the winter?

• Two reasons: 1.The sun’s rays are more direct

during summer 2.The days are longer. Opposite for winter!

WinterSolstice

Dec. 21, 22

SpringVernal Equinox

March 20,21

SummerSolsticeJune 21

Fall Autumnal Equinox

Sept. 22,23

Seasons

153,000,000 km

148,000,000 km

April

May

July

August October

November

January

February

Which hemispherewould have

theseparticular seasons?

Not to scale

The Earth's seasons are not caused by the differences in the

distance from the Sun throughout the year (these differences are

extremely small). The seasons are the result of the tilt of the Earth's

axis.

• Summer is warmer than winter because the Sun's rays hit the Earth at a more direct angle during summer than during winter and also because the days are much longer than the nights during the summer. During the winter, the Sun's rays hit the Earth at an extreme angle, and the days are very short. These effects are due to the tilt of the Earth's axis.

SolsticesThe solstices are days when the Sun reaches its farthest northern and southern declinations. The winter solstice occurs on December 21 or 22 and marks the beginning of winter (this is the shortest day of the year). The summer solstice occurs on June 21 and marks the beginning of summer (this is the longest day of the year).

Equinoxes

Equinoxes-sunlight shines equally on the northern and southern hemispheres(equal hours of daylight & nighttime)The vernal equinox occurs in late March (20-21); the autumnal equinox occurs in late September (22-23).

WinterSolstice

Dec. 21, 22

SpringVernal Equinox

March 20,21

SummerSolsticeJune 21

Fall Autumnal Equinox

Sept. 22,23

Seasons

153,000,000 km 148,000,000 km

April

May

July

August October

November

January

February

Which hemispherewould have

theseparticular seasons?

Not to scale

Following are some common misconceptions regarding the seasons.

• Often students who believe that the seasons are caused by earth’s distance to the sun think that earth orbits the sun in an elongated elliptical path and that this is what causes earth’s distance from the sun to vary enough to cause the seasons. In fact, earth’s orbit, while elliptical, is nearly a perfect circle.

• Others may think that the sun is not in the center of earth’s orbit, thus causing earth to be closer or farther away from the sun at different times. While it’s true that the sun is at one focus of earth’s elliptical orbit, the fact that the orbit is nearly a perfect circle means that the distance from earth to the sun remains nearly constant all year.

• Even people that know that earth’s tilt has something to do with the seasons might think that there is something about the tilt that causes earth to be much closer to the sun at certain times of the year. In fact, the tilt does not make any significant difference in the distance of earth to the sun.

• Results from Earth’s 23.5° tilt on its axis and its orbit relative to the Sun – Equinox – sunlight shines equally on the northern and southern

hemispheres(equal hours of daylight & nighttime)– Solstice – sunlight is at its maximum in one hemisphere and a

minimum in the other hemisphere (winter-shortest # of daylight hours; summer-highest # daylight hours)

http://astro.unl.edu/classaction/animations/coordsmotion/sunsrays.html

• http://www.learner.org/jnorth/tm/mclass/eclipticsimulator.swf