The Moon's orbit is slightly ______ compared to the Earth's orbit and so lunar eclipses do not occur every month.

The Moon's orbit is slightly ______ compared to the Earth's orbit and so lunar eclipses do not occur every month.

• A. Circular
• B. Equatorial
• C. Tilted
• D. Eccentric

Answer: (C)

The Moon’s orbital plane is tilted relative to Earth’s orbital plane by about 5 degrees, so its path around the Sun isn’t perfectly aligned with the shadow Earth casts. Because of this tilt, most full moons occur above or below Earth’s shadow, so a lunar eclipse doesn’t happen every month. Eclipses can only occur when the full Moon happens near one of the two points where the Moon’s orbit crosses the ecliptic (the nodes), so that it actually passes through Earth’s shadow. If the Moon’s orbit were in the same plane as Earth’s, a lunar eclipse would be possible with every full Moon; the tilt is what keeps them from happening each month.

The Moon’s orbital plane is tilted relative to Earth’s orbital plane by about 5 degrees, so its path around the Sun isn’t perfectly aligned with the shadow Earth casts. Because of this tilt, most full moons occur above or below Earth’s shadow, so a lunar eclipse doesn’t happen every month. Eclipses can only occur when the full Moon happens near one of the two points where the Moon’s orbit crosses the ecliptic (the nodes), so that it actually passes through Earth’s shadow. If the Moon’s orbit were in the same plane as Earth’s, a lunar eclipse would be possible with every full Moon; the tilt is what keeps them from happening each month.