The Seasons, Solstices and Equinoxes

Vocabulary Books: 1-4 pages 13-17

Earth’s Orbital Motion Seasons Orbit Revolution Rotation Ecliptic Siderial Year Tropical Year Seasons Orbit: a course, range, or curved path that is followed Revolution: the Earth’s path around the sun Rotation: the movement of earth around its axis Ecliptic: the apparent path of the sun on the celestial sphere over the course of a year Siderial year: the time required for the Earth to reach exactly 1 orbit around the sun Tropical year: the year that our calendar measures Seasons:

????? What causes the seasons? Why is it cold in the winter and warm in the summer in North America?

Our Earth

What month are we closest to the sun, what month are we farthest?

Earth has an elliptical (not perfectly round) orbit around the sun. Aphelion – farthest Away Perihelion- closest Perhelion: when the earth is closest to the sun. Aphelion: when the earth is farthest from the sun. Notice how we are farther at the height of summer! SIDEREAL YEAR: one complete orbit (period) 365.256 days

Recall that the Earth is tilted on its axis by 23 Recall that the Earth is tilted on its axis by 23.5 degrees As earth orbits around the sun, it tilts toward the sun or it tilts away from the sun.

Because Earth is at an angle, the SUN SEEMS to travel at an angle. This apparent path of the sun is called the ECLIPTIC. All the planets stay very near the ecliptic in their orbits.

Solstice When the sun is farthest north or south of Earth on the ecliptic North = summer solstice South = winter solstice

Winter Solstice

Summer Solstice Earth’s Axis

When Earth’s tilt isn’t towards or away from sun Equinox When Earth’s tilt isn’t towards or away from sun Sun crossing celestial equator Spring (vernal) sun moving north of equator Fall (autumnal)  sun moving south of equator Day is equal in length to night

Another way to look at this: The yellow dot is the North Pole Another way to look at this: The yellow dot is the North Pole. Notice how the pole is ‘to the side’ of the center of the sun during equinoxes, and directly centered on the sun during solstices.

Seasons So… has Polaris always been our ‘north star”? The earth is not a perfect circle, it’s a sphere. So….. It wobbles, like a top as it rotates (spins) This wobble is called precession. So… has Polaris always been our ‘north star”?

U.S. has Time Zones 6 Why are time zones not always straight lines if they follow longitude meridians?

World Time zones 24 hours in a day = 24 time zones Where do they start?

Earth is a circle, 360° divided by 24 hrs in a day… Time Zones Time zones are separated by Time Meridians. There are 24 time meridians. Earth is a circle, 360° divided by 24 hrs in a day… Each time zone is 15° of longitude (one hour of travel = 15° longitude) .

It is also colder during the winter because of the angle of the Earth’s tilt. Sunlight can’t hit with full force, but is spread out and weakened, so less able to heat up the Earth.

This tilt of our axis also accounts for the different heights of the sun at noon.