It's all about the tilt! As Earth revolves around the sun, either the Northern Hemisphere or Southern Hemisphere is tipped toward the sunlight, depending on the time of year. But on the equinoxes, the Earth's axis is not tilted toward or away from the sun. Look again to see how this affects the "sunrise" line:
December 21
In December, the sunlight line is angled so sunlight arrives at different longitude locations at the same time. Here, sunlight is arriving in the NE United States and in Central America at the same time.
 March 21
On the equinox, the sunlight line is straight up and parallel to the Earth's axis. As a result, sunlight arrives at the same time all along a single longitude line, not at several different longitudes.

The Living Earth® image is a composite of Earth satellite imagery developed by The Living Earth, Inc.
From: The Earth and Moon Viewer