This cheat sheet covers how the Moon changes appearance during its monthly cycle and how the Moon and Sun help create ocean tides. Students need these ideas to connect what they see in the sky with predictable patterns on Earth. It also helps explain why phase names, Moon positions, and tide patterns are connected.
The reference is useful for reviewing diagrams, vocabulary, and common cause-and-effect relationships in astronomy.
The Moon’s phase depends on how much of its sunlit half we can see from Earth. A full lunar phase cycle takes about 29.5 days, moving from new moon to full moon and back to new moon. Tides are caused mostly by the Moon’s gravity, with the Sun also adding an important effect.
Spring tides happen when the Sun, Earth, and Moon line up, while neap tides happen when the Sun and Moon pull at right angles.
Key Facts
- The Moon does not make its own light; it reflects sunlight from the Sun.
- A complete cycle of lunar phases takes about 29.5 days from one new moon to the next new moon.
- Waxing means the lit part of the Moon appears to grow larger each night, and waning means it appears to shrink.
- The main phases in order are new moon, waxing crescent, first quarter, waxing gibbous, full moon, waning gibbous, third quarter, and waning crescent.
- A full moon occurs when Earth is between the Sun and Moon, so the Moon’s sunlit side faces Earth.
- A new moon occurs when the Moon is between the Sun and Earth, so the side facing Earth is mostly dark.
- Most coastal places have about two high tides and two low tides each lunar day, which is about 24 hours and 50 minutes.
- Spring tides occur at new moon and full moon, while neap tides occur at first quarter and third quarter.
Vocabulary
- Lunar phase
- A lunar phase is the shape of the sunlit part of the Moon as seen from Earth.
- Waxing
- Waxing means the visible lit part of the Moon is increasing from night to night.
- Waning
- Waning means the visible lit part of the Moon is decreasing from night to night.
- Spring tide
- A spring tide is a tide with the greatest difference between high and low water, caused when the Sun, Earth, and Moon line up.
- Neap tide
- A neap tide is a tide with the smallest difference between high and low water, caused when the Sun and Moon pull at right angles.
- Tidal range
- Tidal range is the height difference between high tide and low tide.
Common Mistakes to Avoid
- Thinking Earth’s shadow causes the regular Moon phases is wrong because phases are caused by our changing view of the Moon’s sunlit half.
- Mixing up waxing and waning is wrong because waxing means the lit portion grows, while waning means the lit portion gets smaller.
- Calling first quarter a half moon can be confusing because first quarter means the Moon is one quarter of the way through its cycle, even though half of its visible disk is lit.
- Assuming spring tides happen only in spring is wrong because spring tides happen during new moon and full moon in every season.
- Forgetting the Sun’s role in tides is wrong because the Moon has the strongest effect, but the Sun’s gravity helps create spring and neap tide patterns.
Practice Questions
- 1 A new moon occurs on March 1. About what date will the next new moon occur if the lunar cycle is about 29.5 days?
- 2 If high tide is at 6:00 a.m. and most places have about two high tides per lunar day, about when is the next high tide?
- 3 Place these phases in order after first quarter: full moon, waxing gibbous, third quarter, waning gibbous.
- 4 Explain why a full moon and a new moon both produce spring tides even though the Moon appears very different from Earth.
Understanding Lunar Phases & Tides
Moon phases are really a changing viewing angle. At every point in its orbit, one half of the Moon is lit by the Sun. Earth observers see different portions of that lit half as the Moon moves around us.
The line between the bright part and dark part is called the terminator. It marks where lunar day changes to lunar night. At a quarter moon, half of the Moon's visible disk is bright.
The name quarter refers to the Moon being about one quarter or three quarters of the way around its orbit, not to the amount of light visible. Drawings should always show sunlight arriving from one direction. This makes phase diagrams much easier to read.
A new moon does not usually cause a solar eclipse, and a full moon does not usually cause a lunar eclipse. The Moon's orbit is tilted slightly compared with Earth's path around the Sun. Most months, the Moon passes above or below the exact line needed for an eclipse.
An eclipse needs a close alignment at the right part of the orbit. This is a useful difference to remember. A phase is a normal monthly viewing pattern.
An eclipse is a less common event involving shadows. Students often confuse the Moon's dark side with Earth’s shadow. Earth’s shadow reaches the Moon only during a lunar eclipse.
Tides happen because gravity is not equally strong across the whole Earth. The Moon pulls more strongly on the side of Earth closest to it than on Earth’s center. It pulls least strongly on the far side.
These small differences stretch the oceans into two broad tidal bulges. One bulge faces roughly toward the Moon. The other is on the opposite side.
As Earth turns, a coastal location moves through these bulges and the lower-water regions between them. This creates the repeating rise and fall of sea level. The Moon moves along its orbit while Earth rotates, so a tide arrives later each day by roughly fifty minutes.
Tides are not giant waves traveling straight across an ocean. They are slow changes in water level over many hours.
Real tides depend on more than the Moon and Sun. The shape of a coastline, the depth of the sea floor, bays, rivers, and local weather can make a large difference. Narrow bays can concentrate water and produce a much bigger tidal range than nearby open coasts.
Strong wind and air pressure can raise or lower water levels beyond the predicted tide. Fishers, sailors, surfers, and coastal communities use tide tables because shallow channels and exposed beaches change through the day. When studying this topic, track positions before memorizing names.
Identify the Sun first, then Earth, then the Moon. Next, decide which side of the Moon receives light and which part faces Earth.
For tides, compare the directions of the Moon’s pull and the Sun’s pull. This shows why their combined effect can strengthen or reduce the tidal range.