Key Takeaways:
- Apparent westward motion of celestial bodies is a consequence of Earth's eastward rotation.
- The Moon's orbit around Earth is eastward, contrary to its apparent westward movement across the sky.
- During a total solar eclipse, the Moon's eastward motion is directly observable as it transits the Sun's disk.
- The Moon's shadow, during a solar eclipse, follows the Moon's eastward trajectory across the Earth's surface.
This eclipse sequence begins at upper left and ends at lower right. During this event, the Moon moves from west to east, across the Sun’s face.
Ben Cooper
Because Earth rotates on its axis from west to east, the Moon and the Sun (and all other celestial objects) appear to move from east to west across the sky. Viewed from above, however, the Moon orbits Earth in the same direction as our planet rotates. So, the Moon actually moves from west to east through our sky, albeit so slowly that we almost never notice it.
During a total solar eclipse, however, we can see the Moon’s true motion as it crosses the Sun’s face from west to east. As this occurs, the Moon’s shadow follows it — moving in the same direction — and tracks a path across Earth’s surface.
NASA has created a helpful video, “Flying Around the Eclipse Shadow,” which illustrates this geometry if you’re still having trouble picturing it. You can watch it online at https://svs.gsfc.nasa.gov/4579.
Alison Klesman
Associate Editor
