Moon Phases and the Sky
Why do we always see the same side of the Moon from Earth?
Because the Moon rotates once on its axis in the same time it takes to orbit Earth.
The Moon takes ~27 days to spin once and ~27 days to orbit Earth once. This is called tidal locking or synchronous rotation, so the same “near side” always faces us.
Why does the Moon have a stronger effect on Earth’s tides than the Sun, even though the Sun is much bigger?
Because the Moon is much closer to Earth, its gravitational pull on our oceans is stronger than the Sun’s pull.
If the Northern Hemisphere is tilted toward the Sun, what season is it there?
What is Summer?
During a solar eclipse, why can some people see a total eclipse while others nearby only see a partial eclipse?
Because the Moon’s shadow has two parts: the umbra (total darkness, total eclipse) and the penumbra (partial shadow, partial eclipse).
If it’s summer in Australia, what season is it in the U.S.?
What is Winter?
If the Moon rises around sunset, what phase is it?
Full Moon
Why: A full Moon happens when the Moon is opposite the Sun in the sky. That means when the Sun sets in the west, the full Moon rises in the east — they trade places.
What type of tide happens during a first-quarter Moon?
What is a Neap Tide?
In December, the Sun’s rays are most direct over which part of Earth?
What is the Southern Hemisphere / Tropic of Capricorn?
During what Moon phase does a lunar eclipse occur?
What is the Full Moon?
Draw the Sun, Earth, and Moon during a first-quarter Moon. Label where you’d see the lit half.
Moon at 90° angle to Earth and Sun.
You see a crescent Moon in the west just after sunset. Is it waxing or waning?
What is a Waxing Crescent?
Why: A waxing Moon is growing bigger each night. If you see a thin crescent right after sunset, it must be waxing, because the waning crescent rises before sunrise (in the morning sky).
You see a full Moon tonight. What kind of tides will happen tomorrow?
What are Spring Tides (very high highs, very low lows)
If it is spring in the Northern Hemisphere, what season is it in the Southern Hemisphere?
What is Fall?
During what Moon phase does a solar eclipse occur?
What is the New Moon?
Imagine you see a waxing gibbous Moon. What kind of tides are happening?
What are spring tides are coming soon; stronger tides
If you see the Moon high in the sky at midnight, what phase is it most likely in?
What is a full moon?
Why: A full Moon is brightest and directly opposite the Sun. That puts it highest in the sky around midnight. Other phases are either earlier or later.
If it’s high tide at noon, about what time will the next high tide be?
Around midnight, ~12 hours later
You are in Alaska, and the Sun barely rises all day. What season is it, and why?
What is Winter, because the North Pole is tilted away from the Sun
Imagine it is a full Moon, but no lunar eclipse happens. Why didnʻt a lunar eclipse occur?
The Moon’s orbit is tiltedabout 5° to Earth’s orbit, so even at full Moon, it may pass above or below Earth’s shadow instead of going through it.
A student in Hawaii notices it is the June solstice. What direction is the Northern Hemisphere tilted?
Toward the Sun
The Moon is a half-circle and rises around noon. Which phase is it?
What is First Quarter?
Why: In first quarter, the Moon is one-quarter of the way through its orbit. It rises at noon, is high around sunset, and sets at midnight. That’s when you see the right half lit up.
Imagine it is a full Moon. A student in Hawaii notices very high tides. At the same time, a student in Africa also notices high tides. How can both sides of Earth have high tide at the same time?
The Moon’s gravity pulls water toward it, making a bulge (high tide) on the side facing the Moon. At the same time, inertia causes water on the opposite side of Earth to bulge outward, creating another high tide. That’s why there are two high tides at once on opposite sides of Earth.
If the days and nights are equal in length everywhere, what is this called and in what months does it happen?
What is the Equinox? It happens in March and September.
Why are lunar eclipses visible to anyone on the night side of Earth, while solar eclipses can only be seen in a small area?
Because Earth’s shadow is huge compared to the Moon’s, so the Moon can be darkened for the entire night side of Earth. The Moon’s shadow on Earth, however, is very small, so only a narrow path sees a solar eclipse.
During lunar eclipses, the Moon turns a reddish color instead of going completely dark. Why does this happen, and what does it tell us about Earth’s atmosphere?
When the Moon passes into Earth’s shadow, some sunlight is bent (refracted) through Earth’s atmosphere. Shorter wavelengths (blue light) scatter, while longer wavelengths (red/orange light) pass through and bend onto the Moon. This makes the Moon appear red — often called a “blood Moon.”
It shows Earth’s atmosphere scatters light and can even affect how bright or dark the eclipse looks depending on dust, pollution, or volcanic eruptions.