Understanding Eclipses

Eclipses come in two main varieties: solar and lunar. A solar eclipse occurs when the moon, while orbiting Earth, moves directly in front of the Sun, blocking its light for a brief period. In contrast, a lunar eclipse happens when the moon moves into the Earth's shadow, which obstructs sunlight from reaching it. Since the moon's glow is simply sunlight reflected, the lunar eclipse results from the Earth positioning itself between the Sun and the moon (as depicted in Figure 1 below).

Figure 1: Configuration of the Earth, Sun, and moon during solar and lunar eclipses. Credit: Wikimedia Commons/Tomruen

The timing in Earth's history is remarkable for witnessing such celestial events. If you've experienced a total solar eclipse (ideally wearing protective eyewear), you might have observed how the moon entirely covers the Sun's disk during these moments. This phenomenon is particularly striking because the moon's orbit around the Earth is elliptical, causing it to sometimes appear larger when it is closer to our planet. This is often referred to as a “supermoon.” When a solar eclipse coincides with a supermoon, the moon's size in the sky matches that of the Sun, allowing for a complete blockage of sunlight.

This alignment is coincidental and becomes increasingly rare as the moon gradually drifts away from Earth. In the distant future, several hundred thousand years from now, the moon will be positioned far enough that total solar eclipses will no longer occur.

Figure 2: Total Solar Eclipse. Credit: NASA/Goddard/Rebecca Roth

In contrast, during a lunar eclipse, the moon takes time to traverse the Earth's shadow and re-emerge. This delay occurs because the Earth's shadow is significantly larger than the moon itself.

Given that the moon completes an orbit around Earth approximately every 28 days, one might wonder why solar and lunar eclipses are not monthly occurrences. The answer lies in the orbital planes of the Sun, Earth, and moon. Earth follows an orbital path around the Sun known as the ecliptic, which resembles a flat surface on which planets orbit at varying distances from the Sun. Meanwhile, the moon's orbit is tilted by about 5° relative to this plane and undergoes a slow precession, as illustrated in the top animation of Figure 3.

Figure 3: Animation of the moon's elliptical orbit around Earth and its precession (grey line). The blue line represents the ecliptic, while the green line indicates the nodes where the two orbits intersect. Credit: Thiagobf, CC BY-SA 4.0, via Wikimedia Commons

This slight tilt is significant enough that, during most full or new moons, the moon's position is either above or below the orbital plane of the Earth-Sun system, thus preventing an eclipse from occurring. For an eclipse to take place, the moon must cross the ecliptic plane, aligning the Sun, Earth, and moon. This precise alignment happens only 2 to 5 times a year.