Earth, sun and moon: phases, eclipses and seasons

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Lesson notes

Earth-Sun-Moon System Overview

The Earth, Sun, and Moon form a system where their relative positions cause observable patterns.
The Moon orbits Earth about once every **29.5 days** (a **synodic month**).
The Moon is **tidally locked** to Earth, so the same side always faces us.
The Sun illuminates half the Moon at all times; we see varying portions of that lit half from Earth.

Lunar Phases: Major Phases

The four **major phases** are: **new moon**, **first quarter**, **full moon**, and **last quarter** (also called third quarter).
At **new moon**, the Moon is between Earth and Sun; the lit side faces away, so it is invisible.
At **first quarter**, we see half of the lit side (right half in Northern Hemisphere).
At **full moon**, Earth is between Sun and Moon; the entire lit side faces Earth.
At **last quarter**, we see the left half lit (Northern Hemisphere).

Lunar Phases: Intermediate Phases

Between major phases are **waxing crescent**, **waxing gibbous**, **waning gibbous**, and **waning crescent**.
**Waxing** means the lit portion is increasing (new → full).
**Waning** means the lit portion is decreasing (full → new).
A **crescent** is less than half lit; a **gibbous** is more than half lit.
Each intermediate phase lasts about **7.38 days** on average.

Eclipses

A **solar eclipse** occurs when the Moon passes directly between the Sun and Earth, casting a shadow on Earth.
Solar eclipses happen only at **new moon**.
A **lunar eclipse** occurs when Earth passes between the Sun and Moon, casting a shadow on the Moon.
Lunar eclipses happen only at **full moon**.
Eclipses do not occur every month because the Moon's orbit is tilted relative to Earth's orbit.

Seasons

Seasons are caused by the **tilt of Earth's axis** (about 23.5°) relative to its orbital plane.
During **summer** in a hemisphere, that hemisphere is tilted toward the Sun, receiving more direct sunlight and longer days.
During **winter**, the hemisphere is tilted away, receiving less direct sunlight and shorter days.
**Spring** and **autumn (fall)** occur when neither hemisphere is tilted toward or away from the Sun.
The **solstices** mark the longest (summer) and shortest (winter) days; the **equinoxes** mark equal day and night.

Phases and Seasons: Northern vs Southern Hemisphere

Lunar phases appear **opposite** in the Northern and Southern Hemispheres (e.g., the lit side is on the right in the North, left in the South).
Seasons are also opposite: when it is summer in the Northern Hemisphere, it is winter in the Southern Hemisphere.
The Moon's highest path in the sky varies with season: e.g., a full moon is highest in winter in the Northern Hemisphere.

Simplified diagram of the Earth-Sun-Moon system showing the new moon phase. The Moon's position relative to Earth and Sun determines the phase seen from Earth.

Earth's axial tilt causes seasons. In June, the Northern Hemisphere is tilted toward the Sun (summer); in December, it is tilted away (winter).

Slides

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Practice questions

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1.What is the term for the apparent shape of the Moon as seen from Earth?
Easy
- ALunar phase
- BLunar eclipse
- CSolar eclipse
- DLunar cycle
2.How long does it take for the Moon to complete one cycle of phases?
Easy
- AAbout 29.5 days
- BAbout 365 days
- CAbout 24 hours
- DAbout 7 days
3.During which lunar phase does the Moon appear completely dark from Earth?
Medium
- ANew moon
- BFull moon
- CFirst quarter
- DWaning crescent
4.What is the correct order of lunar phases from new moon to full moon?
Medium
- ANew moon, waxing crescent, first quarter, waxing gibbous, full moon
- BNew moon, waning crescent, first quarter, waning gibbous, full moon
- CNew moon, first quarter, waxing crescent, waxing gibbous, full moon
- DNew moon, waxing gibbous, first quarter, waxing crescent, full moon
5.If the Moon is at first quarter phase, what is its approximate angular separation from the Sun as seen from Earth?
Hard
- A90°
- B0°
- C180°
- D270°
6.What causes the changing phases of the Moon?
Easy
- AThe changing position of the Moon in its orbit around Earth, affecting how much of its sunlit side we see
- BEarth's shadow falling on the Moon
- CThe Moon rotating on its axis
- DClouds blocking parts of the Moon
7.Which term describes the Moon when its illuminated portion is increasing from new to full?
Medium
- AWaxing
- BWaning
- CGibbous
- DCrescent
8.In the Northern Hemisphere, during which season does the full moon appear highest in the sky?
Hard
- AWinter
- BSummer
- CSpring
- DAutumn

Revision notes

## Earth, Sun, and Moon System ### Lunar Phases - The Moon's phases are caused by its orbit around Earth, changing the portion of the sunlit side we see. - **Major phases**: new moon (0% illuminated), first quarter (50%, right half lit in Northern Hemisphere), full moon (100%), last quarter (50%, left half lit). - **Intermediate phases**: waxing crescent (between new and first quarter), waxing gibbous (between first quarter and full), waning gibbous (between full and last quarter), waning crescent (between last quarter and new). - **Cycle**: A lunar month averages 29.5 days from one new moon to the next. - **Waxing** = illuminated area increases; **waning** = decreases. - **Memory aid**: "DOC" – in Northern Hemisphere, first quarter looks like a D, full like O, last quarter like C. ### Eclipses - **Solar eclipse**: Moon passes between Sun and Earth, blocking sunlight. Occurs at new moon. - **Lunar eclipse**: Earth passes between Sun and Moon, casting shadow on Moon. Occurs at full moon. - Eclipses don't happen every month because the Moon's orbit is tilted relative to Earth's orbit. ### Seasons - Caused by Earth's **23.5° axial tilt** relative to its orbital plane. - **Summer solstice**: Northern Hemisphere tilted toward Sun, longest day. - **Winter solstice**: Tilted away, shortest day. - **Equinoxes**: Tilt is sideways, equal day and night. - **Key**: Seasons are NOT due to distance from Sun (Earth is actually closest in January). ### Quick Facts - The Moon is tidally locked: same side always faces Earth. - A lunar day (sunrise to sunrise on Moon) equals one lunar month. - Earthshine: faint light on the dark part of a crescent Moon, caused by sunlight reflecting off Earth.

Lunar phase: The apparent shape of the Moon's sunlit portion as seen from Earth, changing over a lunar month.
New moon: Phase when the Moon is between Earth and Sun; the side facing Earth is dark (0% illuminated).
Full moon: Phase when Earth is between Moon and Sun; the entire near side is illuminated (100%).
Waxing: Describes phases when the illuminated area increases (from new to full).
Waning: Describes phases when the illuminated area decreases (from full to new).
Solar eclipse: Event when the Moon blocks the Sun's light from reaching Earth, occurring at new moon.
Lunar eclipse: Event when Earth's shadow falls on the Moon, occurring at full moon.
Axial tilt: The 23.5° tilt of Earth's axis, causing seasons as Earth orbits the Sun.
Equinox: Twice a year when day and night are equal length; marks the start of spring or autumn.
Solstice: Twice a year when the Sun reaches its highest or lowest point at noon; marks longest and shortest days.

Flashcards

What causes the phases of the Moon?

The phases of the Moon are caused by the changing relative positions of the Earth, Moon, and Sun, which affect how much of the Moon's sunlit side is visible from Earth.

How long does it take for the Moon to complete one cycle of phases?

One complete cycle of lunar phases, called a lunar month, averages about 29.5 days.

What are the four major lunar phases?

The four major lunar phases are new moon, first quarter, full moon, and last quarter (also called third quarter).

What are the four minor lunar phases?

The four minor phases are waxing crescent, waxing gibbous, waning gibbous, and waning crescent.

What does 'waxing' mean in terms of the Moon?

Waxing means the illuminated portion of the Moon is increasing, from new moon toward full moon.

What does 'waning' mean in terms of the Moon?

Waning means the illuminated portion of the Moon is decreasing, from full moon toward new moon.

During which phase is the Moon between Earth and the Sun?

During the new moon phase, the Moon is between Earth and the Sun, so the side facing Earth is not illuminated.

During which phase is Earth between the Sun and the Moon?

During the full moon phase, Earth is between the Sun and the Moon, so the entire sunlit side of the Moon faces Earth.

What is a solar eclipse?

A solar eclipse occurs when the Moon passes directly between the Sun and Earth, blocking the Sun's light from reaching part of Earth.

What is a lunar eclipse?

A lunar eclipse occurs when Earth passes directly between the Sun and the Moon, casting a shadow on the Moon.

Why don't we have a solar eclipse every new moon?

The Moon's orbit is tilted relative to Earth's orbit, so the Moon usually passes above or below the Sun from our perspective, not directly in front of it.

What causes Earth's seasons?

Seasons are caused by the 23.5° tilt of Earth's axis as it orbits the Sun, which changes the angle and duration of sunlight at different latitudes throughout the year.

When is the Northern Hemisphere tilted toward the Sun?

The Northern Hemisphere is tilted toward the Sun during summer (around June solstice), resulting in more direct sunlight and longer days.

What is the relationship between the Moon's rotation and its orbit?

The Moon is tidally locked to Earth, meaning it rotates once on its axis in the same time it takes to orbit Earth (about 27.3 days), so the same side always faces Earth.

What is a lunar month?

A lunar month is the time between successive recurrences of the same lunar phase, averaging about 29.5 days.

What is earthshine?

Earthshine is the dim illumination of the Moon's night side caused by sunlight reflecting off Earth's surface.

How does the Moon's appearance differ between the Northern and Southern Hemispheres?

The Moon's phase orientation is opposite: in the Northern Hemisphere, the right side is illuminated during waxing; in the Southern Hemisphere, the left side is illuminated.

What is a gibbous moon?

A gibbous moon is when the illuminated portion is more than half but not full, appearing bulging outward.

What is a crescent moon?

A crescent moon is when the illuminated portion is less than half, appearing as a thin sliver.

What is the difference between a new moon and an old moon?

A new moon is the beginning of the waxing cycle when the Moon is between Earth and Sun; an old moon is the last visible waning sliver before the new moon.

Why does the Moon appear to change shape?

The Moon appears to change shape because we see different amounts of its sunlit half as it orbits Earth.

What is the angle between the Sun and Moon during a first quarter moon?

During a first quarter moon, the Sun and Moon are at a 90° angle as seen from Earth, so half the Moon's visible side is illuminated.

What is the angle between the Sun and Moon during a full moon?

During a full moon, the Sun and Moon are opposite each other (180° apart) as seen from Earth.

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