The Moon’s phases follow a familiar sequence: new Moon, waxing crescent, first quarter, waxing gibbous, full Moon, waning gibbous, last quarter, and waning crescent. A day-by-day observing calendar helps you know what shape to expect and when it’s easiest to follow the Moon’s changes.
The visible illuminated portion of the Moon changes as it orbits Earth, so its phase affects how it looks in the sky. Understanding this sequence is useful not only for planning observations: it helps you notice how the lunar disk changes day by day, from a thin crescent to a full circle and back to a sliver.
| Phase | What’s visible | What to look for |
|---|---|---|
| New Moon | The illuminated side faces mostly away from Earth | A thin crescent after the new Moon |
| First quarter | About half the disk is illuminated on the right in the Northern Hemisphere | The terminator and the terrain along the light boundary |
| Full Moon | Almost the entire disk is illuminated | Large dark regions, including the Sea of Tranquility |
| Last quarter | About half the disk is illuminated on the left in the Northern Hemisphere | The terminator and the terrain along the light boundary |
- ≈29,5 суток Length of a synodic month—the cycle from one new Moon to the next
- 4 главные фазы New Moon, first quarter, full Moon, and last quarter
- ≈ половина поверхности Fraction of the Moon illuminated by the Sun under ordinary lighting geometry
Why does the Moon change phases roughly every 29.5 days?
The Moon changes phases roughly every 29.5 days because that is how long it takes to complete a full cycle relative to the Sun and Earth—from one new Moon to the next. This interval is called a synodic month.
The Moon does not produce its own visible light: its surface reflects sunlight. The Sun illuminates roughly half the Moon at almost all times, and the phase depends on how much of that illuminated half we can see from Earth. So it is not the Moon as a whole that changes in brightness, but its visible shape in the sky.
The sequence of phases
Over the course of a synodic month, the phases follow this order: new Moon, waxing crescent, first quarter, waxing gibbous, full Moon, waning gibbous, last quarter, and waning crescent. After the waning crescent, the cycle returns to the new Moon.
When should you observe a crescent, a quarter, or a full Moon?
Observe a crescent around the new Moon, a half-lit disk at first and last quarter, and an almost fully illuminated Moon at full Moon. To choose an evening, check the phase: it shows how much of the visible disk is illuminated and where the boundary between light and shadow lies.
A guide for the Northern Hemisphere
- New Moon: The Moon’s illuminated side faces mostly away from Earth, so the thin crescent is usually not visible. The exact alignment of the Moon and Sun does not by itself mean a solar eclipse will occur.
- First quarter: About half the disk is visible, illuminated on the right for an observer in the Northern Hemisphere. The boundary between light and shadow brings the terrain into sharp relief.
- Full Moon: Almost the entire illuminated side faces Earth. The bright disk makes large dark regions easy to see, but small surface features appear less distinct.
- Last quarter: Again, about half the disk is visible, now illuminated on the left in the Northern Hemisphere.
To see a crescent, look around the time of the new Moon, when the thin illuminated portion becomes visible; for contrast between light and shadow, try the first or last quarter. The full Moon is better for observing large dark regions than fine surface details.
What surface features are visible along the terminator?
Crater rims, mountain ranges, and surface irregularities are easiest to see along the terminator—the boundary between the illuminated and dark parts of the lunar disk—because the low Sun casts long shadows near this line. That’s why both the date and time matter, not just the Moon’s phase: the terminator moves from day to day.
Craters and shadows
Along the terminator, oblique light brings out the terrain: dark areas inside craters sit next to illuminated rims, and mountain ranges appear as bright lines against the shadows. As the boundary moves, the illuminated side changes, revealing details that were previously hidden in darkness.
During the waxing crescent and from first quarter to full Moon, the lighting on the western side of the disk changes; after the full Moon, conditions shift toward the eastern side. To compare observations, note the phase, date, time, and name of a prominent feature—for example, a crater or mountain range. This makes it easier to see how its appearance changes as the terminator moves.
What’s best to observe at full Moon?
At full Moon, large, high-contrast lunar features are best to observe: the dark lunar maria, including the Sea of Tranquility, and the bright areas next to them. The almost fully illuminated disk is easy to make out with the naked eye or binoculars, making this phase a good introduction to the overall pattern of the lunar surface.
At full Moon, sunlight falls on most of the visible disk, so the terrain casts less distinct shadows. As a result, small crater details are harder to see than during phases when the terminator—the boundary between light and shadow—passes close to the area you’re interested in.
Full Moon is useful as a reference point for observations: compare the dark Sea of Tranquility with the bright areas of the disk and note how the contrast is distributed. But the full Moon is not universally the best choice: to study shadows and fine crater details, choose a phase when the terminator is nearby.
What can prevent you from seeing the phase you want?
Clouds, the Moon’s position relative to the Sun and horizon, and confusion between the visible part of the disk and the illuminated part of the lunar sphere can all make it harder to see the Moon’s phase. An observing calendar shows the expected illumination, but it cannot guarantee clear skies or good visibility.
Weather and the Moon’s position above the horizon
Clouds can hide the Moon in any phase. A thin crescent appears in the sky not far from the Sun, so an unobstructed horizon and twilight are important for observing it; choose your viewing direction so you don’t look at the Sun.
- Clouds: They can hide the Moon whether it’s a crescent, at quarter, or full.
- Thin crescent: Look for it low above an unobstructed horizon at twilight, bearing in mind how close it is to the Sun.
- New Moon: A solar eclipse does not happen at every new Moon—the Moon must be near a node of its orbit.
A common misconception about the quarter phase
The expression “half the Moon is visible” refers to the first- or last-quarter phase: from Earth, half the disk appears illuminated. At the same time, the Sun illuminates about half of the Moon’s entire surface, not just the part we see from Earth.
How can you make your own Moon-observing calendar?
To make your own Moon-observing calendar, mark the dates of the four main phases—new Moon, first quarter, full Moon, and last quarter—then add notes about the target and conditions for each outing. This will show how the Moon’s illumination and the appearance of its surface change over the course of the cycle.
What to record in your observing log
For each observation, note the date, local time, and target—for example, the Sea of Tranquility or a crater near the terminator, the boundary between the illuminated and dark parts of the lunar disk. Also record visibility conditions, the instrument you used, and your observing location: these details will help you distinguish a change in phase from cloud cover or a difference in viewpoint.
Compare entries made several days apart: this makes it easier to track how the terminator moves across the surface. Over a synodic month—about 29.5 days—the Moon’s illumination completes a full cycle, from new Moon to the next new Moon. Comparing notes over this period helps reveal the recurring phases and changes in the terrain along the terminator.
Frequently asked questions
How long does a complete cycle of Moon phases last?
Why are craters easier to see along the terminator?
During which phase does the Moon look like half a disk?
Why isn’t the full Moon always best for observing craters?
Does a solar eclipse happen at every new Moon?
