There are three types of shadows, namely, the umbra, penumbra, and antumbra. Each of them is responsible for different eclipses that occur on celestial bodies.
So what exactly is the penumbra shadow? It is the lighter outer part of a shadow, and it is a factor that causes partial solar eclipses, penumbral lunar eclipses, and partial lunar eclipses.
You can consider it a shadow within a shadow. This type of shadow is considered a half-shadow since it is not the darkest shadow; that would be the umbra shadow. So let’s see how the penumbra shadow works.
The Penumbra Shadow
Any opaque object illuminated by a light source will cast shadows into space as the object blocks some of the light. Shadows generally have three areas, the umbra, the penumbra, and the antumbra.
The umbra is the darkest central part of a shadow, while the penumbra is the lighter outer part of the shadow. The antumbra is where the lighter part of the shadow begins from where the umbra ends.
Each of these types of shadow can cause an eclipse. Here is what type of eclipse these shadows can create:
The umbra is responsible for the total lunar eclipses, total solar eclipses, and partial eclipses. The penumbra causes partial solar eclipses, penumbral lunar eclipses, and partial lunar eclipses. The antumbra is responsible for only one type of eclipse, the annular solar eclipse.
The partial lunar eclipse happens when the Earth’s penumbra veils a part of the Moon’s visible surface, which isn’t covered by the umbra.
The penumbral lunar eclipse is the event in which Earth’s penumbra covers all of the Moon or only a part of it, and the umbra of the shadow isn’t there.
The penumbral lunar eclipse is the most subtle out of all the three types of lunar eclipses. It is much more difficult to observe than a total lunar eclipse or a partial lunar eclipse.
When is the Next Partial Solar Eclipse?
The next partial solar eclipse will occur on June 21, 2020. Observers might witness it from Africa, southeast Europe, most of Asia, and in northern Australia.
Another partial solar eclipse will occur in 2022, April 30, and it will be visible from South/West South America, Pacific, Atlantic, and Antarctica.
When is the Next Partial Lunar Eclipse?
We will witness a partial lunar eclipse on November 19, 2021. This partial lunar eclipse will be visible from Nort America, South America, Australia, and most of Europe and Asia.
When is the Next Penumbral Eclipse?
The next penumbral eclipse (of the Moon) will happen on November 29, 2020. It will be visible from North America, and it will begin at 11:30 pm.
How Dark is the Moon’s Penumbra?
The Moon’s penumbra obscures only part of the Sun’s disk. The penumbra has a much lighter shade than the umbra, where the Sun is completely covered.
However, the penumbra shadow’s light level isn’t uniform. It is much darker in the areas adjacent to the umbra at the edges. The variation is caused by the changing magnitude of the eclipse in different areas within the penumbra.
How Large is the Moon’s Penumbra?
You might believe that the largest shadow is the umbra; however, the Moon’s umbra covers only small Earth areas, whereas its penumbra can envelop whole continents and oceans.
This is the reason for why partial solar eclipses occur more often than total solar eclipses. For example, the eclipse, which will occur on April 8, 2024, is a total solar example. Still, its penumbra will cover northern Colombia, all the way to Alaska and Greenland, covering North America, Central America, most of the Caribbean, and massive parts of the northern polar regions.
Why the penumbra is so large is related to the Moon and Earth’s distance and the fact that the Moon is smaller than the Sun. As the Moon grows in distance, its umbra shadow’s diameter decreases, so the Moon becomes tinier.
This results in a V-shaped umbra. Like the Moon, our Earth always casts a penumbra into space, responsible for penumbral lunar eclipses.
Please take a look at the Sun during sunrise and sunset; you are actually witnessing, as long as it’s behind the horizon, the Earth’s penumbra. This happens twice a day in some regions; however, penumbral lunar eclipses are rarer since they require the Moon to enter Earth’s penumbra.
However, these events are fairly unspectacular, and it may even be not easy to distinguish from a normal Full Moon.
Penumbra During Planet Transits
When Mercury or Venus passes in front of the Sun, Earth travels through these planet’s penumbra, but it only lasts for a couple of minutes. This is because these planets appear much smaller than our Sun.
When it comes to the antumbra, it may take several hours. The antumbra’s diameter increases with growing distance from the objects casting the shadow, and thus it is quite wide when it reaches Earth.
Did you Know?
- Eclipse shadows travel at great speed, at around 5,000 mph at the poles, and about 1,100 mph at the equator. The width of the path of totality is at most 167 miles / 268 kilometers wide.
- In the worst case, only two eclipses are occurring each year on Earth. In the best case, only five eclipses can occur per year.
- Identical eclipses are quite rare, and they occur once every 18 years or so. This is known as the Saros cycle.
- We only notice a total solar eclipse when the Moon covers more than 90% of the Sun.
- NASA is always working on predicting lunar eclipses. Their list of predictions extends until the year 2100.
- We won’t experience total eclipses forever. Millions of years in the future, our Moon will be too far away to completely occlude the Sun.
- When our Earth experiences an eclipse, we continue to receive at least 92% of the amount of sunlight.
- Eclipses don’t happen just on Earth; they can occur on any planet. Some planets with many moons, such as Jupiter, experience chaotic eclipses.