March 20, 2019 is the vernal equinox, first day of spring in the Northern Hemisphere. The sun's arc is at the equator in an apparent journey north. Days and nights are equal, 12 hours of daylight and 12 hours of darkness everywhere on the globe.
Earth's axis points in the same direction as it orbits the sun. Sunlight hits the earth broadside at the spring equinox. The sun appears higher. The Northern Hemisphere tilts toward the sun, and the top half of the globe warms up.
Earth revolves around the sun in 365 1/4 days. Its orbit is an elipse. Earth is closest to the sun at 91 million miles on January 2 and fartherest from the sun at 94 million miles on July 4. The Northern Hemisphere experiences summer when Earth is fartherest from the sun because it tilts toward the sun. The arc of the sun is overhead at the equator at both equinoxes.
The arc of the sun passes overhead at the equator twice during a year, at vernal and autumnal equinoxes. It passes 23 1/2 degrees north of the equator the first day of summer and 23 1/2 degrees south of the equator the first day of winter.
Equinox means equal nights.
The 5 major latitude circles are: Arctic Circle. Tropic of Cancer. Equator. Tropic of Capricorn. Antarctic Circle.