There are two main types of eclipses, solar and lunar eclipses. A solar eclipse occurs when the Moon moves between the Sun and the Earth and temporarily blocks the Sun, casting a shadow on the Earth. A Luner eclipse is when the Earth passes between the Sun and the Moon and blocks out the Sun, casting a shadow on the Moon. To understand eclipses, we first must understand how the Moon dances with the Earth and the Sun.
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Our Moon
The Moon makes a full circle around the Earth once a month. As it does, its orientation to the Sun changes and the amount of its surface we can see lit up by sunlight changes. This is how we get the phases of our Moon, from New Moon to Full Moon and back again. Below is a diagram comparing how the Moon looks from the Earth’s point of view and how it would look from a bird’s eye view in space.
Image Credit: NASA
Still a bit confused? Here is a great video on the Moon’s phases.
Looking at the diagrams above, you can see the Moon passes between the Sun and Earth (New Moon phase) once a month and has the Earth between it and the Sun (Full Moon phase) once a month. So why do we not see a solar and a lunar eclipse every month?
This is because the Moon’s orbit around the Earth is not flat.
Image Credit: YouTube Space Videos
If the Moon’s shadow is too high or too low when it is between the Earth and the Sun, we will just see a New Moon, not a solar eclipse. The same goes for the Earth, if the Earth’s shadow is too high or too low when it is between the Moon and the Sun, we will see a Full Moon, not a lunar eclipse.
For either eclipse to occur the alignment must be just right.
Lunar Eclipse
As we discovered above, a lunar eclipse happens when the Moon passes into the Earth’s shadow.
There are three types of lunar eclipses, penumbral eclipse, partial lunar eclipse, and total lunar eclipse. Types of lunar eclipses depend on what part and how much of the Earth’s shadow the Moon is passing through.
Penumbrial Eclipse
This is the hardest lunar eclipse to see, and if you don’t know it’s happening, you probably won’t even notice it. A penumbral eclipse occurs when the Moon travels through the Earth’s penumbra (view diagram above), or the faint outer part of the Earth’s shadow. When passing through the penumbra the Moon will dim ever so slightly.
Partial Lunar Eclipse
Sometimes, due to an imperfect alignment, the Moon will only pass through part of the Earth’s umbra (view diagram above), the darkest part of it’s shadow. A shadow will grow and appear on the Moon and then recede, without ever fully covering the Moon.
Total Lunar Eclipse
A total lunar eclipse will occur when the Moon passes entirely into the darkest part of the Earth’s shadow, the umbra. While in Earth’s shadow, the Moon will turn a reddish hue. Lunar eclipses are sometimes called “Blood Moons” because of this red color.
Why Does the Moon Turn Red?
There is a phenomenon that makes our sky blue during the day but turn red at sunset. It’s called Rayleigh Scattering. Light travels in waves and the different colors of light have different physical properties. Blue light has a shorter wavelength, this makes it easier to be scattered by particles in Earth’s atmosphere.
Red light has a longer wavelength and will travel right through the Earth’s atmosphere with little to no scattering. When the Sun is overhead, it is 90 degrees to the Earth’s surface, this is when it has the shortest amount of atmosphere to travel through before getting to our eyes.
We see a blue sky because we are seeing the blue light that did not get scattered away in this short distance. When the Sun is setting, it must pass through more of the atmosphere before reaching our eye. With this longer distance to travel, the blue light gets scattered away and we are left seeing the oranges and reds that did not get scattered away.
Image Credit: NASA Goddard Space Flight Center/Scientific Visualization Studio
During a lunar eclipse the Moon turns red because the red light is the only light that makes it all the way through the Earth’s atmosphere to the lunar surface.
Thankfully for us, lunar eclipses are quite common, occurring between 2-5 times a year. A lunar eclipse can last up to six hours. Approximately 29% of lunar eclipses are total lunar eclipses.
Solar Eclipses
The counterpart to the lunar eclipse is the solar eclipse. A solar eclipse occurs when the Moon passes between the Earth and the Sun perfectly and covers the Sun, casting a shadow onto Earth’s surface.
A solar eclipse can only happen in the New Moon phase. As discussed above, due to the tilt of the Moon’s orbit we do not see a solar eclipse every time the Moon is in the New Moon phase. Like a lunar eclipse, there are different types of solar eclipses, depending on the alignment of the Moon and Sun. The three types of solar eclipses are partial, annular, and total solar eclipses.
Partial Solar Eclipse
A partial solar eclipse occurs when the Moon only covers part of the Sun. This can be due to the Moon and Sun not lining up perfectly, or the viewing angle here on Earth. On the day of a total eclipse, you will only see a partial eclipse if you are not in the direct path of the Moon’s shadow.
Annular Solar Eclipse
An annular solar eclipse is a mix between a total and partial eclipse. Like a total eclipse, the centers of the Sun and Moon will align. But like a partial eclipse, the Moon will only cover part of the Sun. In an annular eclipse the part of the Sun that remains uncovered is a large ring around the Moon.
If the Moon and Sun’s centers are aligned, why do we not see a total eclipse? This is because the Moon’s path around the Earth is not a perfect circle, it is an ellipse. When the Moon is further from the Earth, it appears smaller, resulting in only an annular eclipse if it passes in front of the Sun.
Total Solar Eclipse
A total eclipse occurs when the Moon, Sun, and Earth are aligned just right, and the Moon blocks the entire Sun for a short period of time. The only light that is left is from the Sun’s corona, as seen below, this light is not directed at Earth, which is why there is a shadow cast on Earth.
The Moon’s shadow will trace a path across the Earth’s surface. This is called the path of totality. Like the Earth’s shadow during a lunar eclipse, the Moon’s shadow has an umbra (darkest part) and a penumbra. If you are in the path of the umbra, you will see a total eclipse. The further you get from the umbra the less of an eclipse you will see.
Those who are lucky enough to be in the path of a total eclipse will see the Moon slowly move in front of the Sun until it reaches totality, and the sky will turn dark. The Moon will then continue to pass and reveal the Sun once again. Totality can last up to seven and a half minutes but is usually shorter.
During totality is the ONLY time it is safe to look at an eclipse without eclipse glasses. Unless the Sun is 100% covered by the Moon, you must wear eclipse glasses to prevent damage to your eyes. You must wear eclipse glasses during a partial and annular eclipse as well.
Not any eclipse glasses will do. Make sure to verify that the product’s label says it is ISO verified. ISO is the International Organization for Standardization Filters. Glasses that are ISO compliant not only reduce visible light to safe levels, but block UV and IR radiation as well.
How often do Solar Eclipses Occur?
Somewhere on Earth will experience a total solar eclipse about every 18 months, but due to the size of the Moon’s shadow, only a small percentage of the population will be able to see any given eclipse. In fact, on average, any given point on the Earth’s surface will experience a total solar eclipse once in 375 years.
The next total solar eclipse will pass over parts of the United States on April 8th, 2024. Below we will discuss everything you will need to prepare for this event. Below is a map of the paths of totality of solar eclipses for the next 12 years.
Do All Planets Experience a Total Solar Eclipse?
While all planets that have moons that orbit in the path of the Sun will experience some sort of eclipse at some point in time, not all planets will experience a total solar eclipse. Only the Earth, Jupiter, Saturn, Uranus, and Neptune have moons that are the right size and the right distance between the Sun and the planet to create total solar eclipses.
Will We Always See a Total Solar Eclipse?
Nope, total solar eclipses are very special, and they are not guaranteed forever. Our Moon is slowly moving away from Earth, about 1.6 inches every year. In the future it will be too far away and appear too small in the sky to fully cover the Sun.
While this will not happen for a very long time, about 600 million years, you should still try and see an eclipse if you can and be part of the small portion of humanity that gets to experience this miraculous event.
Just don’t forget to use your eclipse glasses!
