1. How do we get our seasons?

2. The AXIS is important! The axis is the imaginary line through Earth from the North Pole to the South Pole. The earth spins (or rotates) on its axis.

3. 3/5/2016The Seasons3 Day Night Cycle = Rotation

4. 3/5/2016The Seasons4 Rotation vs. Revolution Rotation is the spin of an object about its axis. – The Earth rotates once a day (once every 24 hours). Revolution is the orbit of one object around another. – The Earth revolves and makes one complete trip around the Sun every 365.26 days.

5. What Causes Earth’s Seasons? Earth’s axis is tilted 23.5 degrees – it always points in the same direction (Polaris, the North Star) as we orbit our Sun once a year This tilt causes the hemispheres to alternate in the amount of our Sun’s light and heat they receive through the year http://www.lpi.usra.edu/education/skytellers/seasons/about.shtml

6. 3/5/2016The Seasons6 Summer in Northern Hemisphere. What season is it in the Southern Hemisphere?

7. More daylight hours, more direct sunlight Northern Hemisphere Summer http://www.lpi.usra.edu/education/skytellers/seasons/about.shtml

8. The Four Seasons: The Effect of the Tilt of Earth’s Rotation Axis

9. Solstices and Equinoxes Equinox: The sun shines equally on both hemispheres. Solstice: A solstice is either of the two times of the year when the sun is at its greatest distance from the equator. Summer solstice = longest day of the year. Winter Solstice = shortest day of the year. – Spring Equinox ~ March 21 – Summer Solstice ~ June 21 – Fall Equinox ~ September 22 – Winter Solstice ~ December 21 The dates of the equinoxes and solstices are only approximate dates. – The actual length of a year is about 365 ¼ days (365 days, 5 hours, 49 minutes), not exactly 365 days. We have to add an extra day to a year every four years to keep the seasons synchronized with the seasons (leap year). Over a longer period of time, we need to skip a leap year to compensate the extra minutes we add in every leap year to keep the calendar in sync. Celestial Equator Ecliptic Plane Spring Equinox Fall Equinox Winter Solstice Summer Solstice

10. True color images June December March September http://www.nasa.gov/vision/earth/features/blue_marble.html

11. Do other planets have seasons? The seasons for other planets may be due to the changing distance to the Sun, not the tilt of their rotation axis…For example, the orbit of Pluto has a very large eccentricity. This means its orbit isn’t a perfect circle. Would it be warmer or colder when it is closest to the sun?

12. Seasons on Other Planets In some cases, the changing distances from the Sun will affect the seasons. In others, the axial tilt will make a huge difference! Uranus is completely on its side.

13. Seasons on Planets Planet Axial Tilt Eccent. Orbit Mercury 0° 0.21 88 days Venus 177° 0.01 224 days Earth23° 0.02 365 days Mars 25° 0.09 686 days Jupiter 3° 0.05 12 years Saturn 27° 0.06 30 years Uranus 98° 0.05 84 years Neptune 30° 0.01 165 years

14. Mars’ Orbit and Seasons At summer solstice, Mars is 153 million miles from the Sun At autumnal equinox, Mars is 134 million miles from the Sun At winter solstice, Mars is 128 million miles from the Sun At spring equinox, Mars is 145 million miles from the Sun Original images from http://photojournal.jpl.nasa.gov/index.html