Keeping Time with the Skies · Lesson 2 of 7
Explaining Moon Phases and Moonrise
“Use sunlight and changing viewpoints to explain Moon phases, their timing, and their connection with tides.”
• Distinguish the Sun-lit half of the Moon from the half facing Earth. • Explain full, new, crescent, half, and gibbous phases using relative positions. • Use a lamp-and-ball model to connect an observation with its cause. • Explain why ordinary Moon phases are different from eclipses. • Calculate a change in moonrise time and interpret the approximate daily delay. • Recognise the connection between the Moon’s position and the repeating pattern of tides.
Two Different Halves to Keep in Mind
The Moon reflects sunlight rather than producing the light by which we see it. At any ordinary moment, sunlight illuminates approximately one half of its surface. However, that Sun-lit half is not always the same half that faces an observer on Earth. Confusing these two halves makes phases seem more mysterious than they are.
Imagine a ball illuminated from one side in a dark room. The side toward the lamp is bright, and the side away from it is dark. Move around the ball and the lamp still illuminates the same side, but you see different proportions of the bright and dark regions. The bright boundary looks straight in one view and curved in others because you are looking at a sphere.
The part of a surface receiving light. For the Moon, the main illumination is sunlight.
As the Moon revolves around Earth, the direction from which we view its illuminated half changes. We see nearly all of it at full Moon, nearly none of it at new Moon, and varying parts between them. The Moon also turns as it revolves, keeping approximately the same face toward Earth. Seeing roughly the same surface features does not mean that those features receive the same amount of sunlight every day.
Positions That Produce the Phases
To explain a phase, consider the positions of the Sun, Earth, and Moon together. The Sun is very far away compared with the Earth–Moon distance, so its light reaches this small system in nearly parallel directions. In the diagram, sunlight comes from the right throughout the orbit.
At position A, the Moon is on the side of Earth opposite the Sun. Its illuminated face is directed toward Earth, giving full Moon. At position E, the Moon is in approximately the Sun’s direction, and its mainly unilluminated face is toward us, giving new Moon. These positions describe directions; the Moon is still much nearer to Earth than the Sun is.
Between these positions, our view includes some illuminated surface and some unilluminated surface. A small visible illuminated fraction makes a crescent. Half of the visible disc illuminated makes a half Moon; more than half makes a gibbous Moon. The amount we see changes continuously, although familiar phase names identify useful points along the cycle.
| Position | Appearance from Earth | Stage of the cycle |
|---|---|---|
| A | Full Moon | Beginning of the full-to-new sequence |
| B | Waning gibbous | Bright part decreasing |
| C | Waning half | About a week after full |
| D | Waning crescent | Approaching new |
| E | New Moon | Bright part mostly faces away from Earth |
| F | Waxing crescent | Bright part increasing |
| G | Waxing half | About a week after new |
| H | Waxing gibbous | Approaching full |
This is the sequence for revolution viewed from above the north side of Earth’s orbit. It is not an instruction that the Moon must look lit on a particular side in every photograph. The apparent orientation can tilt with location and time. Observers in different places see nearly the same phase at the same moment, even if their views are rotated.
The Moon completes a revolution relative to distant stars in about 27.3 days, while a new-to-new or full-to-full phase cycle takes about 29.5 days. During that revolution, Earth also moves around the Sun, so the Moon must travel a little farther to regain the same Sun–Earth–Moon arrangement. Use 29.5 days when calculating phase-cycle months.
Problem
Positions C and G both show half of the visible disc illuminated. Why are they different stages of the cycle?
- 1.At C, the Moon is moving through the full-to-new part of the sequence. The bright fraction is decreasing.
- 2.At G, it is moving through the new-to-full part. The bright fraction is increasing.
- 3.C is waning half and G is waxing half. Equal illuminated fractions do not imply the same direction of change.
Test the Explanation with a Ball and Lamp
A physical model lets you connect position and appearance without waiting a month. Your head represents Earth, a small soft ball represents the Moon, and a lamp or a torch held by an adult represents the Sun. Each part has a role, so the model’s result can be interpreted rather than simply watched.
Fix the ball securely on a short stick. In a dark, open space, hold it at arm’s length with the lamp roughly three metres away. Keep the ball slightly above your head so that your head does not cast a shadow on it. Start with the ball toward the lamp, corresponding to E, and look at the side of the ball facing you.
Keeping your arm outstretched, turn slowly anticlockwise when viewed from above. Observe E, F, G, H, A, B, C, D, and E again. At E the face toward you looks dark; at A the bright face looks nearly complete. Between them, the bright boundary changes shape. Record the appearance at each position and compare it with the orbital diagram.
The ball does not change shape, and the lamp does not keep turning on and off. The change comes from the part of the illuminated surface visible to you. If the model produces a dark patch when your head blocks the lamp, lift the ball a little: that patch models a shadow event rather than an ordinary phase. The slightly raised ball also shows why a perfectly flat diagram needs interpretation.
Problem
A student puts the ball directly behind their head and sees it become dark instead of full. What should change in the model?
- 1.The ball is opposite the lamp, so its illuminated side should face the student.
- 2.However, the student’s head is blocking the lamp and casting a shadow on the ball.
- 3.Raise the ball slightly above the head while maintaining the opposite direction. It then models full Moon without the head’s shadow.
Why Moonrise Gets Later
Earth’s rotation brings different directions of the sky above our horizon during a day. But while Earth turns, the Moon also advances along its orbit. After Earth has turned through approximately one daily cycle, the Moon is farther along its path, so an observer generally needs some extra rotation to face its rising direction again.
The average delay in moonrise from one day to the next is about 50 minutes. It is not an exact timetable: the local change varies with date, latitude, and the Moon’s path. A quoted time from one place or year cannot be used as a prediction for every observer. Check local dated moonrise records when planning an observation.
Problem
A dated example lists moonrise at 14:23 on 7 April 2025 and 15:17 on 8 April 2025. Calculate the delay.
- 1.From 14:23 to 15:00 is 37 minutes.
- 2.From 15:00 to 15:17 is another 17 minutes.
- 3.The total is 37 + 17 = 54 minutes. This is close to the approximate 50-minute average, rather than exactly equal to it.
This movement also changes when the Moon is above the horizon. Waxing phases can be visible during the afternoon and evening; waning phases can remain visible after sunrise. Near full Moon, rising near sunset and setting near sunrise makes a long night-time appearance possible. Phase, sky position, and visibility time are different observations of the same changing geometry.
Phases, Eclipses, and the Direction of Sunlight
It is tempting to explain every dark part as Earth’s shadow. The ball model shows why this is unnecessary: part of the side facing us simply faces away from the Sun. A shadow falling onto the Moon is a different event, requiring a more exact alignment.
During ordinary phases, approximately half the Moon is illuminated and we see different amounts of that half. In a lunar eclipse, the Moon passes through Earth’s shadow. In a solar eclipse, the Moon blocks the Sun from view for some places on Earth.
A lunar eclipse can occur only near full Moon, with Earth between the Sun and Moon. A solar eclipse can occur only near new Moon, with the Moon between Earth and Sun. Yet most full and new Moons do not produce eclipses: the Moon’s orbital plane is tilted, so it usually passes above or below the exact line needed. The explanation of phases does not require an eclipse every month.
A crescent provides another test of reflected sunlight. Its bright outer curved edge faces toward the Sun’s direction. Use the general direction of the Sun and a short imaginary path across the sky to the Moon: that path meets the illuminated side first. The line joining the crescent’s two tips runs across the apparent diameter of the disc. A rotated crescent still follows the same illumination rule; do not stare at the Sun to test it.
A Related Pattern at the Seashore
The Moon’s motion is connected with another repeating observation: tides. Tides are the rise and fall of sea-water levels. Coastal records show that this cycle is related to the Moon’s position and phase, even though local coastlines and sea conditions also affect the details.
The repeating rise and fall of sea-water levels, strongly influenced by the Moon’s gravitational effect and also by the Sun.
A corresponding high or low tide often occurs about 50 minutes later on the next day, echoing the daily delay associated with the Moon’s motion. This is a broad pattern, not a promise that every coast has the same tide times or heights. Compare actual local tide records before making a claim about a particular shore. The useful connection here is that observations of the sky and sea can reveal related natural cycles.
Quiz
Why can a spherical Moon appear crescent-shaped?
In the A–H orbital model, which position gives new Moon?
Why is the model ball held slightly above the observer’s head?
A moonrise record changes from 14:23 to 15:17 the next day. What is the delay?
Why does a lunar eclipse not occur at every full Moon?
Which statement about tides is most accurate?
Practice Problems
- Explain the difference between the illuminated half of the whole Moon and the illuminated part of the face visible from Earth.
- Use the A–H diagram to explain the change from waning half to new Moon to waxing half.
- Describe the lamp-and-ball investigation, including the role of each object, an observation, and the conclusion.
- A moonrise record lists 18:40 and 19:28 on consecutive dates. Calculate the delay and compare it with the approximate daily average.
- Explain why a lunar eclipse requires full Moon but does not occur at every full Moon.
- Explain how the Sun-facing side of a crescent can help you check a drawing of its illumination.
- Compare a local tide table on consecutive days. Identify a corresponding tide and discuss why an approximate pattern is different from an exact prediction.
Key Takeaways
• The Sun illuminates approximately half the Moon; our changing view of that half produces ordinary phases. • Full Moon lies approximately opposite the Sun, while new Moon lies near the Sun’s direction. • A lamp, ball, and observer model explains phases when accidental shadows are avoided. • The Moon advances in its orbit while Earth rotates, producing an average moonrise delay of about 50 minutes. • Earth’s shadow causes lunar eclipses, not the normal phase cycle; orbital tilt makes eclipses occasional. • A crescent’s illuminated outer edge faces the Sun’s direction. • Tides have a repeating pattern linked with the Moon, with local variations in timing and height.