Why Eclipses Don’t Happen Every Month

What is an Eclipse

An eclipse occurs when the Sun, Earth and Moon align in a straight line causing one celestial body to cast a shadow on another. A solar eclipse happens when the Moon passes between the Sun and Earth, blocking the Sun’s light. A lunar eclipse occurs when the Earth comes between the Sun and the Moon, causing the Moon to pass through Earth’s shadow.

The Moon’s Orbit and Tilt

The Moon orbits the Earth approximately every 29.5 days, producing new and full moons each month. However, the Moon’s orbit is tilted about 5 degrees relative to Earth’s orbit around the Sun (the ecliptic). This slight tilt means that during most new and full moons, the Moon passes above or below the Sun or Earth rather than perfectly in line with them.

Why Eclipses Are Not Monthly

Because of this orbital tilt, the alignment needed for an eclipse does not occur every month. Even though new and full moons happen regularly, the Sun, Earth and Moon only align perfectly along the Moon’s orbital plane occasionally. Only when a new moon or full moon occurs near the points where the Moon’s orbit crosses the Earth’s orbital plane (called nodes) does an eclipse take place.

Eclipse Seasons

The times when the Sun is near the Moon’s nodes are called eclipse seasons. Each eclipse season lasts about 34–37 days and during this period, one or more eclipses may occur. There are typically two eclipse seasons per year which is why eclipses happen only a few times a year rather than every month.

Summary

In short, eclipses are rare because the Moon’s orbit is tilted relative to Earth’s orbit. The precise alignment required for a solar or lunar eclipse occurs only during eclipse seasons. This explains why, despite having a new moon and full moon every month, we do not witness eclipses monthly.

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