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Earth, Sun &
Moon Positions

Explore how the relative positions of Earth, the Sun, and the Moon create the patterns we observe: day and night, lunar phases, and eclipses.

Earth & Space Science

What Are Relative Positions?

A relative position describes where one object is compared to another object. In space science, we track how Earth, the Sun, and the Moon are arranged in relation to each other at any given time.

These positions are not random. Gravity holds each body in a predictable orbit, so the arrangement changes in a regular, repeating cycle we can model and predict.

Meet the Three Bodies

★ The Sun
A star at the center of our solar system. Earth and Moon both orbit around it. Diameter: about 109 times Earth's.
★ Earth
Our planet orbits the Sun once every 365.25 days and rotates on its axis once every 24 hours.
★ The Moon
Earth's natural satellite. It orbits Earth roughly once every 27.3 days and does not produce its own light.

Why Positions Matter

The way Earth, the Sun, and the Moon line up at any moment determines several phenomena we observe every day and month:

1
Day and night: caused by Earth's rotation relative to the Sun.
2
Moon phases: the changing amount of sunlit Moon we see depends on the Moon's position relative to Earth and the Sun.
3
Eclipses: occur only when all three bodies align in a nearly straight line.
4
Tides: the gravitational pull of the Moon and Sun on Earth's oceans shifts as positions change.

The Orbital Model

Sun Earth Moon Earth's orbit Moon's orbit (Not to scale: sizes and distances are greatly simplified)

Earth's Rotation: Day & Night

Sun Day Night Earth's axis Rotation You

How Moon Phases Work

The Moon does not produce its own light: it reflects sunlight. As the Moon orbits Earth, the amount of its sunlit side that we can see changes. These changes are called lunar phases.

Sunlight Earth New Wax Cr 1st Qtr Wax Gb Full Wan Gb 3rd Qtr Wan Cr

Key Phases Explained

New Moon
Moon is between Earth and Sun. The sunlit side faces away from us: we see darkness.
First Quarter
Moon is at a 90-degree angle from the Sun relative to Earth. We see the right half lit.
Full Moon
Earth is between Sun and Moon. The entire face we see is fully illuminated.
Third Quarter
Moon is at 90 degrees on the opposite side. We see the left half lit.

Solar vs. Lunar Eclipse

Solar Eclipse Lunar Eclipse
Alignment Sun – Moon – Earth Sun – Earth – Moon
What is blocked? Moon blocks sunlight Earth blocks sunlight from Moon
Moon Phase New Moon Full Moon
Visibility Narrow path on Earth Visible from entire night side
Duration Up to ~7.5 minutes (totality) Up to ~1 hour 45 minutes

Eclipse Alignments

Solar Eclipse Sun Moon Earth Moon's shadow Lunar Eclipse Sun Earth Moon Earth's shadow (Not to scale)

Waxing vs. Waning

Waxing
The illuminated portion is growing larger each night. Happens from New Moon to Full Moon. The lit side appears on the right (in the Northern Hemisphere).
Waning
The illuminated portion is shrinking each night. Happens from Full Moon to New Moon. The lit side appears on the left (in the Northern Hemisphere).

How to Model Positions

1
Place the Sun at the center. All light originates from this point.
2
Draw Earth's orbit as a large circle (or ellipse) around the Sun.
3
Position Earth somewhere on its orbit. Mark which side faces the Sun (day) and which faces away (night).
4
Draw the Moon's orbit as a smaller circle around Earth.
5
Position the Moon on its orbit. Shade the half facing away from the Sun. The visible phase is what an observer on Earth would see.

Example 1: Identifying a Phase

Sunlight Earth Moon ? Observer

Example 2: Which Eclipse?

Sun Moon Earth Moon's shadow falls on Earth Order: Sun – Moon – Earth = Solar Eclipse

Example 3: Predicting from a Model

Scenario: A model shows the Moon at a 90-degree angle to the right of the Sun-Earth line, and the right half of the Moon (as seen from Earth) is illuminated. What phase is this, and what comes next?

1
Identify the position: Moon is 90 degrees from the Sun-Earth line. Right half is lit.
2
Match the phase: Right half lit + 90 degrees = First Quarter.
3
Predict what's next: The Moon continues orbiting. The lit portion keeps growing (waxing gibbous), heading toward a Full Moon in about 7 days.

Common Mistakes

✕ Moon makes its own light
The Moon only reflects sunlight. Its brightness depends entirely on its position relative to the Sun.
✕ Earth's shadow causes phases
Phases are caused by the angle of sunlight, not Earth's shadow. Earth's shadow only matters during a lunar eclipse.
✕ Eclipses happen every month
The Moon's orbit is tilted \(\sim 5^\circ\), so perfect alignment is rare: only 2 to 5 eclipses per year.

Summary of Positions & Events

Event Position What We See
Day / Night Earth rotates relative to Sun Sunlit side = day; opposite = night
New Moon Sun – Moon – Earth Moon appears dark
Full Moon Sun – Earth – Moon Moon appears fully lit
Solar Eclipse Sun – Moon – Earth (aligned) Moon blocks Sun briefly
Lunar Eclipse Sun – Earth – Moon (aligned) Moon turns reddish in Earth's shadow

Question 1 of 5

During a full moon, what is the order of the three bodies along the line from the Sun?

Question 2 of 5

What causes day and night on Earth?

Question 3 of 5

A solar eclipse can only occur during which lunar phase?

Question 4 of 5

Why don't eclipses occur every month?

Question 5 of 5

If the Moon is "waxing," which statement is true?