Sun’s annual motion and the Seasons Living on Earth may be expensive, but it includes and annual free trip around the Sun.

Review Question Describe how the stars move through the sky if you were at the earth’s north pole. Where is the north celestial pole and the celestial equator in the sky?

Star trails…

Review Question Describe how the stars move through the sky if you were at the earth’s equator if you are facing north, south, east and west. Where in the sky is the north celestial pole, the south celestial pole and the celestial equator in the sky?

Star trails…

Discussion Which star (A, B, C, or D) will spend the least amount of time above the horizon during the course of a day?

How long do stars on the celestial equator spend above the horizon?

If the camera was pointing east, which way are the stars moving?

If the camera was pointing east, was the picture taken in the northern or southern hemisphere?

Solar and sidereal days Solar day – 24 hours Sidereal day – 23 hours 56 minutes is the actual rotation period of the Earth

Discussion About what time is it in this picture?

Discussion In what constellation will the Sun be in at 6 pm?

Discussion In what constellation will the Sun be in one month for this picture?

Celestial Coordinates Declination (Dec) – measured in degrees minutes and seconds, from the celestial equator. Right Ascension (RA) – measured in hour minutes and seconds from the vernal equinox.

The Seasons Due to the Earth’s rotation axis being tilted by 23.5 degrees from perpendicular to the plane of its orbit.

Near edge-on view

Conservation of angular momentum Anything that spins on an axis or revolves around another object has angular momentum. Conservation of angular momentum requires that the rate of spinning remains constant with time. Also, the axis of rotation of any spinning object remains in a fixed direction in space.

Discussion Why is it hard for people to learn to ride a bike, but once they learn it is considered easy?

Discussion Where is the Sun today on the previous picture?

The seasons and ellipticity The Earth’s orbit is nearly circular – distance from the Sun varies by only 3% Earth is closest to the Sun in January and farthest from the Sun in July Summer in northern hemisphere is winter in southern hemisphere

Why is summer warmer? The Sun, being above the celestial equator, remains in the sky longer during the summer, the longest daylight time occurring on the summer solstice. The Sun rays hit the earth more directly during the summer months, i.e. the summer hemisphere receives more energy per square meter.

Discussion If the Earth’s rotation axis were exactly perpendicular to the ecliptic, would we still experience seasons?

Which planet has most the extreme seasons?

Discussion Another student tells you that the seasons are caused the Earth being closer to the Sun in the summer and farther from the Sun in the winter. What evidence could you use to refute this claim?

The arctic circle The arctic circle is the northern latitude at which on the summer solstice the Sun never sets and on the winter solstice the Sun never rises above the horizon.

Discussion The tropics mark the farthest points north and south where the Sun can appear at the zenith. What is the latitude of the tropics?

Discussion What is the latitude of the arctic circle?