Moon Phases, Eclipses, and Tides
The Moon is Earth’s closest neighbor, and its motion produces some of the most familiar sights in the sky: the changing shape of the Moon, the rare drama of an eclipse, and the daily rise and fall of the tides. All three come from the same simple geometry of Sun, Earth, and Moon. Understanding it makes these events easy to predict.
This lesson covers moon phases, eclipses, and tides.
Moon phases are the changing shapes we see as the Moon orbits Earth and sunlight lights different portions of it. Eclipses happen when the Sun, Earth, and Moon line up. Tides are the rise and fall of the sea caused by the Moon’s gravity. All three depend on the positions of the Sun, Earth, and Moon.
What causes the phases of the Moon?
The Moon does not make its own light; it reflects sunlight. As the Moon orbits Earth, we see different amounts of its lit half, and these are the phases. When the Moon is between Earth and the Sun, its lit side faces away from us and we see a new moon. When Earth is between the Moon and the Sun, we see the whole lit face, a full moon. The in-between phases, crescent and gibbous, are the growing and shrinking slices we see along the way.
What causes eclipses?
An eclipse happens when the Sun, Earth, and Moon line up so one casts a shadow on another. In a solar eclipse, the Moon passes between the Sun and Earth, blocking the Sun and casting the Moon’s shadow on Earth. In a lunar eclipse, Earth passes between the Sun and Moon, so Earth’s shadow falls on the Moon. Eclipses are rare because the Moon’s orbit is slightly tilted, so the three bodies do not line up every month.
| Event | Alignment |
|---|---|
| New moon | Moon between Earth and Sun |
| Full moon | Earth between Moon and Sun |
| Solar eclipse | Moon blocks the Sun |
| Lunar eclipse | Earth’s shadow falls on the Moon |
How does the Moon cause tides?
The Moon’s gravity pulls on Earth’s oceans, tugging the water into a bulge on the side facing the Moon (and another on the far side). As Earth rotates, coastlines pass through these bulges, producing high tides, with low tides in between. This is why most coasts have two high and two low tides a day. The Sun adds to the effect, and when the Sun and Moon line up, tides are especially strong.
Watch: A Short Video Lesson
Astronomy with Mr Guerin walks through this skill clearly in a few minutes. It is a helpful companion to the reading above:
A routine for Moon questions
- Remember the Moon reflects sunlight; phases show its lit portion.
- New moon: Moon between Earth and Sun. Full moon: Earth between Moon and Sun.
- Solar eclipse: Moon blocks Sun. Lunar eclipse: Earth’s shadow on Moon.
- Tides come from the Moon’s gravity pulling the oceans.
- All three depend on Sun-Earth-Moon positions.
Practice questions
- Where does the Moon’s light come from?
- What phase occurs when Earth is between the Moon and Sun?
- In a solar eclipse, what blocks the Sun?
- What causes tides?
- Why are eclipses relatively rare?
- True or false: the Moon produces its own light.
Answers:
- It reflects sunlight.
- A full moon.
- The Moon.
- The Moon’s gravity pulling on Earth’s oceans.
- Because the Moon’s orbit is tilted, so the three bodies rarely line up exactly.
- False. It reflects the Sun’s light.
Where this fits
This lesson builds on Earth, Moon, and Sun and connects the Moon’s pull to tides in the ocean. It leads into the wider solar system. Find all topics on the ASVAB General Science Learning Hub.
Recommended Prep Books
These study guides and practice books help you keep building momentum as you prepare:
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