The Changing Seasons, Equinoxes And Solstices
March 19 at 9:30 P.M. PDT (March 20, 0430 GMT) is the date and time of the Spring Equinox in the Northern Hemisphere (Autumnal or Fall Equinox in the Southern). It marks the transition from winter into spring and the changing of the seasons.
Let’s pause for a moment to consider why we have seasons. There is a common misconception that the Earth’s seasons are caused by our distance from the Sun. Actually the Earth’s orbit is nearly circular with only a small difference between its closest and furthest points from the Sun. Seasons are caused by the tilt of the Earth. Let’s take a look.
The Earth moves in two ways. It spins like a top and it travels in orbit around the Sun. The imaginary line extending through both poles and out into space is called the axis. As the Earth moves in orbit around the Sun the axis is not straight up and down. It is tilted. Without this tilt there would be no seasons on Earth. The axial tilt of 23.5 causes the seasons.
Figure 1 shows the Sun’s light striking the Earth on the Northern Summer Solstice. As you can see the Northern Hemisphere is tilted towards the Sun and receives more solar energy than does the Southern Hemisphere. Figure 2 shows the Sun’s light striking the Earth on Southern Summer Solstice. It shows that the Northern Hemisphere is tilted away from the Sun and receives less Sunlight.
So the Solstices are the days when one of the Earth’s hemispheres is pointed towards the Sun. But what are the Equinoxes?
Figures 3, 4, 5, and 6 show another view of the Earth. In these images we are looking down on the Solar System from a position far above the Sun. In these images the Earth’s size has been greatly exaggerated to allow us to see details. If the Earth were drawn to scale it would be the size of the period at the end of this sentence. These images show the relative position of the Earth as it orbits the Sun. The circle represents the orbit of the Earth around the Sun and the green line is the imaginary line of the Earth’s axis.
Figures 3 shows the Northern Summer Solstice and figure 4 shows the Southern Summer Solstice. The Northern Summer Solstice takes place on June 21 and the Southern Summer Solstice takes place six months later on December 21st.
Figures 5 and 6 show the position of Earth on the Equinoxes with 5 showing Northern Spring and 6 showing Northern Fall. On this day the Earth’s Axis and the Earth’s orbit are close to parallel. As a result the length of the day and the length of the night are equal.
Figures 7 and 8 take another view from space. This time a close up of Earth looking at the line between day and night, called the terminator. Figure 7 shows the terminator on the Summer Solstice, figure 8 shows the terminator on the Spring Equinox. You can see in this image that the line of the terminator and the line on the ecliptic are aligned with each other on the Equinox. This means that the Sun will rise directly in the East no matter where on Earth it is observed. It also means the length of the Day and the length of the night will be close to 12 hours each. This gives the word its meaning equinox: from Latin, aequi (equal) + noct (night).
For those of us in the Northern hemisphere the sunrise will keep moving further north of due east until the summer solstice. The length of the days will continue to increase.