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Earth-Sun motion and seasons


It is colder in the winter than in the summer. We have different seasons throughout the year. But why?

Earth spins on its axis – but it’s axis is not perpendicular to the plane of its orbit around the sun.

Instead it is tilted 23.5 degrees from the perpendicular.

As the Earth moves around the Sun, the axis keeps pointing in the same direction.

On June 21 or 22 each year the axis is such that the Northern Hemisphere is “leaning” 23.5 degrees toward the sun. This is the summer solstice, first day of summer.

Six months later, in December, when Earth has moved to the opposite side of its orbit, the axis points  23.5 degrees away from the sun. This is the winter solstice, the first day of winter.

On days between these extremes, Earth’s axis is leaning at amounts less than 23.5 degrees to the rays of the sun.

Because the axis remains pointed toward the North Star as Earth moves around the sun, the position of Earth’s axis to the sun’s rays is constantly changing.

If the axis were not tilted then we would not have seasonal changes.

How about if we lived in the southern hemisphere?

“Our first day of winter (December 21), it is the first day of summer for places like South Africa, Australia, and Argentina. This is because when the northern hemisphere is tilted away from the sun, the southern hemisphere is tilted toward the light and vice versa. Whichever pole is tilted toward the sun experiences summer on that solstice, and the other hemisphere experiences winter.”

  • Mr. Gruszka’s Earth Science GIFtionary

Winter Seasons

“This shows the orientation of the Earth on June 21. On this day, the north pole is tilted toward the sun and the south pole is tilted away from it. Because of this, June marks the beginning of the summer for places like NYC and also the beginning of winter for other places like Australia. “

Summer seasons

This motion causes the angle of the noon sun to vary over the course of a year.

For example, a midlatitude city like New York, located about 40 degrees north latitude, has a maximum noon sun angle of 73.5 degrees when the sun’s vertical rays reach their farthest northward location in June. Six months later, New York has a minimum noon sun angle of 26.5 degrees.

Tilt of the Earth's atmosphere

Tarbuck Lutgens


The equinoxes occur midway between the solstices.

September 22 or 23 is the date of the autumnal equinox in the Northern Hemisphere.

March 21 or 22 is the date of the spring equinox for the Northern Hemisphere.

On these dates, the vertical rays of the sun strike the equator (0 degrees latitude) because Earth is in a position in its orbit so that the axis is tilted neither toward nor away from the sun.

Solstices and Equinoxes


So consider what happens when you are near the north pole, above the Arctic circle.  During summer time your part of the Earth is always pointing towards the sun.

Arctic summer Sun doesn't set gif



Learning Standards


8.MS-ESS1-1b. Develop and use a model of the Earth-Sun system to explain the cyclical pattern of seasons, which includes Earth’s tilt and differential intensity of sunlight on
different areas of Earth across the year.

College Board

ESM-PE.2.5.2 Construct a representation of Earth’s orbital path and rotation, and identify the length of time each takes. Identify key positions (solstices and equinoxes; perihelion and aphelion) of Earth throughout its orbit, and link these positions to seasonal changes.

Benchmarks for Science Education, American Association for the Advancement of Science

Because the earth turns daily on an axis that is tilted relative to the plane of the earth’s yearly orbit around the sun, sunlight falls more intensely on different parts of the earth during the year. The difference in intensity of sunlight and the resulting warming of the earth’s surface produces the seasonal variations in temperature. 4B/H3** (BSL)

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