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Below, check out a visualization of the April 8, 2024, total solar eclipse!

What's the difference between a lunar eclipse and a solar eclipse?

Solar Eclipse

A solar eclipse happens when the Moon gets in the way of the Sun's light and casts its shadow on Earth. That means during the day, the Moon moves over the Sun and it gets dark. Isn't it strange that it gets dark in the middle of the day?

This total eclipse happens about every year and a half somewhere on Earth. A partial eclipse, when the Moon doesn't completely cover the Sun, happens at least twice a year somewhere on Earth.

But not everyone experiences every solar eclipse. Getting a chance to see a total solar eclipse is rare. The Moon's shadow on Earth isn't very big, so only a small portion of places on Earth will see it. You have to be on the sunny side of the planet when it happens. You also have to be in the path of the Moon's shadow.

On average, the same spot on Earth only gets to see a solar eclipse for a few minutes about every 375 years!

Caution! Eye Safety During a Total Solar Eclipse
Except during the brief total phase of a total solar eclipse, when the Moon completely blocks the Sun's bright face, it is not safe to look directly at the Sun without specialized eye protection for solar viewing.

For safety information, visit the NASA Eclipse Safety Page.

Lunar Eclipse

During a lunar eclipse, Earth gets in the way of the Sun's light hitting the Moon. That means that during the night, a full moon fades away as Earth's shadow covers it up.

The Moon can also look reddish because Earth's atmosphere absorbs the other colors while it bends some sunlight toward the Moon. Sunlight bending through the atmosphere and absorbing other colors is also why sunsets are orange and red.

During a total lunar eclipse, the Moon is shining from all the sunrises and sunsets occurring on Earth!

Why don't we have a lunar eclipse every month?

You might be wondering why we don't have a lunar eclipse every month as the Moon orbits Earth. It's true that the Moon goes around Earth every month, but it doesn't always get in Earth's shadow. The Moon's path around Earth is tilted compared to Earth's orbit around the Sun. The Moon can be behind Earth but still get hit by light from the Sun.

Because they don't happen every month, a lunar eclipse is a special event. Unlike solar eclipses, lots of people get to see each lunar eclipse. If you live on the nighttime half of Earth when the eclipse happens, you'll be able to see it.

Remembering the Difference

It's easy to get these two types of eclipses mixed up. An easy way to remember the difference is in the name. The name tells you what gets darker when the eclipse happens. In a solar eclipse, the Sun gets darker. In a lunar eclipse, the Moon gets darker.

Related Resources for Educators

Launchpad: Solar Eclipses

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Before we get into the subject of gravity and how it acts, it's important to understand the difference between weight and mass.

We often use the terms "mass" and "weight" interchangeably in our daily speech, but to an astronomer or a physicist they are completely different things. The mass of a body is a measure of how much matter it contains. An object with mass has a quality called inertia. If you shake an object like a stone in your hand, you would notice that it takes a push to get it moving, and another push to stop it again. If the stone is at rest, it wants to remain at rest. Once you've got it moving, it wants to stay moving. This quality or "sluggishness" of matter is its inertia. Mass is a measure of how much inertia an object displays.

Weight is an entirely different thing. Every object in the universe with mass attracts every other object with mass. The amount of attraction depends on the size of the masses and how far apart they are. For everyday-sized objects, this gravitational pull is vanishingly small, but the pull between a very large object, like the Earth, and another object, like you, can be easily measured. How? All you have to do is stand on a scale! Scales measure the force of attraction between you and the Earth. This force of attraction between you and the Earth (or any other planet) is called your weight.

If you are in a spaceship far between the stars and you put a scale underneath you, the scale would read zero. Your weight is zero. You are weightless. There is an anvil floating next to you. It's also weightless. Are you or the anvil mass-less? Absolutely not. If you grabbed the anvil and tried to shake it, you would have to push it to get it going and pull it to get it to stop. It still has inertia, and hence mass, yet it has no weight. See the difference?

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Your Weight on Other Worlds | Exploratorium

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Before we get into the subject of gravity and how it acts, it's important to understand the difference between weight and mass. We often use the terms "mass" and "weight" interchangeably in our daily speech, but to an astronomer or a physicist they are completely different things.

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Want to melt those years away? Travel to an outer planet!

This Page requires a Javascript capable browser

TO DO AND NOTICE

Fill in your birthdate below in the space indicated. (Note you must enter the year as a 4-digit number!)

Click on the "Calculate" button.

Notice that your age on other worlds will automatically fill in. Notice that Your age is different on the different worlds. Notice that your age in "days" varies wildly.

Notice when your next birthday on each world will be. The date given is an "earth date".

You can click on the images of the planets to get more information about them from Bill Arnett's incredible Nine Planets web site.

WHAT IS GOING ON?

The Days (And Years) Of Our Lives

Looking at the numbers above, you'll immediately notice that you are different ages on the different planets. This brings up the question of how we define the time intervals we measure. What is a day? What is a year?

The earth is in motion. Actually, several different motions all at once. There are two that specifically interest us. First, the earth rotates on its axis, like a spinning top. Second, the earth revolves around the sun, like a tetherball at the end of a string going around the center pole.

The top-like rotation of the earth on its axis is how we define the day. The time it takes the earth to rotate from noon until the next noon we define as one day. We further divide this period of time into 24 hours, each of which is divided into 60 minutes, each of which is broken into 60 seconds. There are no rules that govern the rotation rates of the planets, it all depends on how much "spin" was in the original material that went into forming each one. Giant Jupiter has lots of spin, turning once on its axis every 10 hours, while Venus takes 243 days to spin once.

The revolution of the earth around the sun is how we define the year. A year is the time it takes the earth to make one revolution - a little over 365 days.

We all learn in grade school that the planets move at differing rates around the sun. While earth takes 365 days to make one circuit, the closest planet, Mercury, takes only 88 days. Poor, ponderous, and distant Pluto takes a whopping 248 years for one revolution. Below is a table with the rotation rates and revolution rates of all the planets.

Why the huge differences in periods? We need to go back to the time of Galileo, except that we're not going to look at his work, but rather at the work of one of his contemporaries, Johannes Kepler (1571-1630).

Johannes Kepler

Tycho Brahe

Kepler briefly worked with the great Danish observational astronomer, Tycho Brahe. Tycho was a great and extremely accurate observer, but he did't have the mathematical capacity to analyze all of the data he collected. After Tycho's death in 1601, Kepler was able to obtain Tycho's observations. Tycho's observations of planetary motion were the most accurate of the time (before the invention of the telescope!). Using these observations, Kepler discovered that the planets do not move in circles, as 2000 years of "Natural Philosophy" had taught. He discovered that they move in ellipses. A ellipse is a sort of squashed circle with a short diameter (the "minor axis") and a longer diameter (the "major axis"). He found that the Sun was positioned at one "focus" of the ellipse (there are two "foci", both located on the major axis). He also found that when the planets were nearer the sun in their orbits, they move faster than when they were farther from the sun. Many years later, he discovered that the farther a planet was from the sun, on the average, the longer it took for that planet to make one complete revolution. These three laws, stated mathematically by Kepler, are known as "Kepler's Laws of Orbital Motion." Kepler's Laws are still used today to predict the motions of planets, comets, asteroids, stars, galaxies, and spacecraft.

Here you see a planet in a very elliptical orbit.
Note how it speeds up when it's near the Sun.

Kepler's third law is the one that interests us the most. It states precisely that the period of time a planet takes to go around the sun squared is proportional to the average distance from the sun cubed. Here's the formula:

Let's just solve for the period by taking the square root of both sides:

Note that as the distance of the planet from the sun is increased, the period, or time to make one orbit, will get longer. Kepler didn't know the reason for these laws, though he knew it had something to do with the Sun and its influence on the planets. That had to wait 50 years for Isaac Newton to discover the universal law of gravitation.

The Gravity Of The Situation

Isaac Newton

Closer planets revolve faster, more distant planets revolve slower. Why? The answer lies in how gravity works. The force of gravity is a measure of the pull between two bodies. This force depends on a few things. First, it depends on the mass of the sun and on the mass of the planet you are considering. The heavier the planet, the stronger the pull. If you double the planet's mass, gravity pulls twice as hard. On the other hand, the farther the planet is from the sun, the weaker the pull between the two. The force gets weaker quite rapidly. If you double the distance, the force is one-fourth. If you triple the separation, the force drops to one-ninth. Ten times the distance, one-hundredth the force. See the pattern? The force drops off with the square of the distance. If we put this into an equation it would look like this:

The two "M's" on top are the sun's mass and the planet's mass. The "r" below is the distance between the two. The masses are in the numerator because the force gets bigger if they get bigger. The distance is in the denominator because the force gets smaller when the distance gets bigger. Note that the force never becomes zero no matter how far you travel. Knowing this law helps you inderstand why the planets move faster when they are closer to the sun - they are pulled on with a stronger force and are whipped around faster!

LINKS
©2000 Ron Hipschman

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Your Age on Other Worlds | Exploratorium

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Want to melt those years away? Travel to an outer planet! This Page requires a Javascript capable browser Fill in your birthdate below in the space indicated. (Note you must enter the year as a 4-digit number!) Click on the "Calculate" button. Notice that your age on other worlds will automatically fill in.

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Kids aren’t the only ones who go through phases: The Moon has a few of its own. Waxing and waning you might have heard of, but what’s a gibbous Moon?

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Spring into action! Find some summer love! Fall into this movie and learn about the seasons! Um. Uh, Winter! Winter, Winter, Winter.

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From the oceans to the valleys, to the landforms, that we love! Tim and Moby talk about our blue, blue planet and its delicate atmosphere.

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A Day on Earth: StudyJams! Science | Scholastic.com

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What causes night and day? This activity will teach students about Earth's rotation and how it impacts our days, nights, and seasons.

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team's incredible timelapse of solar system to scale - cnn video

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Team's incredible timelapse of solar system to scale - CNN Video

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A group of friends set out to put together the first scale model of the solar system depicting the correct proportions of each planet's orbital paths.

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A Day on Earth: StudyJams! Science | Scholastic.com

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What causes night and day? This activity will teach students about Earth's rotation and how it impacts our days, nights, and seasons.

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The Solar Eclipse In Varanasi - Wonders of the Solar System - Series 1 Episode 1 Preview - BBC Two

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More about this episode: http://www.bbc.co.uk/programmes/b00qyxfb http://bbc.co.uk/solarsystem In this spell-binding series, Professor Brian Cox visits some of the most stunning locations on Earth to describe how the laws of nature have carved natural wonders across the Solar System.

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The Universe: The Phases of the Moon | History

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The phases of the moon have been used to track time since the dawn of mankind.