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Why Do We Have Different Seasons? | California Academy of Sciences

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The tilt of Earth's axis is about 23.5 degrees, but the tilt of the moon's axis is only about 1.5 degrees. As such, the moon virtually has no seasons. This means that some areas are always lit by sunlight, and other places are perpetually draped in shadow.

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From one night to the next the appearance of the moon varies. It's tempting to connect the changes in the moon with changes on Earth. That's one reason why so many cultures have seasonal names associated with the moon: "the harvest moon," "the frost moon" and "the flower moon," for example. But tempting as it is, it's incorrect to connect the moon with seasonal variations. Whatever influence the moon has on Earth's seasons is not only minimal, but on timescales of thousands of years. Annual seasonal variations on Earth are solely due to the Earth's orbit and the tilt of its axis.

The Earth orbits the sun along an almost circular path. At the same time, the Earth spins around its axis once every day. It takes about 365 and a quarter days for the Earth to complete a circuit -- and that's the definition of a year. The plane in which the Earth orbits is called the ecliptic. The axis around which the Earth rotates is not perpendicular to the ecliptic. That is, the Earth's axis of rotation is tilted with respect to the direction it moves. That means that at some times of year -- some places on the Earth's orbit -- the sun is aimed more directly at the northern hemisphere and sometimes aimed more directly at the southern hemisphere. When your part of the Earth is tilted toward the sun, it's spring and summer, and when your hemisphere is tilted away from the sun it is autumn and winter. That's the cause of the seasons.

All the while the Earth is rotating and orbiting, the moon is doing the same thing. The moon orbits the Earth once every 29 and a half days -- that's the time from one full moon to the next. The 29 and a half does not fit evenly into 365 and a quarter. That means that full moons do not always occur on the same dates from year to year. You can look at a lunar calendar to see the variations in the dates of the full and new moons. Over the course of your life, the dates of the full moon have shifted by weeks, and the seasons have not changed.

Just because the moon doesn't affect the seasons doesn't mean the phases of the moon have no effects. Full moons mean something to people, so much so that full moons get special names -- names connected to the seasons. Among the names for seasonal moons in both Native American and English traditions you can find those named the Pink Moon, the Egg Moon; the Flower Moon, the Milk Moon, the Sturgeon Moon, the Grain Moon, the Hunter's Moon, the Harvest Moon, the Beaver Moon and the Frost Moon. Those names are connected to seasonal activities, but they do nothing to change the flow of the seasons. The Harvest Moon, for example, is the full moon closest to the autumnal equinox -- the beginning of fall. Traditionally, farmers used the light of the Harvest Moon to extend their reaping hours -- but that has no connection to the actual beginning of fall, which can come before or after the Harvest Moon.

The Earth's axis is tilted at 23.5 degrees from the ecliptic. That angle is responsible for the seasons. But the tilt angle is not fixed. The moon's gravitational pull on the Earth causes precession -- a small, 21,000-year cyclic change in the angle of the Earth's tilt. Without the moon's pull, precession would be even slower, but it would be larger. Larger precession means a greater change in the tilt of the Earth's axis, which means that the nature of the seasons would change. Not much to worry about, though, because those changes would take billions of years -- that is, if Earth didn't have a moon.

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From one night to the next the appearance of the moon varies. It's tempting to connect the changes in the moon with changes on Earth. That's one reason why so many cultures have seasonal names associated with the moon: "the harvest moon," "the frost moon" and "the flower moon," for example.

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High and low tides are caused by the Moon. The Moon's gravitational pull generates something called the tidal force. The tidal force causes Earth—and its water—to bulge out on the side closest to the Moon and the side farthest from the Moon. These bulges of water are high tides.

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High tides and low tides are caused by the Moon. The Moon's gravitational pull generates something called the tidal force. The tidal force causes Earth-and its water-to bulge out on the side closest to the Moon and the side farthest from the Moon. These bulges of water are high tides.

As the Earth rotates, your region of Earth passes through both of these bulges each day. When you're in one of the bulges, you experience a high tide. When you're not in one of the bulges, you experience a low tide. This cycle of two high tides and two low tides occurs most days on most of the coastlines of the world.

More About Tides

Tides are really all about gravity, and when we're talking about the daily tides, it's the Moon's gravity that's causing them.

As Earth rotates, the Moon's gravity pulls on different parts of our planet. Even though the Moon only has about 1/100th the mass of Earth, since it's so close to us, it has enough gravity to move things around. The Moon's gravity even pulls on the land, but not enough for anyone to tell (unless they use special, really precise instruments).

When the Moon's gravity pulls on the water in the oceans, however, someone's bound to notice. Water has a much easier time moving around, and the water wants to bulge in the direction of the Moon. This is called the tidal force.

Because of the tidal force, the water on the side of the Moon always wants to bulge out toward the Moon. This bulge is what we call a high tide. As your part of the Earth rotates into this bulge of water, you might experience a high tide.

One thing to note, however, is that this is just an explanation of the tidal force-not the actual tides. In real life, the Earth isn't a global ocean, covered in an even layer of water. There are seven continents, and that land gets in the way. The continents prevent the water from perfectly following the Moon's pull. That's why in some places, the difference between high and low tide isn't very big, and in other places, the difference is drastic.

That explains the first high tide each day, but what about the second high tide?

The ocean also bulges out on the side of Earth opposite the Moon.

Wait, what?

If the Moon's gravity is pulling the oceans toward it, how can the ocean also bulge on the side of Earth away from the Moon? It does seem a little weird. It's all because the tidal force is a differential force-meaning that it comes from differences in gravity over Earth's surface. Here's how it works:

On the side of Earth that is directly facing the Moon, the Moon's gravitational pull is the strongest. The water on that side is pulled strongly in the direction of the Moon.

On the side of Earth farthest from the Moon, the Moon's gravitational pull is at its weakest. At the center of Earth is approximately the average of the Moon's gravitational pull on the whole planet.

To get the tidal force-the force that causes the tides-we subtract this average gravitational pull on Earth from the gravitational pull at each location on Earth.

Tidal force =
Moon's gravitational pull in a specific location on Earth -
Moon's average gravitational pull over the whole Earth

The result of the tidal force is a stretching and squashing of Earth. This is what causes the two tidal bulges.

These two bulges explain why in one day there are two high tides and two low tides, as the Earth's surface rotates through each of the bulges once a day.

Does anything else affect tides?

The Sun causes tides just like the Moon does, although they are somewhat smaller. When the earth, Moon, and Sun line up-which happens at times of full Moon or new Moon-the lunar and solar tides reinforce each other, leading to more extreme tides, called spring tides. When lunar and solar tides act against each other, the result is unusually small tides, called neap tides. There is a new Moon or a full Moon about every two weeks, so that's how often we see large spring tides.

Wind and weather patterns also can affect water level. Strong offshore winds can move water away from coastlines, exaggerating low tides. Onshore winds can push water onto the shore, making low tides much less noticeable.

High-pressure weather systems can push down sea levels, leading to lower tides. Low-pressure systems-brought on by strong storms and hurricanes-can cause tides than are much higher than predicted, so watch out!

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High tides and low tides are caused by the Moon. The Moon's gravitational pull generates something called the tidal force. The tidal force causes Earth-and its water-to bulge out on the side closest to the Moon and the side farthest from the Moon. These bulges of water are high tides.

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Just like the Earth, half of the Moon is lit by the Sun while the other half is in darkness. The phases we see result from the angle the Moon makes with the Sun as viewed from Earth. We only see the Moon because sunlight reflects back to us from its surface.

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We have a slightly different view of the Moon each night. We describe how the Moon looks with the eight Moon phases, or shapes:

🌑 New

🌒 Waxing Crescent

🌓 First Quarter

🌔 Waxing Gibbous

🌕 Full

🌖 Waning Gibbous

🌗 Third Quarter

🌘 Waning Crescent

If you have looked into the night sky, you may have noticed the Moon appears to change shape each night. Some nights, the Moon might look like a narrow crescent. Other nights, the Moon might look like a bright circle. And on other nights, you might not be able to see the Moon at all. The different shapes of the Moon that we see at different times of the month are called the Moon's phases.

Why does this happen? The shape of the Moon isn't changing throughout the month. However, our view of the Moon does change.

The Moon does not produce its own light. There is only one source of light in our solar system, and that is the Sun. Without the Sun, our Moon would be completely dark. What you may have heard referred to as "moonlight" is actually just sunlight reflecting off of the Moon's surface.

The Sun's light comes from one direction, and it always illuminates, or lights up, one half of the Moon - the side of the Moon that is facing the Sun. The other side of the Moon is dark.

On Earth, our view of the illuminated part of the Moon changes each night, depending on where the Moon is in its orbit, or path, around Earth. When we have a full view of the completely illuminated side of the Moon, that phase is known as a full moon.

But following the night of each full moon, as the Moon orbits around Earth, we start to see less of the Moon lit by the Sun. Eventually, the Moon reaches a point in its orbit when we don't see any of the Moon illuminated. At that point, the far side of the Moon is facing the Sun. This phase is called a new moon. During the new moon, the side facing Earth is dark.

The eight Moon phases:

🌑 New: We cannot see the Moon when it is a new moon.

🌒 Waxing Crescent: In the Northern Hemisphere, we see the waxing crescent phase as a thin crescent of light on the right.

🌓 First Quarter: We see the first quarter phase as a half moon.

🌔 Waxing Gibbous: The waxing gibbous phase is between a half moon and full moon. Waxing means it is getting bigger.

🌕 Full: We can see the Moon completely illuminated during full moons.

🌖 Waning Gibbous: The waning gibbous phase is between a full moon and a half moon. Waning means it is getting smaller.

🌗 Third Quarter: We see the third quarter moon as a half moon, too. It is the opposite half as illuminated in the first quarter moon.

🌘 Waning Crescent: In the Northern Hemisphere, we see the waning crescent phase as a thin crescent of light on the left.

The Moon displays these eight phases one after the other as it moves through its cycle each month. It takes 27 days for the Moon to orbit Earth. That means the Moon's cycle is 27 days long.

Here's what the Moon looks like right now from Earth:

Interested in learning more about the Moon?

Related Resources for Educators

Moon Phases Simulation Viewed from Earth and Space
Our World: Moon Phases
Make a Moon Phase Calendar and Calculator

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We have a slightly different view of the Moon each night. We describe how the Moon looks with the eight Moon phases, or shapes: 🌑 New 🌒 Waxing Crescent 🌓 First Quarter 🌔 Waxing Gibbous 🌕 Full 🌖 Waning Gibbous 🌗 Third Quarter 🌘 Waning Crescent If you have looked into the night sky, you may have noticed the Moon appears to change shape each night.

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The Moon is Earth's only permanent natural satellite. ...
The Moon is the second-densest satellite. ...
The Moon always shows Earth the same face. ...
The Moon's surface is actually dark. ...
The Sun and the Moon are not the same size. ...
The Moon is drifting away from the Earth. ...
The Moon was made when a rock smashed into Earth.

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Sometimes when the moon orbits Earth, it moves between the sun and Earth. When this happens, the moon blocks the light of the sun from reaching Earth. This causes an eclipse of the sun, or solar eclipse. During a solar eclipse, the moon casts a shadow onto Earth.

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Editor's note: The last total solar eclipse occurred on Dec. 26, 2019, and was visible from Saudi Arabia, India, Sumatra and Borneo, with a partial eclipse visible in Australia and much of Asia. Read our full coverage here . A solar eclipse occurs when the moon gets between Earth and the sun, and the moon casts a shadow over Earth.