Junior Science | Earth, Moon and Solar System: Tutor Prep Guide (Phases Every 3.7 Days, Four Errors, Exit Check)

2026-11-13

Tutor lesson-prep guides · Junior Science

Students can say that Earth goes round the Sun, yet answer why summer is hot with because Earth is closer, and go quiet when asked why phases take 29.5 days while the Moon orbits in 27.3. Three models dismantle three errors: rotation for day and night, revolution plus tilt for seasons, relative position for phases and eclipses.

Objectives — Students can: (1) describe rotation (about 24 hours, giving day and night, sun rising in the east) and revolution (365.25 days); (2) use the 29.5-day synodic month against the 27.3-day sidereal month to explain phases and compute about 3.7 days per phase; (3) order new moon, first quarter, full moon and last quarter with their rising and setting times; (4) distinguish a solar eclipse (new moon, Moon in the middle) from a lunar eclipse (full moon, Earth in the middle); (5) compute sunlight travel time from 1 AU at 300000 km per second, and explain seasons with the 23.5 degree tilt.

01 Prerequisites (3-minute check)

If any of these fails, fix it before the lesson starts.

Skill	Quick check	Pass mark
Direction and angle	Seen from above the North Pole, which way does Earth spin?	Anticlockwise, west to east
Speed and time	Distance divided by speed gives what?	Time
Straight-line light	What causes a shadow?	Light travels in straight lines and is blocked by an opaque object

02 Lesson flow (45 minutes)

Time	Stage	Tutor does	Student does
0-3 min	Prior-knowledge check	Ask spin direction, the time formula, shadow cause	Oral answers, gaps written on the first notebook line
3-12 min	Two-sphere model	Use a lamp, a globe and a small ball for rotation then revolution	Each group turns the model and names what moves
12-24 min	Phases and eclipses	Place the Moon at eight positions and name each phase and time	Fill an eight-box phase table with lunar dates
24-33 min	Calculations and seasons	Show AU to light-time, equatorial speed and the tilted axis	Work three examples with units
33-40 min	Independent practice	Four items, circulate and question only	Peer-check and name the faulty step
40-45 min	Exit check	Five quick oral questions; anyone short stays behind	Oral answers; all five correct before leaving

03 Teaching script (read it out)

Scene 1: Rotation and revolution: separate the two motions

Rotation first: hold the lamp still and spin the globe. Students track Hong Kong, facing the lamp at noon, away at midnight, a quarter turn at sunrise or sunset. One turn is about 24 hours, and seen from above the North Pole it runs anticlockwise, west to east, which is why the Sun rises in the east.

Revolution next: walk the globe round the lamp once, 365.25 days. The extra 0.25 is the leap year. Then the equatorial speed: 40075 km divided by 24 hours is about 1670 km per hour, which surprises students sitting still, and makes the point that fast is not the same as felt.

Students repeat: rotation gives day and night, revolution gives the year, and the two must not be merged.

Scene 2: Phases: 29.5 days against 27.3

Put the small ball at eight positions with the lamp fixed. Half the ball is always lit and half always dark, and the changing shape is the proportion of the lit half we can see, not Earth shadow. Say that twice.

The eight phases: new moon (day one, lost in the Sun glare), waxing crescent (days three and four, low in the west at dusk), first quarter (days seven and eight, overhead at dusk, right half lit), waxing gibbous, full moon (day fifteen, up all night), waning gibbous, last quarter (days twenty-two and twenty-three, rises around midnight, left half lit), waning crescent.

Now the two periods: the Moon orbits Earth in 27.3 days measured against the stars, but Earth has meanwhile moved about 27 degrees round the Sun, so the Moon needs another 2.2 days to return to the same position relative to the Sun. The phase cycle is therefore 29.5 days, and 29.5 divided by 8 gives 3.7 days per phase.

Scene 3: Eclipses and seasons: three errors to kill

A solar eclipse needs the Moon between Sun and Earth, so it happens only at new moon, and the shadow falls on a narrow track, which is why few people see any given one. A lunar eclipse needs Earth between Sun and Moon, so it happens only at full moon, and half the night side of Earth can watch, which is why lunar eclipses are commoner.

Seasons: walk the tilted globe round the lamp, keeping the axis pointing the same way. At the June solstice the northern hemisphere leans towards the Sun, so sunlight is more direct and days are longer; December reverses it. Say it flatly: seasons do not come from distance, and the northern summer actually happens near aphelion.

Finish with light travel: 1 AU is 150 million km, light runs at 300000 km per second, so 150000000 divided by 300000 is 500 seconds, about 8 minutes 20 seconds. For the Moon, 384400 divided by 300000 is 1.28 seconds. We always see the Sun as it was eight minutes ago.

04 Question types and worked steps

Type	Example	Steps	Answer
Phase identification	What does the Moon look like on days seven and eight, and when is it visible?	First quarter rises at noon, overhead at dusk, right half lit	First quarter, southern sky at dusk, right half illuminated
Phase interval	If the synodic month is 29.5 days, how far apart are phases?	29.5 divided by 8	About 3.7 days
Light travel time	How long does sunlight take to reach Earth?	Time equals distance over speed, with matching units	500 seconds, or 8 minutes 20 seconds
Eclipse reasoning	Why is a solar eclipse always at new moon?	All three bodies must align with the Moon in the middle	Only at new moon does the Moon sit between Sun and Earth

Sanity check (teach this): Sanity check to teach: after light time, ask how many minutes 8 minutes 20 seconds is (not 8 hours). After phases, ask why a full moon does not bring a lunar eclipse every month (the lunar orbit is tilted about five degrees, so the Moon usually passes above or below Earth shadow). A sunlight answer of 8 seconds means a factor of 60 has been lost.

05 Independent practice (3 questions)

Question	Answer	What to watch
(1) How long is one rotation, and in which direction?	About 24 hours, west to east, anticlockwise from above the North Pole	Does not confuse it with revolution
(2) With a 29.5-day synodic month, how long is each phase?	About 3.7 days	Does not divide 27.3 by 8
(3) How long does sunlight take to reach Earth?	500 seconds, 8 minutes 20 seconds	Keeps kilometres consistent
(4) During a lunar eclipse, what is the order of the three bodies?	Sun, then Earth, then Moon, so it must be full moon	Places Earth in the middle

06 Common mistakes and what to say

Mistake	Student writes	Say this
Seasons caused by distance	Summer is hot because Earth is closer to the Sun.	The orbit is nearly circular and the distance change is a few percent. The real cause is the 23.5 degree tilt: summer has more direct sunlight and longer days.
Phases caused by Earth shadow	The Moon goes dark in Earth shadow, and that is a phase.	Phases are the changing proportion of the lit half that we see. Earth shadow causes a lunar eclipse, a few times a year, not a nightly change.
Mixing sidereal and synodic months	The Moon orbits in 27.3 days, so phases repeat every 27.3 days.	27.3 days is measured against the stars. Earth moves too, so the Moon needs 29.5 days to line up with the Sun again, a difference of 2.2 days.
Swapping eclipse orders	A lunar eclipse is when the Moon blocks the Sun.	The Moon blocking the Sun is a solar eclipse. A lunar eclipse is Earth blocking sunlight from reaching the Moon. One is new moon, the other full moon.

07 Homework (paper, section and question numbers)

Science subject index: revisit linked chapters — From the index, revisit the light and forces chapters for shadows and gravity
Moon observation log — For seven nights at the same time, sketch the phase and position and note how far east it shifts
Calculation practice — Do questions 1 to 4 on the sheet, writing the formula and unit for each

08 Exit check (last 5 minutes)

Question	Answer	Decision
How long are rotation and revolution?	About 24 hours and 365.25 days	Swapping them means re-run act one
How long is the phase cycle?	29.5 days, the synodic month	27.3 alone needs the sidereal distinction
Is a solar eclipse at new or full moon?	New moon	Full moon means re-run act three
What causes the seasons?	The 23.5 degree tilt changing sunlight angle and day length	Distance as the answer needs correction
How long does sunlight take to reach Earth?	About 500 seconds, 8 minutes 20 seconds	8 seconds means a unit slip

09 Tutor FAQ

There is no globe or lamp. How do I teach phases?

Use a desk lamp as the Sun, a tennis ball as the Moon and a student head as Earth. Walk the ball round the head and the changing lit proportion appears in under three minutes, better than any diagram.

Why is there not an eclipse every month?

The lunar orbit is tilted about five degrees to Earth orbit, so at most new and full moons the three bodies are nearly but not exactly aligned and the shadow passes above or below. That is why eclipses come a few times a year, and why each is visible from different places.

How many numbers must be memorised in this chapter?

Five: 24 hours, 365.25 days, 29.5 days, 1 AU at 150 million km, and the 23.5 degree tilt. Everything else, including 3.7 days, 8 minutes 20 seconds and 1.28 seconds, should be derived, not memorised.

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A TutorZone lesson-prep guide. Written by the TutorZone team; questions and lesson flow are original and are not taken from any school or publisher material.