Pic of the day #37 (Earth and Sun)

Pic src: Fundamentals of Geophysics (William Lowrie)

Left Figure: Kepler’s first two laws of planetary motion: (1) each planetary orbit is an ellipse with the Sun at one focus, and (2) the radius to a planet sweeps out equal areas in equal intervals of time.
Right Figure: The relationship between the ecliptic plane, Earth’s equatorial plane and the line of equinoxes. The equatorial plane is tilted at the same angle to the ecliptic, which it intersects along the line of equinoxes. During the annual motion of the Earth around the Sun, this line twice points to the Sun: on March 20, defining the vernal (spring) equinox, and on September 23, defining the autumnal equinox. On these dates day and night have equal length everywhere on Earth. The summer and winter solstices occur on June 21 and December 22, respectively, when the apparent motion of the Sun appears to reach its highest and lowest points in the sky.

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