Eclipses
Our Moon takes centre stage in some of the most spectacular events in the sky! Here we tell you more about solar and lunar eclipses, and why we on Earth are especially lucky!
"Sköll is the wolf that follows the shining god to the forest's edge; and another is Hati, he is Hróðvitnir's son, he shall run before the bright bride of heaven." Grímnismál (translation of verse 39)
There are as many myths and legends about eclipses as there are cultures. And perhaps that is not so surprising, for it is hard not to be affected when the sky suddenly darkens and the Earth is bathed in a reddish glow. The poem above describes the giant wolves Sköll and Hati, who according to Norse mythology chase the Sun and Moon in an attempt to swallow them. According to legend, eclipses occur when the wolves briefly catch hold of the two celestial bodies, and when they finally succeed in devouring their prey, it will be a sign of the end of the world, Ragnarök.
When humans tell stories and myths about how the gods influence nature, it is often an attempt to explain the inexplicable. The more we explore and learn about nature, the more we find that the world is magical and wonderful even without gods! Today astronomers know much more about the magic behind eclipses.
Eclipses are in fact shadows! Enormous shadows that form when the Moon or Earth blocks the Sun's light, casting each other into darkness. So let us take a closer look at these remarkable shadows!
Additional info:
See when this year's eclipses will take place here:
The Night Sky - What You Can See
Read more about the Sun, the Moon, and the Earth here:
The Moon slides in front of the Sun during a total solar eclipse, revealing the Sun's glowing corona, the only moment when this outermost layer of the Sun's atmosphere becomes visible to the naked eye. (Credit: NASA)
Solar eclipse
A solar eclipse occurs when the Moon moves between Earth and the Sun. When this happens, the Moon blocks the Sun's light and casts a shadow onto Earth's surface. Since the Moon is smaller than Earth, its shadow only covers part of Earth, meaning not everyone around the globe will be able to see a solar eclipse!
If you are standing in a place that lies directly within the Moon's shadow, you will experience what we call a total solar eclipse: the Moon blocks the entire Sun, and it becomes completely dark on Earth even though it is the middle of the day! If you have the right equipment to protect your eyes (very important! Read "Protect your eyes" on the side of the page), you can look up at the sky and see the Moon as a large dark sphere moving in front of the Sun until it covers it completely. The Moon will then continue past, and it will become light again. The entire solar eclipse from start to finish can take a couple of hours, but the Sun will only be completely covered for a few minutes.
PROTECT YOUR EYES
The sun's light is dangerous for your eyes.
Read here about solar eclipse glasses and how to make your own solar telescope.
Protect your eyes during the solar eclipse
The Moon's shadow appears as a dark spot on Earth's surface during a solar eclipse, seen from space. Only those standing within that small patch of darkness experience the dramatic darkness of a total eclipse. (Credit: NASA/NOAA, Deep Space Climate Observatory)
If you are standing somewhere on the edge of the Moon's shadow, it will not block the Sun completely, and you will instead experience a partial solar eclipse. Here the Moon only covers part of the Sun, so it does not become completely dark. If you have the right equipment (still very important to protect your eyes), you can look up at the Sun and Moon, and it will look as though a large bite has been taken out of the Sun where the Moon is blocking it!
An illustration of how the Moon's shadow falls on Earth to create a solar eclipse. The inset shows the three types of solar eclipse as seen from Earth: partial, total, and annular. (Credit: CPH Planetarium, Nanna Bach-Møller)
Our solar eclipse is completely unique!
Earth is not the only place where solar eclipses occur. Many of the planets in the Solar System have their own moons, most of them in fact have many moons, and every time one of these moons moves in front of the Sun, a solar eclipse will occur. And yet our solar eclipses here on Earth are very special! By a remarkable cosmic coincidence, our Moon happens to lie at exactly the right distance from us so that it appears almost the same size as the Sun in the sky, even though the Sun is vastly larger. So when we have a total solar eclipse, the Moon fits almost perfectly in front of the Sun.
This means the Moon can cover the Sun itself without covering the outermost part of the Sun's atmosphere that we call the corona. During total solar eclipses, the corona stands out as a spectacular glowing halo around the Moon's shadow, and this is simply a sight you cannot see anywhere else in the Solar System!
A solar eclipse on Mars, captured by the Perseverance rover. Mars' moon Phobos is much smaller and less round than our Moon, giving Martian eclipses a very different look from the ones we experience on Earth. (Credit: NASA/JPL-Caltech)
The reason the Moon and Sun appear the same size is that the Moon is both 400 times smaller than the Sun (in diameter) and 400 times closer to us than the Sun. If the Moon were smaller or further away, it would not cover the entire Sun, and if it were larger or closer, it would cover everything and we would not be able to see the Sun's edge and corona. This lucky coincidence arose entirely by chance, and the Moon does not always fit so perfectly in front of the Sun. Sometimes the Moon is a little further from Earth in its orbit, and is just barely too small to cover the Sun, and we get what is called an annular solar eclipse.
The Sun's corona, its outermost atmosphere, blazing into view during a total solar eclipse. This breathtaking sight is only visible from Earth, where by a remarkable cosmic coincidence the Moon fits almost perfectly in front of the Sun. (Credit: ESA)
In fact, we will not always be able to see total solar eclipses. The Moon is very slowly moving away from Earth, drifting outward at 3.8 cm per year. So once the Moon was so close to Earth that the Sun's corona could not be seen during eclipses, and in around 600 million years the Moon will be so far away that it can no longer cover the Sun completely.
We know of no other planet whose moon fits so perfectly in front of its star, so we are truly fortunate here on Earth. Some astronomers therefore joke that if we ever finally meet aliens, we can invite them to Earth's solar eclipse so they can enjoy the beautiful sight with us, a little like travelling to other countries to see their spectacular natural wonders. Just look at the top image to see what a solar eclipse looks like on Mars, where the potato-shaped moon Phobos creates a very different and rather less elegant spectacle!
Lunar eclipse
A lunar eclipse occurs when Earth lies between the Moon and the Sun, so that Earth blocks the Sun's light as seen from the Moon.
Normally when the Moon is on the side of Earth facing away from the Sun, it is fully illuminated by the Sun's light, and we have what is called a full moon. So when an eclipse takes place, Earth gradually blocks the Sun's light and the beautiful full moon is darkened by Earth's shadow. Unlike a solar eclipse, a lunar eclipse can be seen from a large area of Earth, namely the entire half of Earth that is experiencing night when the eclipse takes place!
An illustration of how Earth's shadow falls on the Moon to create a lunar eclipse. The inset shows the three stages as seen from Earth: the full moon before the eclipse, a partial eclipse, and the red Blood Moon of a total eclipse. (Credit: CPH Planetarium, Nanna Bach-Møller)
How the lunar eclipse looks depends on whether Earth, the Moon, and the Sun are perfectly aligned, or slightly offset from each other. If the Moon is slightly offset from Earth and the Sun, we get what is called a partial lunar eclipse, where Earth's shadow only covers part of the Moon. If they are perfectly aligned, we get a total lunar eclipse, where the entire Moon lies within Earth's shadow. When this happens, Earth blocks almost all of the Sun's light, and the Moon can be bathed in a reddish glow, what we call a Blood Moon.
The Blood Moon occurs because some of the Sun's light passes through Earth's atmosphere and is redirected onto the Moon. Sunlight appears white when you look at it, but it is actually a mixture of all the colours you can see in a rainbow. The blue part of the light is scattered more in Earth's atmosphere, while the red part passes through and reaches the Moon. This is the same effect that makes the sky blue and sunsets red. You could therefore say that it is the light from Earth's sunset that colours the Blood Moon red.
A total lunar eclipse, turning the Moon a deep blood red as it passes through Earth's shadow. The red colour comes from sunlight filtering through Earth's atmosphere. (Credit: NASA)
When Earth, the Moon, and the Sun align
Eclipses occur when Earth, the Moon, and the Sun align, and this requires the Moon's orbit to be positioned in a very specific way. Earth and the other planets in the Solar System all travel in nearly circular orbits around the Sun. All the orbits lie neatly alongside each other, a little like the grooves on a vinyl record, so if you looked at the Solar System from the side, it would be as flat as a pancake! We say that the orbits all lie in the same "plane." The Moon does not orbit the Sun, but orbits Earth, and it takes approximately one month to travel around Earth, which is why we call it a "month." The Moon's orbit lies in nearly the same plane as Earth's own orbit, but not exactly. It is tilted slightly.
If the Moon's orbit lay in exactly the same plane as Earth's, we would have eclipses every month: every time the Moon was closest to the Sun in its orbit, it would block the Sun and create a solar eclipse. And every time the Moon was furthest from the Sun, it would lie in Earth's shadow and we would see a lunar eclipse.
Earth's orbit around the Sun and the Moon's orbit around Earth, seen from above (left) and from the side (right). The slight tilt of the Moon's orbit relative to Earth's is what prevents us from having eclipses every single month. Note that the sizes and distances shown are not to scale. (Credit: CPH Planetarium, Nanna Bach-Møller)
The reason we do not see this is that the Moon's orbit is tilted slightly relative to Earth's, by 5.1 degrees to be precise. And that is enough to ensure that most of the time the Moon is not perfectly aligned with Earth and the Sun. Occasionally however, the orbits align perfectly, so that the Moon will position itself perfectly between the Sun and Earth 2 to 5 times per year, creating a solar eclipse, and will lie perfectly in Earth's shadow 1 to 2 times per year, giving us a lunar eclipse.
