winter solstice- also known as midwinter, is an astronomical phenomenon marking the day with the shortest period of daylight and the longest night of the year.
summer solstice- solstice that marks the onset of summer, at the time of the longest day, about June 21 in the northern hemisphere and December 22 in the southern hemisphere.
vernal equinox- the equinox in spring, on about March 20 in the northern hemisphere and September 22 in the southern hemisphere.
autumnal equinox- the equinox in autumn, on about September 22 in the northern hemisphere and March 20 in the southern hemisphere.
23.5 degrees-The four positions of Earth where the 23.5 degree tilt marks the beginning of each season. Seasons on Earth are caused by its 'tilt' or spin axis of 23.5° off perpendicular to the orbital plane.
Northern and and Southern Hemispheres-The planet's four hemispheres are each shaded a dark gray. The Equator, that imaginary horizontal line at 0º degrees latitude at the center of the earth, divides the earth into the Northern and Southern Hemispheres.
seasons-each of the four divisions of the year (spring, summer, autumn, and winter) marked by particular weather patterns and daylight hours, resulting from the earth's changing position with regard to the sun.
