Seasons PDF

SEASONS: True color images December March June September 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...

SEASONS: True color images December March June September 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 The Four Seasons: The Effect of the Tilt of Earth’s Rotation Axis  Solstices and Equinoxes Equinox: “equal days” The Earth’s axis is perpendicular the the sun Solstice: A solstice is either of the two times of the year when the sun is at its greatest distance Ecliptic Plane from the equator. This is when the Earth’sWinter axisSolstice is Spring Equinox pointed directly at the sun Spring (vernal) Equinox ~ March 21 Summer Solstice ~ June 21 Celestial Equator Fall (autumnal) Equinox ~ September 21 Winter Solstice ~ December 21 Fall Equinox The dates of the equinoxes and solstices are only approximate dates. Fall Solstice The actual length of a year is about 365 ¼ days (365 days, 5 hours, 49 minutes) We have to add an extra day to a year every four years to keep the seasons synchronized with the seasons (leap year). Northern Hemisphere Summer More daylight hours, more direct sunlight Height of Sun for Canada Winter: Sun stays low in the sky. Spring: The Sun moves higher in the sky than in winter. Summer: The Sun travels high Fall: The sun travels to a medium-height in the sky  Daylight Hours Across the Globe Time is indicated as number of hours (h) Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Miami 10.5 h 11 h 12 h 12.5 h 13 h 14 h 14.5 h 14 h 12.5 h 12.5 h 11 h 11.5 h Brisbane 14.5 h 13.5 h 13.5 h 12.5 h 11 h 11 h 11 h 11.5 h 11.5 h 12.5 h 14 h 14.5 h Nairobi 12 h 12 h 12 h 12 h 12 h 12 h 12 h 12 h 12 h 12 h 12 h 12 h Punta Arenas 17.5 h 15.5 h 13.5 h 11.5 h 10 h 8.5 h 8.5 h 9.5 h 11.5 h 13 h 15 h 16.5 h Nome 5h 7h 10 h 13.5 h 17.5 h 21.5 h 22 h 18 h 15.5 h 11.5 h 8.5 h 5.5 h Singapore 12 h 12 h 12 h 12 h 13 h 13 h 12 h 12 h 12 h 12 h 12 h 12 h Cape Town 15 h 14 h 13.5 h 12.5 h 11.5 h 10 h 10.5 h 11 h 11.5 h 12.5 h 14 h 14 h Seattle 9h 10 h 11 h 13.5 h 15 h 17 h 16.5 h 15.5 h 13.5 h 11.5 h 10.5 h 9.5 h Vostok 24 h 24 h 19 h 14.5 h 0h 0h 0h 0h 7h 15 h 24 h 24 h http://www.lpi.usra.edu/education/skytellers/seasons/activities/light.shtml Tilt of Earth’s Spin Axis 23.5o tilt The Earth’s spin axis is tilted by 23.5 degrees off vertical with respect to the “ecliptic plane” (plane of the Earth’s orbit around the sun) The spin axis remains essentially parallel to itself during the course of the year Summer Solstice – 21 June tropic of Cancer arctic sunlight circle equator antarctic sunlight circle  Winter Solstice – 21 December tropic of Cancer arctic sunlight circle equator sunlight antarctic circle tropic of Capricorn Vernal Equinox – 21 March arctic circle sunlight tropic of Cancer equator tropic of Capricorn sunlight antarctic circle  Temperature Effect Summer days are longer and the sun is more intense (due to the more direct illumination angle). Thus summer is hotter than winter. There is a lag of the seasons when comparing the dates of the solstices with the actual extremes in temperature because it takes time to heat up the oceans and atmosphere at the onset of summer and to cool them off at the onset of winter. If the Earth’s spin axis were not tilted by some angle, we would have no seasons.