The motions of Earth, the Moon and the Sun
FoundationEarth's rotation causes day and night, its orbit around the Sun makes a year; the Moon orbits Earth, and phases depend on the sunlit part we see.
- Rotation causes day and night
- One revolution takes about 365 days
- Moon phases change with the visible sunlit fraction
§Text Passage
1Every day the Sun rises in the east and sets in the west, and it looks as if the Sun circles us — but in fact it is Earth that is turning. Earth spins once about every 24 hours, from west to east: the half facing the Sun has daytime, the half turned away has night. Because Earth never stops spinning, day and night keep alternating, and sunrise and sunset are the natural result of this rotation.
2Earth also orbits the Sun, taking about 365 days for one journey — a year. The seasons arise because the axis is tilted by about 23.5 degrees: the angle at which sunlight strikes a place shifts through the year, so in summer the sunlight is more direct and the days longer, while in winter the light slants and the days are shorter. The seasons are not caused by the Earth-Sun distance changing, and the Southern Hemisphere's seasons are the opposite of ours.
3The Moon circles Earth about once every 29.5 days and gives off no light of its own, only reflecting sunlight. As it moves, the angle of the sunlit part we see keeps changing, and in turn we see the new moon, first quarter, full moon and last quarter. The Mid-Autumn Festival falls on the night of a full moon, and for thousands of years people have built lunar calendars on the phases of the Moon to guide farming and festivals.
4When the Moon moves between the Sun and Earth and blocks the sunlight, a solar eclipse occurs; when the Moon slips into Earth's shadow and darkens, we see a lunar eclipse. Tides arise mainly from the Moon's gravity pulling the seas, and most coasts see roughly two high tides and two low tides a day. These motions explain day and night, the seasons, moon phases and eclipses, and underpin calendars, tide prediction and space mission planning; scientists often demonstrate them with models, bringing the vast universe into the classroom.
1Summary 重點摘要
Earth's rotation causes day and night, its orbit around the Sun makes a year; the Moon orbits Earth, and phases depend on the sunlit part we see.
2Vocabulary cards Vocabulary
The spinning of an object around its own axis.
The movement of an object around another object along an orbit.
The path followed by an object moving under gravity.
The changing shape of the Moon's sunlit part as seen from Earth.
The imaginary line through Earth that it spins around, tilted by about 23.5 degrees.
When the Moon moves between the Sun and Earth and blocks the sunlight.
When the Moon moves into Earth's shadow and darkens.
The periodic rise and fall of sea level, caused mainly by the Moon's gravity.
3Step by step Steps
- Day and nightEarth spins once about every 24 hours, from west to east; the half facing the Sun has daytime while the half turned away has night. As Earth never stops turning, day and night keep alternating, and the Sun 'rising in the east and setting in the west' is an illusion created by this spin. This rhythm governs human life: our schedules for rest, school and work are all arranged around the alternation of day and night.
- Year and seasonsEarth orbits the Sun in about 365 days, and with its axis tilted some 23.5 degrees, the angle at which sunlight strikes each place shifts through the year, giving rise to the seasons. The seasons come from this changing angle, not from the distance to the Sun swinging nearer and farther: in summer the sunlight is more direct and the days longer, and winter is the opposite. Hong Kong's hot summers and cool winters show the pattern, while the Southern Hemisphere experiences the seasons in reverse.
- Moon phasesThe Moon emits no light of its own and only reflects sunlight; in about 29.5 days it circles the Earth, and as the visible sunlit fraction changes we see, in turn, new moon, first quarter, full moon and last quarter, the cycle repeating month after month. The Mid-Autumn Festival falls on the night of a full moon, and since ancient times people have arranged festivals and farming and fishing around the phases of the Moon, making them the astronomical phenomenon closest to daily life.
- Eclipses and the calendarEclipses come from a clever line-up of the heavenly bodies: when the Moon slips between the Sun and the Earth and blocks the sunlight, we get a solar eclipse; when it moves into the Earth's shadow and darkens, we get a lunar eclipse. Tides arise mainly from the Moon's gravity, which makes the sea rise and fall in a daily rhythm. Ancient people went further and built the lunar calendar on the phases of the Moon to guide farming and festivals — Mid-Autumn moon-gazing is dated by exactly that calendar.
- Solar versus lunar eclipseWhen comparing the two eclipses, the key is the line-up: in a solar eclipse the Moon sits in the middle, between the Sun and the Earth, and its shadow sweeps across the Earth, darkening the day for a few minutes; in a lunar eclipse the Earth is in the middle and the Moon dims as it enters the Earth's shadow. A lunar eclipse is visible from the whole hemisphere facing away from the Sun, while a solar eclipse shows only along the narrow track of the shadow, which is why solar eclipses feel far rarer — and proper eye protection is essential when watching one.
- Why tides rise and fallTides arise mainly from the Moon's gravity pulling on the oceans: the water bulges on the side facing the Moon and on the opposite side as well, so most coasts see roughly two high tides and two low tides a day. Around new and full moon, the Sun and the Moon line up with the Earth and their pulls add together, producing extra-large spring tides. Fishermen, boat operators and coastal engineers all work to tide tables — astronomy and daily life are closely bound together.
4Diagrams Diagrams
5Worked example Example
Example: why does a full Moon not mean the Moon produces light?
- The Sun supplies the light
- The Moon reflects sunlight
- At full Moon, most of the sunlit side facing Earth is visible
- Different viewing angles create the phases
- Answer: The Moon makes no light; it only reflects the Sun's. At full Moon it sits on Earth's far side from the Sun, so the whole Earth-facing half is lit and looks round and bright — but every ray is reflected sunlight, not the Moon's own; this is the master key to all phases: only the viewing angle changes, never the light source — do not mix the two up.
6Common mistakes Mistakes
7Quick check Quick check
Q1Is day and night caused mainly by Earth's rotation or revolution?
Q2Where does the Moon's light come from?
Q3What two things mainly cause the seasons?
Q4During a solar eclipse, which body lies in the middle?
Q5How long do one rotation and one revolution of Earth take?
8Recap Recap
- Rotation causes day and night
- One revolution takes about 365 days
- Moon phases change with the visible sunlit fraction
- Seasons result from axial tilt together with revolution
- The Moon reflects sunlight and produces none of its own