Dave and I are almost done with our journey. After spending an entire year in the Boundary Waters Canoe Area Wilderness, we plan to exit on Friday! Did you know that we began our year on a special day last year? It was the Fall Equinox. An equinox is one of two days each year when the length of day and night are the same. This year’s Fall Equinox is about to happen, on September 22 to be exact. In this Notes from the Trail we’ll learn about equinoxes and why the seasons change.
What is an equinox?
An equinox happens twice a year. Another name for the Fall Equinox is the Autumnal Equinox. It is the first day of fall. The length of the day and night are the same on this day, because the sun is shining directly on the Equator. The Spring Equinox happens in March. It is called the Vernal Equinox. These two pictures help to understand how this happens.
Here you can see how the sunlight hits the Earth at different points in its orbit around the Sun. Image by Colivine. http://commons.wikimedia.org/wiki/File:Orbital_relations_of_the_Solstice,_Equinox_%26_Intervening_Seasons.svg
Right now in the Northern Hemisphere the length of daylight is decreasing every day. What time does the sun rise where you are? When does it set?
The Earth orbits the Sun
Imagine the Earth orbiting (or spinning around) the Sun. It takes one year for the Earth to make a full circle around the Sun. The North Pole is the top of the Earth and the South Pole is the bottom of the Earth. Imagine a straight line running through the Earth from the North Pole to the South Pole. This is called the Earth’s axis. The Earth rotates on its axis.
This is what causes night and day. When it is nighttime where you are, you are on the side of the Earth facing away from the sun and it is dark outside. The Earth keeps spinning, eventually the Sun rises and it is daytime.
What causes the seasons?
Now that we can imagine the Earth rotating on its axis and orbiting the sun, tilt the axis a bit. The Earth’s axis is tilted 23.5 degrees. The Earth never stands upright, it always leans to the side.
It is this tilt that causes seasonal changes. As the Earth orbits the Sun the northern half (or Northern Hemisphere) receives more sunlight for half of the year. This when it is summer in the Northern Hemisphere and winter in the Southern Hemisphere.
Which Hemisphere do you live in?
Eventually the Earth reaches a point in its orbit where the top and bottom receive equal sunlight– the equinox! This would be the Fall Equinox in the Northern Hemisphere. On the same day, the Spring (or Vernal) Equinox happens in the Southern Hemisphere.
What is summer like where you live? What is winter like?
Then the Earth continues its orbit and the Southern Hemisphere gets more sunlight than the Northern Hemisphere. In other words, summer happens in the Southern Hemisphere and winter happens in the Northern Hemisphere.
I can tell that fall is coming to Minnesota and the Boundary Waters. Loons’ feathers are changing and they are getting ready to migrate. Large flocks of Canada geese are already flying in V formation, heading south. Leaves on maple trees, birch trees and aspen trees are starting to change color. Squirrels are busy caching food for the winter.
What changes are taking place around you as fall happens in the Northern Hemisphere and spring happens in the Southern Hemisphere?
The Reason for the Seasons Lesson Plans: http://education.nationalgeographic.com/education/activity/the-reason-for-the-seasons/?ar_a=1
Sun and Earth Lesson Plans: http://education.nationalgeographic.com/archive/xpeditions/lessons/07/g35/seasons.html?ar_a=1
How Equinox’s Work: Video https://www.youtube.com/watch?v=CjbgzL2Axok
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